Talk:Rocket

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Former featured article candidate Rocket is a former featured article candidate. Please view the links under Article milestones below to see why the nomination failed. For older candidates, please check the archive.
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Current status: Former featured article candidate
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edit·history·watch·refresh Stock post message.svg To-do list for Rocket:
  • Clarify military terminology

Mach Number[edit]

It looks to me like the Mach number referenced in Spaceflight is calculated at sea level. In reality, the Mach number in the exhaust jet is typically much lower, as the temperature of the gas is very high.

I would very much rather not get into any kind of edit war over this, but I keep seeing this error over various Wiki articles and decided its best to start fixing it in this one. Various folks have been calculating the jet exhaust Mach number using atmospheric conditions; this is an extremely incorrect way to calculate the Mach number. This website shows how to calculate it using various area ratios and k. By this short example, it can be seen that obscene area ratios would need to be used to create jet exhaust Mach numbers as high as quoted in this article. http://www.nakka-rocketry.net/articles/noz_example1.pdf

I am not an authorative source and am willing to discuss, but please find me some sources or math that show that the jet exhaust will have hypersonic (> Mach 5) jet exhaust.

Here is a tutorial by NASA on the matter as well. https://www.grc.nasa.gov/www/k-12/airplane/isentrop.html

Kees08 (talk) 01:05, 16 May 2016 (UTC)

Just to be clear - you don't disagree with the exhaust velocity being up to ~4500 m/s, just with the local mach number within the hot exhaust stream? Maybe it makes sense to just drop the mach number from the sentence, since mach number does indeed change with temperature and the exhaust in a supersonic bell is far from isothermal. VQuakr (talk) 06:23, 16 May 2016 (UTC)
Correct, and thats a reasonable solution. Mach number in the jet is ballpark Mach 3 to Mach 5, depending on a lot of variables. Kees08 (talk) 06:35, 16 May 2016 (UTC)
Ummmm it's an adiabatic expansion. Kees08 you really don't know anything about this, do you? I have never ever heard of a rocket jet being as high as 3 even; 5 would be completely impossible. It's almost like you're bullshitting or something. If you're talking about the speed of the exhaust, it's around twice the speed of sound at the throat, depending on the nozzle coefficient which is usually about between 1.7-2.2. If you're talking about the speed of sound relative to the air around it, it's 10-15 as the article said, but you just took this out for what seems to be no good reason. So far as I can tell, you're not interested in edit warring, unless the article doesn't say what you want it to say, while apparently you don't actually know anything about the topic.GliderMaven (talk) 23:12, 16 May 2016 (UTC)
Quick reply, I'll reply in more detail when I get time. I didn't remove it from the article, another user did based on the discussion that had occurred on this page. All I ever did was add a tag to the article to alert readers there was a discussion occurring here about the issue. I would appreciate if you didn't remove those tags without discussing it on the talk page in the future.
As for the Mach number, a properly designed de Laval nozzle is subsonic prior to the throat (M < 1), sonic at the throat (M = 1), and supersonic after the throat (M > 1). No one in industry ever refers to the velocity of the jet relative to the velocity of the air it is in because that is an irrelevant fact. Check out the A/A* graph for a little more clarity. http://www.nakka-rocketry.net/th_nozz.html
I am also a little confused as to why you mentioned that it is an adiabatic expansion. You said that, but then didn't really relate it to anything we were talking about. The fact that the calculations are done with the isentropic equations leads to the results I pointed out in the first paragraph.
I would also ask that you don't result in personal attacks, such as accusing me of 'bullshitting' and 'not knowing anything about the topic,' and instead provide sources or calculations to prove your position.
Take a look at the three links I posted above and the article on de Laval nozzles. I look forward to your reply, let me know if there is any more confusion on the matter, thanks! Kees08 (talk) 06:09, 18 May 2016 (UTC)
The confusion is apparently all yours. The specification of the exhaust jet velocity relative to the air as a mach number is entirely appropriate for several reasons. 1) the jet is actually travelling through the air as it leaves the nozzle, and actually has a mach number around 10-15. 2) it's being compared with orbital velocity and the fact that it's comparable to orbital velocity is important, since if (for example) it was much smaller than orbital velocity, no rocket would be able to make orbit 3) this is an article on rocket vehicles, not rocket engines; it is therefore inappropriate to compare the exhaust speed with some internal speed which has no external consequences. If you have more lack of knowledge or understanding please do feel free to use this talk page as an education service.GliderMaven (talk) 14:23, 28 May 2016 (UTC)
@GliderMaven: your tone reflects poorly on yourself. Fix it please. Back to the content, why are we using mach numbers as a stand-in for velocity, anyways? Just report the typical rocket exhaust and LEO velocity in m/s and be unambiguous. VQuakr (talk) 16:46, 28 May 2016 (UTC)

Recommendation to remove Oberth effect from article[edit]

I think this section fits the definition of straying from the topic. It is much more suited to the Orbital maneuver article, and in fact is already located there. I think the outline of this article needs reorganized in general, but I believe this would be a good start. Thoughts, opinion, objections? Kees08 (talk) 04:45, 25 May 2016 (UTC)

No, that would be a bad start. Oberth effect applies strongly to upper stages of two stage rockets, they have far more performance than there is chemical energy in the upper stage.GliderMaven (talk) 13:53, 25 May 2016 (UTC)

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