# Talk:Scramjet

## Original, unstructured (and unsigned) entries

Why not give a more descriptive account on the subject. Please fix clarity of topic

For the layman, it would be nice to have a simple summary of the characteristics of a scramjet:

- speed range in mph - operating heights (air densities) - speculation on the sort of craft it might be applicable to.

I think some of these things might be in there, but not for a simple overview for those of us who are not wise in the world of physics.

Can someone please expand on the "obvious" problems with practical/commercial applications of scramjets? What kind of acceleration are we talking about, what would be the maximum practical speeds for passenger aircraft, etc? Speaking as an uninitiated guy, I can't say that I quite understand yet, and a quick two or three sentence expansion on those problems would fix the problem. Now especially relevant, with the hypersonic flight tests back in the news. User:MrZaius 22:54, 16 Nov 2004 (UTC)

Nice Job! Congrats to everybody who helped contribute to this article so far. It was badly needed. Mackerm 23:28, 9 Apr 2004 (UTC)

On second thought, much of this material has been covered at hypersonic. Maybe it's best to simply redirect there. Mackerm 00:01, 10 Apr 2004 (UTC)
I did the opposite and moved most of it here, leaving hypersonic as just a definition page

## Math

From the article:

This would provide a 700% (7x) efficiency boost to a space vehicle that used it.

No, a 100% effectivity boost gives 2x efficiency, so 700% gives 8x. Which of "700%" and "7x" is the original figure? Thue | talk 20:48, 15 Jul 2004 (UTC)

They're talking about ISP, I presume. The talk about oxygen is seriously misplaced, so I have edited this down; LOX is dirt cheap; the tankage is extraordinarily lightweight (1% the mass of the contents) and the overall impact of carrying the oxidiser is minor.

The real point of a SCRAMJET is that the higher ISP in the atmosphere reduces the propellent usage enough, and operates to high enough speed that it might enable SSTO.

The problems of SCRAMJETs include that the whole vehicle tends to melt; the engine, tends to melt; and the extra speed it gives is relatively small compared to orbital speed (about mach 25).

You also spend longer in the draggy atmosphere, so you spend more fuel to save LOX. But LOX is dirt cheap (\$0.2/kg) compared to hydrogen (\$5/kg) or kerosene (<\$1/kg).

--Wolfkeeper 22:33, 21 Aug 2004 (UTC)

Just a note on the costs. Combust hydrogen and oxygen at high temperatures and the result is a mix of OH and H20. This means that you need between 1/16 and 1/8 of the mass of hydrogen compared to oxygen.
Um. No. The optimum ratio for maximum ISP- which is quite distinct from the stochiometric ratio, is at about 5.5:1 of LOX to LH.
Launch vehicles typically use between 6:1 and 8:1 to optimise the *vehicle* performance rather than the exhaust performance.
Well, maybe you can correct me, but I thought that over-stoichiometric operation was desirable due to the cooling requirements of the combustion chamber.
Overstochiometric is used because it pushes up the hydrogen content of the exhaust gases. For technical reasons hydrogen goes faster for a given energy than other elements or molecules, so it turns out that running fuel rich gives much better ISP. It does also cool the combustion somewhat, but that's not why.
Since I was talking about theoretical performance (since there are no working scramjet lifters today), comparison at stoichiometric is entirely relevant if you want to compare the theoretical advantages of the two types of engine
That's nonsense. Rockets are dogs at stochiometric. You can only sensibly compare things if you co-optimise, i.e. run each at their own individual optimums and then compare the results.
- Of course, apart from anything else, rockets have the practical advantage that they actually work now :-)
i.e. by your reckoning for the \$5 of hydrogen you need between \$1.60 and \$3.20 of LOX.
No, at most \$1.60. Note how much cheaper the LOX is than the hydrogen. --WolfKeeper
A great part of kerosene is C12H24 (for RP-1), and assuming that the final products are between CO/CO2 and OH/H20, you need between 36 and 64 atoms of oxygen to combust one molecule of kerosene, or a weight ratio of between 1:3.4 and 1:6.1 kerosene:oxygen for a price point between \$0.68 to \$1.36 for your oxygen to combust one kilogram of kerosene.
Yes, and the fuel costs are still cheaper than using hydrogen (even though you need more hydrocarbon based propellent than hydrogen). And, AFAIK scramjets can't run on kerosene since it isn't such a good coolant, hydrogen is a good coolant.
Most kerosene proposals I've seen involve running the fuel through the walls and using the heat of the flow to crack it into ethylene/propane/ethane/hydrogen and similar before injecting the fuel as a gas. This gives the required reaction rates, and there's plenty of research on running scramjets on these products. As for the cooling, I think the best answer is that it depends on running times, building materials, aspiration or film cooling, combustion temperature and flight path to such an extent that it's just not possible to say whether kerosene is usable or not. Additionally, the cooling improves if you inject more fuel than stoichiometric, which is one reason why rockets run rich.
Of course, I'm being devils advocate here because the greatest savings aren't in pure fuel costs, but the comparison is not quite as bad as you make it out to be.

## ISP

High ISP means more mass to orbit for a given vehicle size.

Only if everything else is equal- which it seriously is not- particularly in this case- scramjets and airbreathing engines in general are *heavy*.
My comment was more to what's the point (ie the potential) of airbreathing engines. The final assessment as to whether the balance is going to come out positive in terms of scramjets is still under heavy debate within the scientific community, and obviously your point is one side of the equation. I wanted to make the point that most people are talking about being able to lift more mass than necessarily having SSTO.

As previously noted, the cost of fuel is much less than the cost of superstructure for a given vehicle. SSTO is a maybe, on one hand you have the cost advantages of reusing a vehicle, on the other hand a reusable vehicle is much more expensive, since leading-edge failures, use of ablatives or even just a hard landing all have to be ruled out, making the vehicle overengineered compared to a single use vehicle. The overall impact of carrying oxidiser is major,

If it's so major, how come rockets make orbit, but airbreathers are still so far away from it?
Why don't quantum computers, flying cars, nuclear rockets, genetic medicines or AI work? Same reason.
They all do *work*. They may not do what you want them to do though. And for different reasons in every case.
No, they all *should* work, but don't due to technical problems.

because the same order of magnitude of oxidiser is needed as fuel, and every extra unit of mass costs exponentially if carried on a rocket.

No, because the fuel mass on a rocket goes down very rapidly initially which means you don't have to carry it around, only the empty tanks which are very light. It's mainly the dry mass relative to the ISP at the end of the burn that counts, and the tanks that are about 1% of the weight of their contents (for dense fuels), and rocket engines have 100:1 thrust/weight ratio so don't give you a lot of dry mass.
Heiser and Pratt have takeoff weight fractions for a rocket: Payload 4%, Empty 7%, Fuel 24%, oxidiser 64%. I'll just get you to look at the rocket equation- (final velocity)= ln [(start mass)/(end mass)]*(exhaust velocity) and you can work out for yourself why reducing the start mass for the same payload gives an exponential decrease in the cost of the takeoff.
Those are percentages. Every extra unit of mass scales up the rocket in *proportion*. It is *not* exponential on increasing the mass. Rockets *are* exponential on delta-v. But so are airbreathers.
Yes but different exponent due to oxidiser not being on-board.

Also Mach 25 is about 7000m/s and an altitude of up to 90km is a significant fraction of orbital speeds. All this means is that a scramjet alone won't do the whole lift to orbit, which we all knew anyway.

7800 m/s more like it.
Mach number is a function of temperature. Go look at the temperatures at those altitudes. I used ~200K, though I think the mean is a bit under that at 90km. M=sqrt[(gamma=1.4)*(R=287)*(T=~200)]
True, but irrelevant. Orbit is mach 25 or so (depending on what altitude you use for 'mach 1'). Scramjets run out of steam at about mach 9-12. You've still got a long way to go.
Entirely relevant, since I was correcting your point. I'd like to see a reference for scramjets running out of steam at Mach 9, but I'm on board with Mach 12. I would comment that pure scramjets run out of steam at (Mach number of your choice) in the same way that turbojets run out of steam at Mach 1.2, which is to say sort-of for a pure version of the concept not augmented by any tricks. That doesn't stop aircraft from flying far faster than that limit.
Turbojets run out of steam at about mach 3.2 - mach 4 or so- when the blades melt. There are paper designs like ATREX or SABRE that are good for mach 5.5; but they require precoolers- and the precoolers, well, nobody has ever built one, although latest research suggests that they are physically possible.
Well, maybe I stand corrected. My understanding was that at a bit above the transsonic region that turbojets are using ram compression and injecting into the bypassed air, effetcively operating as a ramjet/turbojet hybrid. My point was mainly that a lot of limitations of any *pure* concept can be overcome.
It depends on whether you consider the inlet to be part of the engine or not. I personally think the inlet counts; if you don't, then no jet engine, except scramjets can do even mach 1.
My point was more that a pure turbojet is freestream-compressor-combustor-turbine, as soon as you start injecting fuel into the bypass air or into the exhaust (afterburner) then you've got a hybrid engine. I believe there are a couple of modern engines which can fly supersonic in a pure turbojet mode, but in v. low ss region. My point was not that it's bad, just that other engine types than scramjet also don't use a pure concept.
Just a comment too: Mach is spelled with a capital M, like Pitot, Pascal and Newton. Not so important here, but for the article a point to note
A scramjet that does mach 8.5 the state of the art IRC. That's only 1/3 of the way to orbit. Some people think that mach 12 may be the top speed. And the scramjet is still heavy; quite possibly too heavy to take to orbit. Sure you can drop off the scramjet, but then you're little better off than if you just had a two stage rocket.
Yes, that's the point. Imagine if your rocket launch, including all stages was 5% more efficient (Dual-mode scramjet-->scramjet-->RBCC). That's still money in the bank, and reduced cost to orbit. This is not an all or nothing proposition.
Yes, but it's the dry mass you pay through the nose for, not the fuel. The scramjet is heavier, hence more expensive. Only if the scramjet enables reuse or is less labour intensive will it be worth it. But we don't know whether it will be- nobody has ever built a practical system. It might well be more labour intensive.
That is certainly an argument, and if you can back it up with some decent references (how much heavier, how much more expensive etc), it ought to go in the article. The people I'm talking to aren't thinking necessarily about reusable vehicles, but that's not necessarily right either.
Yes, well- I like missiles, really I do, except when one is pointing at me.
And most of the *rocket* people I know *are* talking about reusable vehicles.
Yes, Funny about how everybody's talking about making reusable vehicles with somebody else's method.
The guys I talk to aren't.
Know why that is? It's because reusable vehicles are an order of magnitude harder to make than throw-aways regardless of the technology used. It's a political thing: "That technology won't work because reusable vehicles are too difficult to make" and it's largely a red herring. The technologies to make reusable vehicles lie more in the HT materials than in the engine technology. If we could make a rocket engine today that didn't require an overhaul between each flight it would be at least as much of a leap forward as having working scramjets.
Actually, I know of atleast two engines that don't require overhauls. The XCOR (http://www.xcor.com) engine, which is stunningly reliable and suffers no degradation (Quote: "we stand beside our engine, but not behind, that would be stupid", and they mean it- quite literally). Also, the all-aluminum engine 'Big Bertha' at http://www.spl.ch/gallery/mixed.html ).
Again I'll have to take your word for it. All I see on these pages are small concept engines for suborbital flight, which often means low ISP. I would just note that it's no big trick to sacrifice efficiency for longetivity.
Actually, nearly all liquid fuelled rocket engines are reusable to one degree or another. They have to be, since they need to be fired multiple times in testing. The thing which often limits reuse is start up and shutdown- the nozzle at the throat is under amazing thermal stresses. But XCOR have shown that 10,000 ignition cycles are doable. Once lit, even existing engines like RL-10 have been run for hours without any failure or observed degradation at all. Compared to a jet engine, that doesn't sound very long, but it only takes about 6 minutes to get to orbit, and individual rockets often only burn for a fraction of that.
I'd be interested in a reference for the RL-10, because as far as I know, it was originally specified for under 10 minutes in a single flight and then a full overhaul, being restartable rather than truly reusable.
The technical challenges to having reusable rockets and reusable scramjets are similar, and it's all about having a material that will withstand 3000K without degrading.
No, in a rocket engine a cool boundary layer forms in the hot gas, and the metal itself never gets hot. 'Big Bertha' uses aluminum that melts at about 600C or so.
Perhaps you could explain to me how that is thermodynamically possible without the injection of a coolant gas (=over stoichiometric operation)?
The classic technique is "regenerative cooling" where you pass the propellent through a jacket *around* the combustion chamber, so it sucks the heat out of the wall, and then the heated propellent gets sent into the injectors. Because you are effectively pumping the heat away from the (*highly* conductive) wall it doesn't fail- the heat eventually goes out the exhaust. Similar ideas might be used with scramjets, but you've got such an enormous wall to cool, so it's much harder to get it to work (and less propellent goes through the engine in a scramjet.)
I know about regenerative cooling. It doesn't create a cool boundary layer in the manner I though you were originally referring to (since the BL is only cooled by the wall). I had thought you were referring to permeable walls, or film cooling, which are both options with scramjets too. Obviously the heat management in a scramjet is different to a rocket because of the relatively low pressures in the combustion chamber and the big area.

## my recent edits

some may object to them. i am User:Xmnemonic, though i cant login to wikipedia for various reasons right now. 141.213.39.67 18:27, 17 Nov 2004 (UTC)

I am user:Xmnemonic. Regarding the CFD wording: fair enough, I was desperately trying to make the CFD comment in question more mature sounding and was a bit hasty in my word choice (can you imagine the phrase "coming to grips" appearing in Britannica?).
On "shock" vs. "shock wave": while the former is far more common in actual technical speech, you cannot deny that "shock" is synonymous with "shock wave" (otherwise I suggest you mention normal shocks in the shock article and remove the shockwave wikilink). You know that shock means shock wave; I know that shock means shock wave. John Anderson knows the terms are equivalent, and uses them alternatively throughout the shock wave section of Modern Compressible Flow (I assume you're familiar with Anderson and his background). But the average, non-aerodynamicist does not know that "shock" means "shock wave." And while using "shock" instead of "shock wave" is understandable to the informed, to those who are using this encyclopedia to for its intended purpose, i.e. to learn new information, using the technical term does nothing other than confuse. Yes, when discussing aerodynamics with fellow aerospace enthusiasts, I always say "shock." Using "shock wave" instead in such context would be foreign to me. But to the majority of this article's audience, "shock" is the unfamiliar term, whereas "shock wave" is not. Neither term is "wrong", as with "delta vee" vs. "velocity change"; here though, in this article, one term I think is stylistically more appropriate than the other. - User:Xmnemonic
After reading this article and accompanying talk page I'm in shock. Not anaphylactic shock just regular shock but no electrical shock and I'm not in a shock wave.--Gbleem 00:22, 19 Nov 2004 (UTC)
I am user 129.247.79.163. I agree that "shock" is synonymous with "shock wave", however since "shock" is the term mostly used by technical writers, I think it should be the one used. Secondly, I would note that all of the shocks we are discussing here are attached/stationary, and this is not the common understanding of a "wave". Thirdly I note that the word "shock" is linked to the article shockwave, which should clear up problems in this direction. However that in the interests of wikimocracy, that if you feel strongly enough about it to edit it back again, I'll both not change it, and attempt to preserve it from future edits. As far as the CFD thing is concerned, the only problem that I had was that it changed the meaning of the sentence, rather than the edit itself. I think your change captures it perfectly.

"A scramjet is reminiscent of a ramjet. In a ramjet operating supersonically, the supersonic inflow of the engine is decelerated to subsonic speeds and then reaccelerated to produce thrust. This deceleration, which is produced by a normal shock, creates a total enthalpy loss which limits the upper operating point of a ramjet engine. At high speeds, the kinetic energy of the freestream air entering the scramjet engine is large compared to the energy released by the reaction of the oxygen content of the air with a fuel (say hydrogen). Thus the heat released from combustion at Mach 25 may be around 10% of the total enthalpy of the working fluid. Depending on the fuel, the kinetic energy of the air and the potential combustion heat release will be equal at around Mach 8. Thus the design of a scramjet engine is as much about minimising drag as maximising thrust.

Changing from subsonic to supersonic combustion makes the control of the flow within the combustion chamber more difficult. Since the flow is supersonic, no upstream influence propagates within the freestream of the combustion chamber. Thus throttling of the entrance to the thrust nozzle is not a usable control technique. In effect, a block of gas entering the combustion chamber must mix with fuel and have sufficient time for initiation and reaction, all the while travelling supersonically through the combustion chamber, before the burned gas is expanded through the thrust nozzle. This places stringent requirements on the pressure and temperature of the flow, and requires that the fuel injection and mixing be extremely efficient."

These paragraphs are not clear to me. It's unclear when we are talking about conventional ramjets and when we are talking about scramjets.--Gbleem 02:02, 19 Nov 2004 (UTC)

I am user 129.247.79.163. When you are talking about the kinetic energy of the incoming air, and the maximum energy which can be released by the fuel, it doesn't matter which one you're talking about, since all engines can be thought of as a black box with air+fuel=>black box=> thrust+drag. Thus the first paragraph is supposed to be an explaination that the scramjet is merely a ramjet in which drag is reduced by keeping the flow at higher velocities, since the thrust is limited by the energy releasable by the fuel. As for the second paragraph, I had hoped that it was clear that it was all about the problems inherent in controlling supersonic internal flow. If you can clarify it, please feel free.

## introduction

I note to Gbleem that i reversed a small part of your introduction edit about the upper limit of scramjet operation. This is because a scramjet without oxidiser input is like a jet engine without an afterburner. So you need to clarify, when you're talking about the upper limit, under which circumstances you're talking about. After all, for a long time there were no jet engines which could go supersonic without an afterburner (see some of the discussion in the previous section), but people had no problem about talking about supersonic jet engines. The last paragraph in the theory section is intended to capture some of the flavour of this line of thinking.
I was assuming a turbojet with an afterburner is still a jet because the oxidizer is air. A scramjet with oxidizer injection should be a scramjet-rocket hybrid. --Gbleem 15:00, 19 Nov 2004 (UTC)

## Concorde

The page claims that the Concorde was the only "production supersonic airliner" to have been operated, yet the Concorde's own page states that it was "one of only two supersonic passenger airliners to have seen commercial service." The supersonic transport page states that the other is the Tupolev Tu-144, which only flew commercially from 1975 to 1978. Perhaps the Tu-144 should be noted? --LostLeviathan 15:41, 19 Nov 2004 (UTC)

## Fuel

During the Hyper-X tests, there was much talk about the potential for scramjet technology to operate using "practically no fuel" once at hypersonic speed. I don't see any mention of this in the article, though, and I'm hardly an expert-- does anyone know anything about this? siafu 1 July 2005 20:00 (UTC)

Two of the links in the "references" section are now broken:

Can suggest the latter should be replaced with something appropriate near http://hapb-www.larc.nasa.gov/, but I don't have the subject expertise to select one. I have too little Wikipedia mileage or personal time to know the best action for flagging the first (seems deleting it might be missing an opportunity to replace it). Huge Bananas 11:18, 9 November 2005 (UTC)

## Suggest Adding Minimum Scramjet Mach Number

An obvious question is what's the lowest workable Mach number for a scramjet. This impacts the type and amount of performance boost needed to reach scramjet operational speed. E.g, if it's Mach 2 that's reachable with a turbojet. If it's above Mach 3 that likely requires a ramjet or variable cycle engine. If it's Mach 5, that might require rockets. I looked around the net but don't see authoritative information on that. Maybe a more knowledgeable person can comment/add that info? Joema 19:53, 19 December 2005 (UTC)

The reason it's not there, is that the minimum Mach number is unclear. The minimum Mach number is limited by the fact that the compressed flow must be hot enough to burn the fuel, and of high enough pressure that the reaction is finished before the air moves out the back of the engine. (When a supersonic flow is compressed it slows down). It is clear that a pure scramjet can operate at Mach numbers of 6-8, but in the lower limit, it depends on what you're allowed to call a scramjet. Certainly there are designs where a ramjet transforms into a scramjet over the Mach 3-6 range. In this range however, the engine is still receiving significant thrust from subsonic combustion of "ramjet" type. On second thoughts, maybe I'll just add this explanation. AKAF 08:05, 20 December 2005 (UTC)

Looked around some more, a few references below. Suggest including a statement something like: "while scramjet minimum Mach varies with design details, the single prototype vehicle actually flown had a minimum Mach of 4."
Is the X-43A a pure scramjet or a combination ram/scramjet? The below article says it's only boosted to Mach 4 before the scramjet takes over. The other references vary, maybe because of what you mentioned.
My purpose in mentioning this is it's an obvious question readers will have. It affects scramjet practical utility. Mach 4 is very high minimum Mach for a practical operational vehicle. It implies mandatory rocket boost, not turbojet. If true the X-30 would have required turbojet power from takeoff to Mach 2-3, then rocket power from Mach 3-4 (unless a variable cycle turbo/ramjet could achieve that), then scramjet power from mach 4-17, then again rocket power from Mach 17-25. Maybe that's one reason why it wasn't built.
Boeing X-43 : "Scramjets only operate at hypersonic speeds in the range of Mach 6 or higher, so rockets or other jet engines are required to initially boost scramjet-powered aircraft to this base velocity."
[1] "initially propelled by a solid rocket booster, the scramjet demonstrator take-over will occur at approximately Mach 4.5 where it then will accelerate to flight speed between Mach 6.0 to 7.0+."
[2] "ram/scramjet engines in the range of flight Mach numbers from about 4 (the minimum Mach number for ramjet operation)" Note: although it says ramjet, from the context it appears to mean scramjet.
[3] : "The prototype scramjet aircraft was dropped from a B-52 aircraft, and then boosted to Mach 4 by a Pegasus rocket." Joema 13:17, 20 December 2005 (UTC)

X-43A is problematic because practically everything except the press releases are classified. For this reason its not a good example. Additionally, a lot of the general press releases are just plain wrong about the flight path. We do know that the X-43A is a dual-mode design (ram/scramjet), but as far as I remember its boosted to about Mach 6/9 (for the Mach 6.5/9.8 test) before separation, and then the engine falls for a couple of seconds before engine start engine start. There are problems with practically all of the popular science articles about it because of the failure to distinguish between stage separation and engine start, let alone all of the factual problems. I think the Mach 4 comment comes from a statement in the NASA release about the concept of a scramjet being to go from Mach 4 [[4]] (Paragraph 4).
Has it been determined yet at what speed the rocket cut off and whether the scramjet itself consequently accelerated to the top speed of 7,546 mph(as per the X-43 page)? I think this would make an important addition to the article.Mytg8 (talk) 18:40, 23 August 2008 (UTC)
Since pure (non-afterburning, non-precooled) turbojets can't go above about Mach 1.5, AKAF 13:49, 20 December 2005 (UTC)
Um. Do you have a source for that claim? I read in a book about Concorde that it was actually capable of reaching Mach 2.0 with its afterburners switched off, but it just took a long while (and wasted fuel, since it spent a long time in the transonic region where there's unusually high drag.) It certainly doesn't have precoolers. And a design for a new version of Concorde was designed to not use afterburners at all IRC.WolfKeeper 21:03, 20 December 2005 (UTC)
they certainly won't be useful for boosting a scramjet. Since afterburning, precooled turbojets aren't significantly more efficient or safer than rockets for accelerating flight, AKAF 13:49, 20 December 2005 (UTC)
Afterburners? Unsafe? Do you have a source for that claim? Concorde used them... Lots of supersonic planes use them.WolfKeeper 21:03, 20 December 2005 (UTC)
a rocket start is likely. It is far more likely that the first couple of generations of scramjet will be as a rocket second stage ie a more efficient rocket, rather than a new turbojet. You only have to look at boeing/airbus current designs to see how far we are away from even a supersonic airliner. AKAF 13:49, 20 December 2005 (UTC)

You can clearly accelerate to and fly at Mach 1.5 without afterburners, since the F/A-22 already does that. Some believe the YF-23A could fly at Mach 1.8 without afterburners, but it's classified so we don't know for sure. The Concorde definitely cruises at Mach 2 without afterburners, although it uses them to reach that.

Re being far away from a supersonic airliner, because of complex airline economic issues, it's likely the next civilian supersonic transport will be a business jet. BTW the Cessna Citation X has marginal supersonic ability right now, but they're prohibited from using that since the plane's FAA certification doesn't allow it. Joema 04:08, 21 December 2005 (UTC)

Ok, so this is an answer to Joema and Wolfkeeper above: Perhaps I should have added a question mark behind the Mach 1.5 value, since my only point is that a turbojet alone isn't going to get you to Mach (4-6) (Appears on further research that you're both right-Sorry). I don't say that jets with afterburners and precoolers are unsafe, just not significantly safer than a rocket engine for that kind of work. Just do a thought experiment: We want to accelerate from standstill to Mach 5. If you want to do it with a jet, you need a complicated combination of Jet/afterburners/ramjet. With a rocket you just need a single engine. I'm not saying that option one is impossible or even undesirable, just that its significantly more technically difficult than adding a rocket engine. In the case of civilian aviation, difficult=unsafe and not all of the technologies which you see in military jets are transferrable to civilian transport since military aircraft are generally considered unsafe by civilian aviation standards (and so (for example) you won't be seeing a Mach 3 civilian transport anytime soon).
To get back on topic, do the paragraphs I added help at all? I realise that the definition of minimum speed is inadequate, but I have a stack of papers on my desk that were about exactly as unhelpful, so I think that's just the state of the art in this field. AKAF 08:16, 21 December 2005 (UTC)

Appreciate the dialog on this. My recommendation would be to say something like "Similar to a ramjet, scramjets have a minimum functional speed. This speed is uncertain due to the largely classified nature scramjet research. However it's likely to be at least Mach 4 for a pure scramjet, with higher Mach numbers more likely. A hybrid ramjet/scramjet would have a lower minimum functional Mach number, and some sources indicate the NASA X-43A research vehicle is a hybrid design."

Mach 4 may be much too low though. I get the impression that the useful speed is more like Mach 7. There's also a big difference between 'functional' and 'useful thrust' i.e. you can have something functioning but still slowing down.WolfKeeper 16:54, 22 December 2005 (UTC)

Stating this at least gives casual readers the concept, without being inaccurate. Because of the wide Wikipedia audience, it's possible by stating this you'll get expert feedback with more definitive sources.

Exactly how to word it, or whether to include it is your call. However it seems such an obvious question my suggestion is to say SOMETHING. This isn't a court of law and we won't be sued. Lots of people reading this don't even know what a scramjet is. I think it's valuable to emphasize the overall characteristics. One of those is a minimum Mach number, similar to a ramjet. Even if you heavily qualify it and give a broad range, it still accomplishes the educational purpose. If an expert reads it and can more tightly bound the minimum Mach number, Wikipedia gives him the ability to suggest or change it. Just stating something increases the liklihood of that happening. That's the great thing about Wikipedia. Joema 00:16, 23 December 2005 (UTC)
I've just added text as suggested by Joema. I hope that this is a simple version, which can be understood by everyone, with the more complicated explaination for why in the "theory" section. I've changes the minimum Mach number to 5, since I really couldn't find anything below that. Certainly there are a number of groups doing full-engine tests in the Mach 5-6 range. Although I agree with Wolfkeeper's comment about 'useful thrust', I think that it's really not productive to talk about here due to the nature of scramjet research. I find it useful to remember There are no examples of free-flying, thrust producing scramjets in the open literature.AKAF 09:50, 23 December 2005 (UTC)

The changes look great, thanks for doing that. Joema 16:28, 23 December 2005 (UTC)

## Metric

Hi everyone. I am from Australia and not Yankee bashing...but....shouldn't metric equivalents of units in this article be provided (if even in brackets)? especially since the recent test was done in Australia which uses metric units? english units can be useful eg. when discussing height of people etc, but for scientific articles, i would think that SI units should be used.

Yes, so add it, anyone can edit. Please remember to keep significant figures approximately the same, so 11000 ft is probably best translated as 3300m or 3400m, depending if you think the original number was rounded or truncated (11000.0 ft =3352.8m). If you feel that you'd like to spend more time at wikipedia, you can create an account, which lets you keep track of things you like, and what you've done. Even if you don't create an account, please sign your posts by typing four tildes (~~~~), which will automatically link to your username. AKAF 08:45, 29 March 2006 (UTC)

## Page Reorganisation April 2006

I made a number of edits to Scramjet lately, trying to do a bit of a cleanup of the whole article. I would appreciate your input. One of the things I did was to split off a page about scramjet programs, since it seems like there might be enough programs for a general overview page. I'd appreciate it if that page could be looked at a bit.

I made an edit to the section on advantages and disadvantages of airbreathing engines, but that whole section still just doesn't seem right. I would like some discussion of the sort to be on the Scramjet page, but it's very confusing, because the arguments are handwavey, without numbers. This lack of numbers mean that it's hard for a layman to understand the arguments presented, and to weigh the advantages and disadvantages. It's even harder because, as for the minimum Mach number discussion, the severity/advantage of some of the options are also not well understood. For instance the fuel density problems and coking due to cracking for active cooling are not discussed, and could probably do with a page for themselves.

I have tried to bring in a realistic example, with some numbers, to try and illustrate the arguments, but the whole section still reads a bit scrappily.

My favourite options here (in order) would be:

1. Make a good quality page on this argument, along the lines of a short review of Heiser and Pratt's "Hypersonic Airbreating Propulsion", with some equations and estimates of the possible ranges of each of the advantages and disadvantages.

2. Reduce the section to a table of advantages and disadvantages plus a case study

3. Try to rewrite the section, somewhat longer, as prose, with a but more discussion of areas of research which apply to the field (materials, fuels, structured, MHD, SSTO, etc)

I make edits to the "recent progress" section to try to get away from nationalistic arguments. I can fully appreciate why these might be attractive, but they were leading to a scrappy article. Any suggestions would be appreciated.

AKAF 07:13, 24 April 2006 (UTC)

Do you have a cite for the Ariane 5 ECA figures improvement if a scramjet was used? Specifically it contains the phrase "immediately increasing the total payload by a factor of 16".WolfKeeper 07:33, 25 April 2006 (UTC)

It's not a cite, it's a calculation. I was trying to make the point that in the _ideal_case_ (which we don't have) that the mass which is now liquid O2, in the main stage, would all be payload. Ideally I'd like to make the following argument:

(Scramjet benefits)= (A+B+C) - (X+Y+Z) (+-) (I+J+K)

Where

A,B,C are the benefits (positive ISP, easier transfer to polar orbit, supersonic cruise etc)

X,Y,Z are the negatives (complexity, drag, new materials, etc)

I,J,K are the unknowns (system performance, system weight, environmental impact, SSTO, etc)

And give some discussion of each of these to illustrate the degree of uncertainty for each.

You just can't do that, because they just don't add. It's a mass budget for a space vehicle. Things tend to multiply if anything due to mass fraction considerations. The vehicles follow radically different trajectories to reach orbit. The ISP of scramjets varies over the flight speed and altitude, and the instantaneous weight goes down over the flight differently, and the scramjet dry mass is much heavier, which means the vehicle gets less speed from the final orbital burn. The final burn for orbit from say Mach 15 to Mach 25 involves the vehicle halving in weight; and most of that weight loss would be LOX. And then there's another burn from mach 0 to say mach 7, that's another factor 1.69 if you use pure rocket. If you use jet engines then the dry mass goes up even more, and you pay that all the way to orbit... it's all very messy. And I haven't even worked out how much hydrogen you would burn on the scramjet.WolfKeeper 10:30, 25 April 2006 (UTC)
Would it have honestly made a difference to my argument if I'd substituted * for + in the discussion? Point is: discussion of numbers, not handwaving. Fine, if the above is your best example, then put it in.
No, because it's original research.WolfKeeper 17:12, 25 April 2006 (UTC)

It's not original research. Having the equation (maximum new payload ignoring all negatives)=(old rocket weight)-(LOX weight), is the statement of barest fact based in physics. In fact what is currently completely missing from this section is any statement of fact. The section is currently unreferenced. There are vague arguments, but not even the vaguest estimates for their relative importance. AKAF 07:25, 26 April 2006 (UTC)

I'll comment that SSTO is scarcely the only option, but it's a favourite strawman, so let's get it in there. The maximum Mach number is another huge point, which should also go in. As should RBCC and exotic fuels for range extension. AKAF 11:26, 25 April 2006 (UTC)

In the Arianne example, I'm making the comment that the boosters and top stage are irreplaceable, so an estimate for the value of the positive benefits is 16 times the payload. The last paragraph is then leaving space for a discussion of the estimated size of the negatives. The reason for this, is that essentially a lot of aerospace "experts" make arguments like in this section that the engines are "very" heavy, when the question as to whether scramjets will ever be practical is very much a question of the hard numbers of exactly _how_heavy_, or that the heating problems are fixable by one method or another without discussing the technical problems of such a system.

I think that any arguments (both positive and negative) need to be very clear as to how important they are. You may believe from my writing that I fall generally on the positive side, but I want a discussion of all sides. I _do_ have a serious problem with handwavey arguments, and arguments which are mathematically dubious. In the category of handwavey would be all sentences including the following: "seems to", "very", "may be" and a whole stack of other weasel words, which you'll notice I've also used :-). I think that these are illustrative of the imprecision of the current section.

An an example of a mathematically dubious argument, consider the paragraph on Lower thrust-weight ratio, which only talks about the engines. I think it is simply silly to talk about (rocket engine vs scramjet engine) when the point that you're trying to make in this section is (rocket vehicle vs scramjet vehicle). Thus, (in the idealised case) you would need to compare (total thrust)/(total weight), where a discussion of the range of values of each of those variables would be beneficial.

AKAF 09:26, 25 April 2006 (UTC)

It wouldn't matter if you were right, the wikipedia rule is no original research. Grab a book, read it; search the web, find a reputable analysis.WolfKeeper 10:30, 25 April 2006 (UTC)
Last time I saw some rough analysis on this, the payload went up by not 16 times, more like 16% extra payload.WolfKeeper 10:30, 25 April 2006 (UTC)
Sigh. Not OR, simply illustrating the point rather better than your analysis of the space shuttle subsystem in the first paragraph. The total payload may well go up by only 16%, or even down by 70% (as in some analyses). The point of the section is the following two points: # What factors affect potential scramjet performance # How important are those factors.
In fact all the figures I've seen say that scramjets don't outperform rockets in any major way; and they're a total nightmare to design, so R&D costs are high. But the real battle is not about performance, it's about costs; and scramjets start out at a disadvantage there too. Still, if a scramjet vehicle is fully reusable, maybe even a three stage vehicle *might* make sense; maybe- maybe not.WolfKeeper 17:12, 25 April 2006 (UTC)

I think it's fair to say rather than "all the figures" is "all the projections", since they are all talking about projections based on an array of assumptions. As an example, I've seen figures between 0.2g (Harry Spencer) to 0.5g (Robert Zubrin) to 10g (Fred Billig) for scramjet acceleration. Now, since only one of those three figures was experimentally measured by an expert in the field, I know which one I believe, but I think that the section deserves some discussion of the sort. Especially as to which set of numbers leads to which projection. I think that something like the discussion at Drake Equation would be appropriate. AKAF 07:25, 26 April 2006 (UTC)

The current discussion addresses point 1 in a broad way, and point 2 not at all. I am questioning the point of having the whole section if that's the best it's possible to do. I put a point under the section for calculation of the mass of the vehicle, and you removed it. Fine. Put something in other than vague qualitative assertions, and I'll be right there with you. AKAF 11:26, 25 April 2006 (UTC)
It's not appropriate to make stuff up; we need to get well founded information from somewhere to stick it in the article.WolfKeeper 17:12, 25 April 2006 (UTC)

I'll dig out some books. I've got copies of [[5]], [[6]], [[7]] and [[8]] and I guess a few dozen papers on the whole-systems design, and about the same on each of the subsystems. I guess if we start with Heiser and Pratt, they go through some reasonable discussion of parameters. If you already have a couple of these, then I can concentrate on the remaining. I'll look at it in the next couple of days. AKAF 07:25, 26 April 2006 (UTC)

## simple discription

would the person that wrote this section use proper scientific language instead of the ambiguous tripe that is currently in that section,

• what dose he meen exactly by ' the fuel is to hot to burn properly? is tis refering to problems with nitrous oxide production, carbonation in the fuel lines, or some sort of material problem
• Blockage!!???#@~@!!! kitchen funels!!???!##@!! i am exasparated the use of such language ;air being 'collected' 'coursing' drag which increases 'quickly' with speed. this is not put very concice

i am assuming you are refering to the use of a converging - diverging pasage. and anothere thing the working fluid dose not 'become even hotter and more compressed' due to combustion, it may get hotter and expand, unless of course you are talking about some sort of detination of the fuel then there may be some sort of pressure rise any way sorry to bitch, will try and carm down and start a more civilised edit ric

Does anyone else think it would be more logical to have the advantages separated some the disadvantages for clarity? For the most part it is fairly easy to tell which is an advantage or disadvantage, but there are a few places where this could be confusing for some readers. Johnwwatson 12:57, 5 August 2006 (UTC)

## Half an engine

This section read "The shockwave of the vehicle itself compresses the expanding gasses..." I don't know much about scramjets, but I went ahead and changed that to "The shockwave of the vehicle itself compresses the inlet gasses..." I reasoned that earlier in the article, at "Simple description" is was discussed that the underside of the vehicle forms the intake and nozzle for the engine. I suspect that both of these versions are true, as the shape of the bottom of the X-43A would seem to indicate that. The engine is not placed on the back of the vehicle, but in the middle. It seems to use the front to compress the inlet gasses into the engine, and then the shockwave and the back may be used to direct the exhaust gasses. That should produce lift from deflection at both stages, and get pretty high efficiency by minimizing the size of the engine. I changed it because in the "History" portion it was noted that the pressure inside the engine must be fairly high in order to have combustion before the gasses leave the nozzle. This seems to indicate that a scramjet has a nozzle, but would probably need the front of the aircraft to produce the necessary compression, hence, it's the front half (the compressing of gases) that wouldn't be performed by the engine. However, it could probably be reasoned either way, so if you know a lot about scramjets, I'd appreciate it if you could expand upon that section. -NorsemanII 07:41, 2 October 2006 (UTC)

## Unreferenced

I've add the Unreferenced, which may be a bit unkind to some of the article, but not to some key sections. Erich (talk) 11:02, 19 June 2008 (UTC)

## Reaction times

I've reverted the edit to reaction times, because it missed the point. Although the full reaction takes microseconds, modeling the reaction requires that all partial reactions are also modeled. The chain branching reactions are particularly sensitive to small amounts of H and O, and these are self-catalysing reactions, meaning that a very small timestep is required for direct modeling. Exactly what timestep is required, will depend on the modeling scheme, but I think that 10e-19 is a good reference number for simple reactions. Consider for example the H+H+M and O+O+M reactions (NASP 10+12), which are exponential for small quantities of mixed fuel. In this case a good fit or small timestep is required for direct modeling, making the chemical reactions "stiff". AKAF (talk) 08:36, 26 June 2008 (UTC)

The edit did not miss the point. There are some chemical reactions that are faster, but *typically* the times will be microseconds. I took away again the sentence which claimed the typical times are of the order of E-19 seconds. This is not only misleading, it is erroneous. Cypoet (talk) 02:37, 27 June 2008 (UTC)
I see your point thus -- A few sentences explaining this could be added if you so wish, but I don't think this belongs in this article. This is specific to chemical modeling and CFD, not scramjets. —Preceding unsigned comment added by Cypoet (talkcontribs) 02:40, 27 June 2008 (UTC)

## Supersonic combustion - Is it still deflagration?

When scramjets burn the air-fuel mixture at supersonic speeds, is it still a deflagration process or is it now a detonation process? --Pavithran(talk) 06:32, 06 January 2009 (UTC) —Preceding unsigned comment added by 202.169.31.178 (talk)

Deflagration. GreyTrafalgar (talk) 22:00, 30 June 2010 (UTC)

## Minimum Mach number

I've reverted the minimum Mach number comment for several reasons. Firstly, there have been several successful experiments of both net thrust producing scramjets and supersonic combustion at Mach 6 (for fully aerodynamic engines). See the NASA/CIAM tests (eg AIAA 2002-4128) and the tests using Borane-based fuels by Billig et al (AIAA 93-2329). So Mach 7 is obviously too high for an absolute minimum, and since there are no production engines of this type, its also probably unfounded to say Mach 7 is some kind of functional minimum. The minimum Mach number is very much a function of the fuel and engine design. It may be that Dr Paull's caveats for the Mach 7 number were not fully explained. As far as I know he's only looking at "green" engine designs (= Hydrocarbon or hydrogen). I think this is a far too complex problem to be fully referenced by a single popular science show. AKAF (talk) 08:07, 16 March 2009 (UTC)

You might be right, but it's unreferenced, and the Mach 7-8 is referenced. Verifiability over truth right?- (User) Wolfkeeper (Talk) 15:11, 16 March 2009 (UTC)
Did you miss the two references which I just gave right there in the text? In my opinion a reference in the scientific literature beats one in a popular science forum any day. AKAF (talk) 15:50, 16 March 2009 (UTC)
Because they're in different circumstances, not really, and note the guy making the statement is a real expert that has actually flown stuff. You should probably add yours as well to the article. Referencing improves, the crowd roars.- (User) Wolfkeeper (Talk) 16:27, 16 March 2009 (UTC)
I have not understood that statement at all. AKAF (talk) 18:28, 16 March 2009 (UTC)
If there are multiple POVs on a particular thing, like minimum mach, then the wikipedia way is to add all of them or at least representative examples, with references.- (User) Wolfkeeper (Talk) 18:46, 16 March 2009 (UTC)

Under this section and subcategory "Propellant" one could read the following.
"An advantage of a hypersonic airbreathing (typically scramjet) vehicle like the X-30 is avoiding or at least reducing the need for carrying oxidizer. For example the space shuttle external tank holds 616,432 kg of liquid oxygen (LOX) and 103,000 kg of liquid hydrogen (LH2). The shuttle orbiter itself weighs about 104,000 kg (max landing weight). Therefore 75% of the entire assembly weight is liquid oxygen."

It is very wrong to say that the oxygen makes up 75% of the space shuttle since nothing is said about the normal takeoff weight of the orbiter (only maximum landing), nothing is said about the weight of the Solid Rocket Boosters that are needed to get the assembly of the launch pad and not even the weight of the external tank that holds the liquid oxygen and liquid hydrogen is taken into consideration.
Of the true "entire assembly" at takeoff the liquid oxygen is much less than 75% of the weight. The external tank weight is either 26,500 kg, 30,000 kg or 35,000 kg depending on tank version used. Each of the two Solid Rocket Boosters weight is 590,000 kg and the orbiter weight with payload is at maximum 109,000 kg with a maximum payload of about 25,000 kg depending on the specific orbiter used.
In other words the oxygen weight is only 30% of the entire space shuttle stack at liftoff.
JasonCW - 15:49, 14 November 2009 (CET) —Preceding unsigned comment added by JasonCW (talkcontribs)

## June/July 2010 Overhaul

Beginning to reorganize, clarify, cite, update, etc.... entire article. If it seems that I'm removing relevant info or links, it's likely they're just temporarily removed as I go from section to section.

Began with the intro. GreyTrafalgar (talk) 21:58, 30 June 2010 (UTC)
Added design principles and basic principles GreyTrafalgar (talk) 14:25, 1 July 2010 (UTC)

## Tags removed

The tags have been on the page for over a year in some cases, without the taggers actually bothering to explain on the talk page what they consider the problem. I think that there is some basic minimum assumption that all pages can/should be edited, so the tags are not useful as a general warning. Moreover, the page ratings of >4.0 show that those rating do not feel that the tags are justified. AKAF (talk) 16:34, 20 September 2011 (UTC)

## Comparison to orbital velocity

I removed the phrase from the intro which said the top speed "is near orbital velocity". There are multiple things wrong with this:

• The notion that a Mach number represents an absolute speed is a layman's misunderstanding. Mach number is defined as the ratio of a speed to the speed of sound; the speed of sound varies with air temperature, and temperature varies with altitude, therefore "Mach (anything)" without the altitude can't be meaningfully related to any speed.
• Orbital speed is similarly ambiguous; it depends on altitude and eccentricity. So trying to equate a Mach number to a fraction of an orbital speed is pretty meaningless without being more specific.
• The wikilink to orbital velocity needed to be disambiguated; the article is titled orbital speed. Speed is the appropriate term to use, because velocity means "a speed in a certain direction." (Another layman's error to assume they are synonymous.)
• The assertion that it's near orbital speed is uncited in the body (as is the top Mach number value.) So this really makes it original research. (If it's somebody else's (reliable source) "OR", it should be cited.)

There's also the issue that a scramjet must have a maximum altitude limit, because it's air-breathing. And a stable orbit is inversely limited: it won't do any good to reach "orbit" at too low an altitude, because air drag will quickly kill it.

Bottom line is, there are so many variables involved, that any meaningful statement comparing the scramjet's top capability to orbit would involve original research, unless some reliable source makes it. JustinTime55 (talk) 16:34, 16 January 2012 (UTC)

## Thrust-to-weight ratio

In the article is assumed thrust-to-weight ration about ~2.0 I don't believe that it is really so low. If I compare with current turbojet engines ( 4.0-7.0 ) and ramjets according to this page ( http://airindustriesresearch.com/siram/ramstats.htm ) could have thrust-to-weight ratio about 30.0 While ramjet/scramjet are both in principle just a specially shaped shock-wave-forming tube with no moving parts the thrust-to-weight of scramjet should be for sure higher than in turbojet and similar to ramjet at its peak of optimal working conditions. While there is no citation of "thrust-to-weight ration about ~2.0" reference, I suspect that this number is misinterpreted - for example it could be thrust-to-weight ratio of the whole airplane ( not just the engine ), or thrust-to-weight ratio at not optimal conditions ( too high or too low velocity ).

As can be seen from the page ( http://airindustriesresearch.com/siram/ramstats.htm ) the thrust-to-weight ratio of this type of engines very much depend on velocity. It could be as low as 7.0 at 0.5 Mach and as high as 30.0 at 3.0 Mach — Preceding unsigned comment added by ProkopHapala (talkcontribs) 15:03, 8 May 2013 (UTC)

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