Talk:Spacecraft propulsion

Metadiscussion

One thing that bothers me about this page is that the title is spacecraft propulsion. A spacecraft is a vehicle that 'flies' in space. The term doesn't cover launch vehicles. And yet, we don't have a page discussing rocket engines (say) in general.

I'm not totally clear what should be done, but there's a structural bug in the Wiki here.

-WolfKeeper

Ok, I propose to move this page to Launch vehicle and Spacecraft propulsion. Anyone have any problem with that?

Wolfkeeper 21:47, 2005 Apr 11 (UTC)

Well, I'm not sure if there are any launch vehicles that don't reach space (or at least, part of them does). But this page focuses on propulsion methods that work in vacuo, with some minor digressions dealing with air-breathing and ground-based systems. However rocket engine redirects here (since the best general explanation of rocket engines is here), and rockets are used on various non-spacecraft (jet fighters, missiles, toy fireworks, experimental land vehicles).

I don't think "Launch vehicle and spacecraft propulsion" is a good idea (because it's not only launch vehicles, and because the grammar is a bit suspect). At the moment, I'd describe the content as "propulsion systems relevant to space travel", which I think is reasonably simplified to "spacecraft propulsion". So, barring a significantly better name, I think we should stick with it.

After all, we have redirects to get people here, and a table of contents to tell people what's here, so I don't think there will be much confusion. --Andrew 00:47, Apr 12, 2005 (UTC)

hello What about calling the page Rocket Propulsion. I suggest this because the major reference text on this subject is Rocket Propulsion elements by sutton, and if its good enough for him to use as a title then perhaps its good enough for us! nasalcherry

A mistake in the article?

Sorry, I'm not certain about it, and my english is too bad to try to correct it:

In the article I see the text: "the energy required for that impulse is proportional to the exhaust velocity"

But I think it's not true.

The energy contained in the exhaust is proportional to the square of the exhaust velocity, instead. It's a kinetic energy after all (E=1/2 m v^2). —Preceding unsigned comment added by Indaco1 (talk • contribs) 00:44, 7 October 2009 (UTC)

-- But the impulse (momentum transfer) scales with the velocity linearly, therefore energy cost per impulse scales linearly with exhaust velocity (130.225.212.4 (talk) 10:44, 25 March 2011 (UTC))

I worked for almost a year on a project that failed because of this distinction - because it was not made clear enough in the source book I had been using. The critical point is that the energy efficiency of a rocket falls as the momentum efficiency increases with rising exhaust velocity. Effectively with a high enough exhaust velocity almost all the kinetic energy goes into the exhaust and almost none into the rocket - the most extreme example of this is 'photonic' drive where basically none of the KE goes into accelerating the ship. (- such a drive would gain momentum from photon pressure the same way that solar sails do) - Lucien86 (talk) 14:05, 16 February 2012 (UTC)

Two of "the same picture"

There are two of the same picture in this article: the one of an ion engine being test-fired. Except they're not the same picture. The same image is in two different resolutions from two different sources. Could someone with more invested with this article and / or the pictures take a look at this? Johndodd 01:57, 15 January 2006 (UTC)

solar sails

moved to Talk:Solar sail

future methods

WRT fusion rockets, would it be more accurate to say that we can't make a fusion reaction self-sustaining? We can sustain one, it just doesn't generate as much energy as we put in? ... I also don't know whether classifying fusion rockets with antimatter drives and space warps is fair . . . we're likely to be able to do controlled fusion well before either of the others.

Robert Merkel

Another possibility you haven't mentioned was powering sunsails with ground-based or satellite-based lasers Robert Merkel

Is that now covered by Beam-powered propulsion ?

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I'm moving warp drive to this page, because I don't think there's really much solid theory behind it; if there is, please add it (or online references to reputable sources). Otherwise, maybe it'd be worthwhile to set up a fictional spacecraft propulsion page?

Warp drives

No one knows how to grab space in order to warp it. It might be possible with extremely odd materials like neutronium, strange matter, or negative mass, but no one knows how to get these.

Warping space is easy, matter does it all the times (we call it gravity). The trick is finding a way to make a propulsion system out of it. --BlackGriffen

To my amazement, I remembered how to spell Miguel Alcubierre's name correctly, and Google found his 1994 warp drive article: http://www.astro.cf.ac.uk/groups/relativity/papers/abstracts/miguel94a.html Now we know how to make a propulsion scheme, the trick is finding the exotic matter required.. -- Malcolm Farmer

discussion

Does a vernier rocket fit in to any of the categroies already listed? Just curious. Also, may I suggest a division of the rockets by category (i.e. chemical, ionic, other).--BlackGriffen

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A vernier is a steering rocket. Typically they will use one of the schemes already listed, usually a monopropellant. Hydrazine is popular.

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Text under "Nuclear Kinetic" read "Bimini Atoll". Changed to "Bikini Atoll" If this is wrong please change back.

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What does lemonade have to do with Zubrin's magsail?

It's a quote from "Entering space", one of his books. According to Zubrin, when he and his collaborator did their drag calculations on the Bussard ramjet and figured out the drag would be greater than the thrust, they realised that they should forget about the thrust entirely and try to maximise the drag. Hence, they found the universe was a cosmic lemon for the ramjet, so they took advantage of what they found (the metaphorical lemonade). --Robert Merkel

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By the way, "Entering Space" quotes a figure of ~100-300 metre diameter for the required mirror size to focus light on a light sail for a sufficient distance to get to between c/10 and c/3. Did Zubrin get his calculations wrong (quite possible), is there some factor being ignored, or is the "Earth-diameter" quote working under different assumptions for the mission parameters? --Robert Merkel

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Dr Zubrin was calculating the size required for the initial acceleration, he relied on another system to slow down. Dr Forward on the other hand used a two stage sail and needed the laser to remain focused out to a target star. --Joshua Benner

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It seems to me that the Mini-Magnetospheric Plasma Propulsion (M2P2) offers one of the best concepts for interstellar travel right now. I emailed the creator of the concept and he denied that it would be used for interstellar travel, stating that interplanetary propulsion was the goal. For some reason he simply side stepped my questions without even considering them. Magnetic sails are being considered for interstellar flight. M2P2 is a form magnetic sail and apparently much more efficient, affordable and manufacturable than any other form I've read about. We could build it today. Why couldn't the M2P2 be coupled with a particle beam (same kind proposed for other magnetic sails) to produce higher thrust than the solar wind can provide. This could potentially get it up to interstellar speeds. Once at a the target solar system it could be easily decelerated by deploying the magnetic bubble against the destination star's solar wind. Here's a link to the website: http://www.geophys.washington.edu/Space/SpaceModel/M2P2/ --[digital_poltergeist]

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I'm thinking that it might be a good idea to break this article up into separate articles, each dealing with one of the propulsion methods described, and turning Spacecraft propulsion into a general overview and list of links. The existing spacecraft propulsion article is getting extremely long, and none of the propulsion types has room for a lot of detail. Anyone object? If not, I'll get to work this evening busting it all up into pieces. --BD

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Nice table. However, although specific impulse generalizes well across all examples of a specific engine type, thrust is somewhat dependant on the actual size of the engine (a really gigantic ion engine might produce more thrust than a teeny-tiny chemical engine, for example). Anyone have any ideas how to describe thrust as a general characteristic? Bryan Derksen

Might be better to talk about thrust-to-weight ratio instead. That can give you a better clue about which technologies might be used for which applications (booster to LEO, interplanetary, interstellar, etc.). ansible

Yes and no. For long journeys, the fuel will weigh far more than the engine, and that's what specific impulse is for. What the column is trying to capture is the spacecraft accelerations that can be expected using this method. Perhaps that's what it should contain? Of course, that's pretty crude too: a small liquid-fuel rocket might be used for attitude control while a big one for launches. The point is to distinguish low-thrust methods like ion drives from drives that can produce accelerations on the order of a gee.

Then again, maybe some propulsion methods have natural size limitations on the engine?

This is especially bad for mass drivers which can produce lots of thrust by throwing kilogram payloads at hundreds of kilometers per second but are huge.

--Andrew 05:06, Apr 17, 2004 (UTC)

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On the page about the Gaseous fission reactor, it says "This can create specific impulses of 20,000 s"; but on this page, the table lists a specific impulse of "1000-2000" s. Could we check some sources? Perhaps a zero just got dropped somewhere? John Owens

units specifying thrust

(moved to Talk:Specific impulse)

I apologize for putting this here, because I know it's not quite the right place, but I couldn't figure out where else to put it (too late at night to devote much coherent thought at the problem....)

Anyway, this wikipedia entry is missing something; a somewhat new Earth to Orbit laser-based drive. The only fancy name I know of for it is for the currently theoretical man-sized version; the "Mercury Lightcraft". Here's the guy who developed it:

http://www.rpi.edu/dept/mane/deptweb/faculty/member/myrabo.html

The drive works by refocusing pulsed laser light from an external generator to explode the air underneath the craft. It also spins to help with stability. The Mercury Lightcraft designs call for an additional "second gear" (which, funnny enough, can't kick in, apparently, until Mach 3) in which superconducting rings within the craft set up a magnetic feild while electrodes in fins on the hull ionize the air, which results in pushing the air down (Added bonus: Eats it's own sonic boom).

Additionally, and the Mercury calls for a space-based laser generator rather than a Earth-bound one, an additional mirror on the nose refocuses the light to eplode the air somewhere above the cone, so that the shockwave reduces the drag (I got to talk with Prof. Myrabo and gathered that he had done experiments to show how effective this was, and that it was really effective). I'm guessing that the effect is nearly the same as "supercavitation", although in air rather than water.

-Narfanator

discussion

Methods need to be checked to make sure that the ones claimed to be in current use actually are. That seems like a useful distinction though. --Andrew 04:58, Apr 17, 2004 (UTC)

--- - There's a bunch of data on rocket economics ("lift costs per pound" for various historical rockets) collecting on page http://en.wikipedia.org/wiki/Talk:Space_elevator that I think need to be moved to some article. Is Spacecraft propulsion, Rocket, Transport economics, or somewhere else the appropriate article to put that data ? -- DavidCary 05:38, 12 May 2004 (UTC)

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there is an error in the forumula for Dv you only multiply by g0 if the Isp is given in kilograms force*second/kilograms, NOT if it's given in Newton*second/kilograms, as said just above and as given in the table for engine performance.

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You asked for comments. The only thing I could see is that altitude control seems to be confused in the text with orientation--BozMo|talk 15:19, 28 Jun 2004 (UTC)

Text from Rocket

The following text was moved in bulk from Rocket; it doesn't really fit here, but it could perhaps be integrated.

In a chemical rocket, the exhaust gasses can be moving at a speed of anywhere from 1500 meters per second (0.94 miles per second) in low-efficiency rockets to 4500 meters per second (2.77 miles per second) in high efficiency engines using liquid oxygen and liquid hydrogen.

In any kind of rocket, the speed of the exhaust, when divided by 9.80665 is referred to as ISP, or specific impulse. This number defines for how many seconds one pound of propellant can generate one pound of thrust. For example, a rocket with an exhaust speed of 4500 meters per second has an ISP of 459. This means that one pound of propellant has 459 pound-seconds of potential thrust; one pound of thrust for 459 seconds or 459 pounds of thrust for one second, and anywhere in between.

The velocity a rocket can attain in a vacuum is determined by the rocket equation

ΔV = Ln(Mf/Me)*(VEx)

Where Ln is the natural logarithm function, Mf is the fully fuelled mass of the rocket and payload, Me is the mass of the rocket when it's fuel is depleted, and VEx is the exhaust velocity in either meters per second or feet per second. The result of the equation will be the rocket's total Delta-V, or change in velocity in the same units used for the exhaust velocity.

An example of Delta-V requirements for different missions: Low earth orbit (LEO): 9500 meters per second (average including atmospheric drag and gravity losses, 7800 meters per second final velocity.) LEO to escape velocity: 3200 meters per second Lunar orbit to Lunar Landing: 2200 meters per second LEO to Mars: 6100 meters per second

Often times the required velocity for the mission is unattainable because the propellant tanks weigh so much as prevent the (Mf/Me), or mass ratio, from being high enough. This problem is solved by dropping smaller propellant tanks after they are empty, therefore shedding excess weight and attaining a higher effective Mass Ratio and permitting a higher Delta-V. Common Mass Ratios are 20/1 for dense propellants such as liquid oxygen and Kerosene, 25/1 for dense monopropellants such as Hydrogen Peroxide, and 10/1 for liquid oxygen and liquid hydrogen. However, Mass Ratio is highly dependant on many factors such as the type of engine the vehicle uses and structural safety margins.

Sometimes the required Delta-V for the mission requires an unrealistically high number of stages, three or four considered the practical limit. In this case the rocket designer must look at rocket engines with a higher ISP.

ion thrusters are capable of exhaust velocities exceeding 40,000 meters per second for Ion engines. These engines have extremely low thrust, and are not suitible for launching rockets from the surface of a planetary body, but are extremely useful and efficient in space.

Nuclear thermal rockets have also been developed, but never put into use. The NERVA engine developed by NASA in the 1960's and 1970's had an exhaust velocity approaching 9500 meters per second, and were capable of launching payloads from the Earth's surface. However, the public phobia of all things nuclear during the Cold War killed the project.

Nuclear pulse propulsions are extremely simple, powerful and efficient. The 30 year old ORION concept proposed dropping nuclear bombs 150 meters behind the vehicle, and using a ten meter wide 'pusher plate' to absorb the explosion energy. The thrust was transmitted to the spacecraft via a system of shock absorbers. Depite the enormous amoutns of energy released, the crew would only experience forces comparable to those felt during a launch the Space Shuttle. The ORION concept had an effective exhaust velocity of over 30,000 meters per second, with later designs projected to be capable of over 100,000 meters per second.

Request for references

Hi, I am working to encourage implementation of the goals of the Wikipedia:Verifiability policy. Part of that is to make sure articles cite their sources. This is particularly important for featured articles, since they are a prominent part of Wikipedia. The Fact and Reference Check Project has more information. If some of the external links are reliable sources and were used as references, they can be placed in a References section too. See the cite sources link for how to format them. Thank you, and please leave me a message when a few references have been added to the article. - Taxman 19:40, Apr 22, 2005 (UTC)

Breakthrough Physics

Perhaps all the BPP stuff should be collapsed into a single article.

That sounds like a very good idea; each of them, and the BPP article itself, is a stub, so they could quite happily be folded in as sections of that article. --Andrew 15:36, May 4, 2005 (UTC)

Couldn't find anything about Mag-Beames: beam for 90-day Mars visit Varan

Thinko

The article mentions bipropellant rockets and solid rockets as if it was a sort of dicotomy. Well, it is not. You can have liquid, solid, gaseous (uncommon because it would have poor energy density) or hybrid rocket engines. Bipropellant rockets just mean there is a fuel and an oxidizer. But there are tripropellant liquid rocket engines (1 oxidizer, 2 fuels) as well as monopropellant rocket engines (using for e.g. hydrazine). Hybrids usually have solid fuel and liquid oxidizer. - Quasarstrider 7 July 2005 22:14 (UTC)

All of these propulsion methods are mentioned, and linked to pages which discuss them in detail, already, in section 3 under the head "propulsion methods. Icelight

Another error: SpaceShipOne has no first stage. The proper wording is: the name of that space launch system is Tier One, the first stage of the launch system is called White Knight and the second stage of the launch system is called SpaceShipOne. - Quasarstrider 7 July 2005 22:20 (UTC)

While those are certainly how Rutan refers to them, for all intents and purposes the White Knight acts as a first stage, providing altitude and some velocity before the ignition of the "second" stage hydrid engine. As that line links directly to the reference article, a reader should be able to clear up any uncertainties for themselves easily. Icelight

This article seems to be terribly confused between what a spacecraft, a space launch system, or a rocket are. - Quasarstrider 7 July 2005 22:22 (UTC) I propose this page is renamed as 'space propulsion'. You could also have 'air propulsion' (e.g. turbojet), 'land propulsion' (e.g. maglev), 'water propulsion' (e.g. pumpjet), etc pages. - Quasarstrider 7 July 2005 22:28 (UTC)

Please see the very first item in this talk page, where this issue has been discussed. Icelight July 8, 2005 15:18 (UTC)

"Worm-Hole rotating ring" is totally made up.

is a jet engine considered a motor?

With all the hype that's happening over at the article motorized bicycle, we/I was wondering. Is a jet engine (or whatever it may be called... rocket?) [such as here] considered a motor? --CyclePat 06:52, 31 October 2005 (UTC) (could you leave me a quick message on my user page saying I got a message if you anser this. Thank you!)

--- I recently saw an interview with a "top engineer" (ie willing to be interviewed), he stated that, engines tend to operate on a thermal cycle (ie otto, carno, etc), while motors don't . But that the naming is not consistent.. note from an engineer Nov 2005

Methods requiring new principles of physics

In the "Methods requiring new principles of physics" section, a sentence reads: "Such methods would be essential for any hope at interstellar spaceflight". I'm inclined to think that should read ... "manned interstellar spaceflight". It's not impossible to envisage a spaceprobe (using one of the "Technologies requiring further research") that could get to Alpha Centuri in say 100 years. That's totally impractical for a manned spaceflight, but for the automated one that's not an impossible goal. -- Finlay McWalter | Talk 17:04, 11 December 2005 (UTC)

I'd have to dissagree with both of those statements - though no currently possible propulsion method is capable of getting a manned ship to another star in a single life time, there is the possibility of generation ships powered by ome of the more efficient methods of propulsion. The Orion Project, maybe, or a Bussard Ramjet? Phædrus 20:18, 14 December 2006 (UTC)

Rocket engines

IMO this section is totally pathetic. There's almost 1 million articles in the wikipedia and we have about 4 paragraphs on something like rocket engines. People have written 2 inch thick books on rocket engines and still not said everything, and we've got this tiny section. We need to link in turbopumps injectors and all kinds of other good stuff.WolfKeeper 02:27, 17 January 2006 (UTC)

• Rocket engine currently just redirects back here. Perhaps you could split the rocket engine section out of this article and put it there to give it more room to grow? Rockets are a very important form of spacecraft propulsion, of course, but this page is supposed to be more general than that so isn't the best place for the most in-depth coverage of the subject. Bryan 06:41, 31 January 2006 (UTC)

Yeah, I was thinking about doing that. But at the moment I'm thinking that putting the meat in the different sections like bipropellant rocket and solid rocket and so forth. I mean, for example, solid rockets don't have turbopumps or injectors, so it's not totally stupid to do it that way (although there are annoying overlaps.)WolfKeeper 12:30, 31 January 2006 (UTC)

At the moment I'm thinking about putting the absolute basics here; De Laval nozzles and efficiency considerations and covering the rest in the other articles.WolfKeeper 12:30, 31 January 2006 (UTC)

Nuclear photonic rocket

Nuclear photonic rocket is up for deletion. It's linked to from this page, so I figured people who watch this talk page might want to comment, here: Wikipedia:Articles for deletion/Nuclear photonic rocket. -Ikkyu2 03:22, 28 January 2006 (UTC)

Maybe if we include the success of the Proton (Russian) designs since 1965 unchanged mainly because of its sucess with an avarage rate of 98% proves the fact that Russian Rocket engines are actually better for comercial use.

Reorganize plez

Yes, I saw the header, but the current version has either been significantly altered from the one which had feature article status (or else I feel it should not have been given that status in the present form). My basic objection is that highly speculative proposals are jumbled up with propulsion methods which are in current use or at least are generally thought to be realizable with current technology. ---CH 04:49, 30 January 2006 (UTC)

This is almost inevitable in any field that has seen both real-world progress and a great deal more speculation. If this article were "automobile propulsion", it might be worth mentioning that there have been rocket-powered cars for setting land-speed records as an interesting tidbit. But you probably wouldn't mention nuclear-powered cars (even though I distinctly remember an edition of Boys' Life that proposed and even illustrated cars zooming along on Super-Autobahns powered by fission reactors.) You definitely wouldn't mention what George Jetson drove to work. Don't get me wrong: I'm fascinated by alternatives to conventional rocketry for certain purposes. I am even working on a book about the history of projectile launch to space ("space gun" tech of various flavors, such as ram accerators, electromagnetic mass drivers, light gas guns, etc.), both as a speculative idea and as on-going (albeit sputtering) R&D. In general, however, I think an article about space propulsion should focus on rocketry, give plausible alternatives a glancing mention with links to other articles, and leave a section on "Theoretically possible but still purely speculative approaches to space propulsion" as end matter. As for stuff that defies known laws of physics, surely Wikipedia has enough about science fiction and video gaming already? Yakushima 11:15, 11 August 2006 (UTC)

Fails to cite sources

This article needs to cite its sources. As it is, it should not be a featured article. I will propose the removal of its featured status if this is not addressed within the next week or so.--SeizureDog 22:39, 4 June 2006 (UTC)

Use of scramjets in space

The article, I believe, does not render full justice to the use of scramjets in space. It only considers it as a propulsion system to get out of the atmosphere. I believe that it should consider the propulsion in space it could provide as well. The method would be quite impractical, as it would require a huge scramjet, but worth mentioning all the same. This information is based off of the August 2006 Scientific American issue. --Pacvenn 18:27, 2 September 2006 (UTC)

Throwing kittens out of the back of a spacecraft might also "propel" it, but would also be quite impractical. The use of air-breathing engines in space, where there isn't any air, probably isn't plausible enough for reference in an encyclopedic article. Icelight 01:04, 20 October 2006 (UTC)

Removed VASIMR from table

The table is only a theoretical construct, in a theoretical section, it's not supposed to show energy usage of real drives.

It's probably a good idea to add a table for electically powered drives though, giving their actual performances.

I also had a question about the VASIMR drive, the table entry that was added showed a minimum ISP of 1000 seconds, but the lowest I've seen was more like ~1500 seconds. For this to go into the article it needs to be referenced.WolfKeeper 17:32, 19 October 2006 (UTC)

i could dig through NASA and industry documents for sources. But if you're suggesting that VASIMR just confuses the table and chapter - i'd agree. So yeah just leave it out, saves me some digging too. Roidroid 12:54, 21 October 2006 (UTC)

The calculations

You know, the ones under the calculations section? Those don't by any chance use calculus do they? Just wondering. -Fruition11 08:17, 7 January 2007 (UTC)

Rocket engines → Spacecraft propulsion

Please see this request for discussion about the rocket engines redirect. Sdsds 21:17, 1 March 2007 (UTC)

The redirect was retargeted. (Sdsds - Talk) 07:48, 27 May 2007 (UTC)

Cleanup

I think this article could use some serious cleanup. It's come a long way since I last ran across it, especially now that most of the fringe BPP stuff has moved into its own article. Still, the refs are too thin and Wolfkeeper seems intent on keeping the Redshift rocket--a fictional propulsion system--and gravitoelectromagnetic nonsense based on a single sentence fragment referring to a forty year old Robert Forward essay I doubt anyone here has actually read. So what do you say? Should we throw up the tag or what?

--Rev Prez 22:21, 8 March 2007 (UTC)

I simply removed your claim of these drives being 'fringe' on NPOV grounds.WolfKeeper 22:52, 8 March 2007 (UTC)

Delta-v

I added delta-v to the propulsion table. The numbers used are half-assed right, they need referencing.

The reason I added this is because without this heading the table is not terribly useful- the exhaust velocity column does not give much information because it fails to consider the effect of mass fraction (and different systems have vastly different mass fractions).

The numbers are probably highly contentious. If you dislike any number feel absolutely free to change it; but please add a reference.WolfKeeper 13:53, 4 August 2007 (UTC)

Yes the table is completely unreferenced and is difficult to read because of the lack of borders. I'm not certain that the section should even be retained. — RJH (talk) 20:45, 4 August 2007 (UTC)

The section should be retained, because it is a useful table. Mere presentation issues like borders are not a good reason to delete something. Adding references is a much better idea.WolfKeeper 22:14, 4 August 2007 (UTC)

Photonic propulsion?

I note recent news items on an advance in Photonic drives (PLT), now up to a demonstrated 30 micro-newtons of thrust with off the shelf components. (Google or see slashdot for more info) But I don't see them in the page here. (Nor can I immediately see how they should be worked into the categories the article already has, or I would add them) Could someone with better writing skills than mine work them in? Nahaj 17:30, 18 September 2007 (UTC)

Is this something different from a lightcraft? --68.0.124.33 (talk) 16:20, 9 January 2008 (UTC)

Yeah. If it's the one I think it is, it's a weird drive, they bounce the light backwards and forwards between two mirrors, and they get a much stronger force that way. Trouble is, they don't seem to have a clue how to turn this into a true propulsion system. It might be useful for keeping a network of craft in their relative positions though, but I wouldn't like to bet they can keep the mirrors and optics accurately enough aligned for that to work either.- (User) WolfKeeper (Talk) 17:27, 22 January 2008 (UTC)

Mentioning momentum wheels for attitude control

I propose removal of the text in the lead that mentions use of momentum wheels for attitude control, as being off-topic. Although these topics are related to propulsion, they are not propulsion per-se. Mentioning them distracts the reader from the on-topic material mentioned in the lead and covered in the article body. (sdsds - talk) 17:33, 11 February 2008 (UTC)

I agree to some extent, but attitude control is one major function of propulsive devices (for angular momentum shedding), and I think it is entirely reasonable to note they are used in conjunction with momentum wheels (albeit briefly).- (User) WolfKeeper (Talk) 18:17, 11 February 2008 (UTC)

nonsensical sentence

Is it just me or is this sentence a bit daft:

"Although solar power and nuclear power are virtually unlimited sources of energy, the maximum power they can supply is substantially proportional to the mass of the powerplant.

What is it that is trying to be said? That these energy sources have a low maximum power:mass ratio? Erich (talk) 02:28, 19 June 2008 (UTC)

moved from article

• If the maximum power is fixed, then for a particular delta-v and a given maximum time to attain that delta-v, then thrusting at the maximum power over the entire time with a constant propellant flow rate and a constant ve achieves the minimum propellant usage.

This seems to be wrong, or at least very incomplete, since the time/delta-v/power are interrelated and the 'constant' propellant flow is not specified.- (User) WolfKeeper (Talk) 15:40, 2 July 2008 (UTC)

The problem with this:

• If the maximum power is fixed, then thrusting at that maximum power, at maximum propellant flow rate, achieves the maximum acceleration (the minimum time to achieve a given delta-v).

is that the paragraph is talking about energy and power use.- (User) WolfKeeper (Talk) 15:43, 2 July 2008 (UTC)

You are right.

My question is: How do I achieve minimum propellant usage with an ion thruster, if I have practically unlimited amounts of energy available?

Problem is that that's not realistic. Infinite Isp gives you minimum propellant, but takes infinite energy.- (User) WolfKeeper (Talk) 16:54, 2 July 2008 (UTC)

My question is: How do I achieve maximum acceleration with an ion thruster, if I have practically unlimited amounts of energy available?

Acceleration is a power thing, not an energy thing.- (User) WolfKeeper (Talk) 16:54, 2 July 2008 (UTC)

I think the answer to these questions are *more* important than the answers to "How do I achieve minimum energy usage" that this article currently highlights. (As far as I know, there are no spacecraft where the energy will run out before the propellant).

Well, no, for obvious reasons. But it's perfectly possible to build one like that ;-)- (User) WolfKeeper (Talk) 16:54, 2 July 2008 (UTC)

Would this clarification be adequate until I (or someone else) work out the exact equation for the particular flow rate?

With current technology, the power source is the limit, not the engine. Building higher Isp engines is not especially difficult. It's the power source that you need that is.- (User) WolfKeeper (Talk) 16:54, 2 July 2008 (UTC)

• If the maximum power is fixed, then for a particular delta-v and a given maximum time to attain that delta-v, then thrusting at the maximum power over the entire time at constant acceleration a = (delta-v)/t -- at the minimum propellant flow rate that gives that acceleration -- achieves the minimum propellant usage.

It's quite possible my source is wrong, and the best way to achieve minimum propellant usage is with some complicated "start at maximum flow rate, then ramp it down according to this function" (which necessarily ramps up v_e if we maintain maximum power to the ion thrusters).

Whatever the correct answers to those questions are, I think this article should contain those answers.

--68.0.124.33 (talk) 16:30, 2 July 2008 (UTC)

If you have a source then we can probably put it back in. It's completely unrealistic though.- (User) WolfKeeper (Talk) 16:54, 2 July 2008 (UTC)

I don't understand. What is unrealistic about an ion thruster connected a RTG? (Or an ion thruster connected to solar panels)?

--68.0.124.33 (talk) 14:50, 3 July 2008 (UTC)

RTGs/solar/anything power sources have specific powers of ~1kW/kg or worse. Although ion thrusters can take as much power as you can throw at them, just using a bigger power supply gets you nowhere fast. The limit case is just the power supply, propellant tank and ion drive and an infinitesimally small payload and it's easy to calculate the performance, and it's not what you would call exciting ;-).- (User) WolfKeeper (Talk) 15:03, 3 July 2008 (UTC)

With current technology, the power source is the limit, not the engine. I agree. So I think this article should put at least as much emphasis on power-limited spacecraft propulsion, than it does on energy-limited spacecraft propulsion.

I agree that the *acceleration* of ion thrusters is pretty weak, directly leading to long *times* to get from point A to point B. However, since real spacecraft have used ion thrusters, I suspect that there are valid reasons to accept weaker acceleration and longer mission times in return for other useful advantages. --68.0.124.33 (talk) 15:52, 7 July 2008 (UTC)

One thing that hasn't been emphasized is the much larger payload fractions that are permitted with high specific impulse engines. That can substantially reduce the cost of a mission. Fast acceleration is mostly only necessary for manned spaceflight. Station keeping missions and other missions with high total delta-v also exist: sample return missions, for example, might require a large total delta-v. Often high specific impulse engines are competing with gravity assists.. maybe the article also needs more detail on these as well.

Probably focusing on the minimum cost way of achieving a certain mission would help to improve the structure of this section.--Dashpool (talk) 10:39, 12 July 2008 (UTC)

Max power?

"However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed"

Where does this come from? I'm not sure I agree. Wouldn't the 'best acceleration' be obtained when the exhaust velocity is as fast as possible. Going 'Full Blast' might be a waste of energy, but unless you're running on a system that has a limited total energy, than that's not an issue... I'm picturing Solar power or Nuclear power where the energy is unlimited in total amount, only limited by how much can be generated at any particular time. I guess if the ship's flying off of batteries, then yes, total system effeciency becomes an issue.

It seems to me that this article overemphasises the importance of having the exhaust velocity match the ship velocity, and it can become confusing to the reader as to what the author's intent. Probably every space ship currently out, and for the foreseeable future is using Solar as a power source, and this is a non issue for them. —Preceding unsigned comment added by 209.195.177.231 (talk) 23:11, 7 November 2008 (UTC)

The thing is, all power sources, including solar and nuclear have a power/mass ratio that they track pretty well indeed. It turns out you really don't have infinite power, it's very, very finite; the mass of the power source is a major consideration.- (User) Wolfkeeper (Talk) 03:24, 8 November 2008 (UTC)

My point is: If you're using nuclear or solar as power, you're going to be spitting out reaction mass at full power, you're not going to want to reduce that power because then your reaction mass efficiency is going to tank. It's either: Spend all the power you have and get the highest delta V per mass, or reduce power and get less delta v... Anyone that's doing anything less than using full power doesn't have their head screwed on right :P —Preceding unsigned comment added by 209.195.177.231 (talk) 17:14, 9 November 2008 (UTC)

No, for the same power source you can either accelerate a small mass flow by a lot, or a large mass flow by a little, or somewhere in-between. Notably, if you accelerate the mass flow to the same speed as your vehicle (relative to the C of M frame of the vehicle+propellant that you launched from), then the exhaust stops, and the vehicle gets practically all of the energy of the power source. So you can size your vehicle's mass flow to match the power source.- (User) Wolfkeeper (Talk) 19:45, 9 November 2008 (UTC)

Thereby wasting reaction mass... You could have received more KPS if you had used the maximum power available.

I'm afraid not; calculating the optimum mix of power and propellant to minimise overall costs is a standard operation that is described in text books. Rocket scientists are not noted for wasting stuff.- (User) Wolfkeeper (Talk) 00:11, 12 November 2008 (UTC)

Worth noting is that most high-isp engines are low thrust. As a result, for a given delta-v, they require a very long burn. This is a direct consequence of the limited power available. The long burn is less efficient, in terms of required delta-v for a given orbital maneuver. This effect is a large piece of why maximizing isp does not always maximize mission capability. As you push the isp higher, the burn for a given delta-v gets longer; the longer burn means that even with the higher delta-v you have reduced maneuvering ability. Chemical rockets normally operate under an assumption of an impulsive (instantaneous) burn; with high isp drives this is not applicable. Contrary to popular opinion, rocket science actually is complicated ;) Evand (talk) 03:04, 12 November 2008 (UTC)

Well said Evand.

Wolf: I just read the message above this one, and now I realize that you either don't get it, or dont want to get it, that user's saying the same thing I am... Which is also the same thing being said everywhere else but here (Read the article on specific impulse). I do think you're doing a disservice by implying that people should somehow strive to run their Ion thrusters at lower than max output... but hay I'm the second person to try and tell you this... There will be more I'm sure. Not worth arguing over. You may lead some people astray, but it's not like people are going out and buying thrusters based on what they read on here so no major harm done I guess. —Preceding unsigned comment added by 209.195.177.231 (talk) 22:05, 12 November 2008 (UTC)

You're still not... getting it. In a rocket launch the Isp is much less than (one of) the optimum(s), the energetic optimum would be about 2/3 of orbital speed, about 2/3 of 7800 m/s = 5200 m/s = 520 seconds (it's a bit rough, the actual optimum would be higher than this, you need to allow for gravity and air drag etc.) So anything you can do to get a higher Isp in a rocket launch brings you closer to the optimum and is all to the good. But if you were to go much above this, it doesn't do you much good at all. Above maybe a 1000 seconds you'll find it's costing you more for the same journey to orbit; all your exhaust is going the wrong way... too fast... over the whole launch and you would be paying for faster exhaust, but you end up going the same speed.- (User) Wolfkeeper (Talk) 20:59, 22 November 2008 (UTC)

Page quality

This used to be one of the better pages in the spacecraft section, now it seems filled with uncited statements, original research, and half/untruths. What's all this about repeatedly trying to imply that having high-power propulsion is bad? This is the way everything is going for propulsion outside of the planetary atmosphere... pretty much all new drives are ION or PLASMA or electric based, hall effect thrusters, VASIMIR,FEEP,PPT,MPD. The general theory these days is to create as much power as possible and use that to propel as little reaction mass as possible as fast as possible, so I'm a little confused as to why this article is trying to downplay that? It definately needs to be cleaned up and everything without citations swept away. (Okay maybe thats a little far but hay it would be an improvement). Way to much synthesis in this article and alot of it is bad synthesis. —Preceding unsigned comment added by Dkelly1966 (talk • contribs) 20:47, 21 November 2008 (UTC)

You say "The general theory these days is to create as much power as possible and use that to propel as little reaction mass as possible as fast as possible". I completely challenge the latter part of that. Exhaust velocities well in excess of 30km/s are routinely achieved, but most missions do not use them. Why is that? It's because they would usually take longer to reach their destination, would use more energy and would cost more. I also challenge the first part- while a higher power/weight ratio is never a bad thing, simply adding more power generation and weight is easy to do but beyond an optimum point, slows you down and reduces your payload.- (User) Wolfkeeper (Talk) 20:16, 22 November 2008 (UTC)

The reason that electric drives are used is because they reduce launch mass, they have a notably small propellant mass. But they're much, much slower than rockets. They're generally less efficient, particularly/mostly at the highest exhaust velocities, and they're also less able to leverage effects such as the Oberth effect.- (User) Wolfkeeper (Talk) 20:25, 22 November 2008 (UTC)

If you haven't already, you need to read chapter 19 in Sutton thoroughly.- (User) Wolfkeeper (Talk) 20:25, 22 November 2008 (UTC)

Table of Methods

Why isn't this in an appropriate table, as is this? Where is the Specific impulse for each of these methods? This section needs some work...08:40, 7 November 2009 (UTC) —Preceding unsigned comment added by 75.70.245.140 (talk)

For whatever it's worth, the new table is harder to read and use. While having the ability to sort is theoretically nice, mixing very theoretical propulsion systems with real ones makes the sorts rather unpractical. And even the theoretical advantage of sorting is lost because most of the data does not sort well. —Preceding unsigned comment added by 90.191.132.56 (talk) 23:39, 3 March 2010 (UTC)

Would be nice to have power requirement in table.

88.159.72.164 (talk) 16:21, 3 July 2010 (UTC)

Correction Needed On Momentum Transfer Without Mass Ejection

About massless propulsion,

"The law of conservation of momentum states that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship“

The statement is not true unless modified with a reference to the energy transferred by radio waves and heat radiators, where no rest mass is ejected. This topic is currently under discussion at NASA and contractors to account for Pioneer 10 and 11 anomalous trajectories.

The effect is small in many cases and sometimes ignored, but will become a design criteria for long missions in the future.

Astrojed (talk) 02:37, 18 August 2010 (UTC)

• You're referring to the Pioneer Anomaly. The presentation that you cite in your recent addition to this article speculates that radio waves and thermal radiation might be an explanation of the anomaly, but other research cited at Pioneer_anomaly#Proposed_explanations specifically excludes radio waves. In any case it's going way beyond the evidence to state that this is "the most common type of massless propulsion" which "must be designed into the stability control systems", nor is it justified to claim that the effect is significant on long missions - in fact the Pioneer Anomaly took so long to discover because it is insignificant. For these reasons I've reverted your addition. andy (talk) 09:04, 18 August 2010 (UTC)

Thanks Andy for the explanation. your points are well takem. I don't intend to continue editing this item. The quoted statement is still not completely true, and goes against ordinary text books. I hope someone will modify it in an acceptable way. For example Halliday and Resnick, PHYSICS PART II, pages 995-996 Energy and Momentum transfer from electromagnetic waves, heat radiation, and light radiation pressure with no mass transferred. Poineer 10 and 11 are still being debated and might be better discussed on the page [ http://en.wikipedia.org/wiki/Pioneer_anomaly ] where "asymmetrical radiation of heat remains a prime suspect." The JPL reference I gave does recommend design criteria for recoil force of heat radiators and radio dish antennas in furute long missions. Small in this case means 250,000 miles off course. Thanks again. Astrojed (talk) 19:53, 18 August 2010 (UTC)

Article needs substantial citations

This article is a good and useful one; however, per WP:V we should not be sourcing assertions with other Wikipedia articles, nor should we have substantive assertions that are unsourced by reliable secondary sources with inline citations. The Table of methods section is a particularly good example of the need for sources. I have tagged just a few of the assertions that need cited with ^{{{citation needed}}}. Does anyone have a good book that might contain a lot of this comparative propulsion info in it? Cheers. N2e (talk) 18:05, 7 February 2011 (UTC)

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Need some rewording / terminology fixes

The section on Effectiveness (3rd paragraph) starts out with "The rate of change of velocity is called acceleration" and finishes with "When launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used." The latter statement is of course untrue given the definition in the former; any acceleration, no matter how small, is by definition enough to overcome gravity. The only thing needed to overcome gravity is velocity -- it does not matter if that velocity is changing (acceleration) or not. I'm not sure how to reword the statement to make it accurate in the context of that paragraph. Perhaps it should just be removed.