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question about approximating isp at different chamber pressures
I'm just wondering where this table came from, is there some equation or formula behind it, or did it just come from a bunch of test data? 24.137.113.9018:55, 13 July 2007 (UTC)[reply]
Just a note
Near the bottom of the page, for the H2O2/N2H4 combo, in a vacuum, it lists the exhast velocity as 3700 m/s. I'm not an expert, but this seems very unlikely, given that LOX/kerosene is 3500, and in general LOX and kerosene are supposed to be better performing than H2O2/N2H4. Could someone who knows what they're doing run those calculations again?
Hmm. I ran it on 'cpropep web'[1], and got a maximum of 322 seconds. I wouldn't consider that to be definitive however. But your observations are very good ones; and I certainly distrust these figures.WolfKeeper22:51, 4 February 2006 (UTC)[reply]
Ok, the 1992 version of Huzel and Huang lists H2O2/N2H4's ISP as: 337.6 seconds. Looks like the original version quoted here was wrong. (An older version of this book was available on the net.)WolfKeeper01:05, 6 February 2006 (UTC)[reply]
Sure enough, I had mistyped the data from H&H for that combination (I typed 377.6 instead of 337.6). Let's not suggest that an earlier version of H&H was wrong unnecessarily!
All those units need to be specified, in all the tables. What's "pressure" in earlier table, for example. Gene Nygaard 17:08, 22 Dec 2004 (UTC)
Hmmm. "Pressure" is psia, which I'm sure you figured out from context. But this does bring up the point that psia is hardly metric. And 1000 psia is 6.894 MPa, which is hardly a nice round number. Iain McClatchie 19:38, 23 Dec 2004 (UTC)
You could just as easily use a nice round 10 MPa as the base pressure (the one multiplied by 1.00 in the first table, and the starting point in the second table), couldn't you? Or even if that is for some reason unsatisfactory, 7 MPa would certainly work just as well as 1000 lbf/in². Why use a halfway conversion, one still requiring reference to English units? Gene Nygaard 08:37, 27 Dec 2004 (UTC)
7 Mpa is not the same as 1000 lbf/in2. The results of the program run are accurate enough that the difference between the two might matter. Iain McClatchie 18:32, 27 Dec 2004 (UTC)
Opening paragraph
Wolfkeeper,
You obviously know the subject, we're arguing over the meaning of words.
"believed" versus "noted".
Nobody knows what someone once believed. That's definitely the wrong word.
The fact that they wrote stuff down supports this position that they believed it :-) Noted has connotations, for example you talk about 'noted researchers'. It sort of implies correctness/importance etc. which is not really true in this case. It's like I once refered to a serving lady as a 'wench', when she was giving me some lip. Whilst formally correct, the other connotations (which I didn't mean at all) could have got me into a lot of trouble :-) Connotations are important. WolfKeeper
The correctness of hydrogen being "ideal": I agree, no propellant is ideal, so the problem word here is "ideal". How about "Many early rocket theorists noted that hydrogen would be a marvellous propellant, since it gives the highest specific impulse."
This is what high specific impulse means, so this phrase is redundant.
not precisely, they are not truly equivalent, but I don't feel strongly.WolfKeeper
"where a hydrogen stage's low fuel mass"
I do not want to confuse Isp and mass. Helium cold-gas thrusters have low fuel mass too, but they have crummy Isp. Isp is what matters, not (directly) mass.
No, that's not correct. For a given stage's delta-v and for a given payload mass a hydrogen fuelled stage happens to weigh less than say, a kerosene one. That's mainly why they use it I believe, because the whole launch vehicle gets a lower GLOW, and hence ends up cheaper. But the fact that hydrogen gives a high Isp is significant, but not on its own enough. For example a theoretical fuel with an Isp 1 second less than hydrogen, but with several times higher density could easily give the same delta-v, but the stage would weigh less due to the better mass fraction.WolfKeeper
"in practice"
Hydrogen is bulky in theory too. Even slush hydrogen is bulky in theory.
Without in any way trying to be evasive: it depends on the theory. A theory is just a model of reality; models always leave things out.WolfKeeper
There are no other proposed hydrogen storage systems besides cryogenic storage. That makes it the only practical storage system. Would you quibble with the phrase "remotely practical"?
Yes. GOX for example has been used on Buran. There's no obvious reason why GH2 couldn't be used for similar purposes; it's much heavier that way, but that doesn't always matter, and gaseous propellants are easily space storable, liquid hydrogen is rather difficult.WolfKeeper
I'm leaving your edits up for now so we can talk about this and resolve it. But I don't agree with all of them.
Some early liquid rockets were powered by ethanol. E.g. V-2. What propellants did Goddard use? Did he manage cryogenic oxygen? Sdsds22:04, 1 April 2007 (UTC)[reply]
This is an excellent article. I like the conciseness and choice of references (e.g. Huzel and Huang as opposed to the overused Sutton). I am a bit perplexed by one of the opening comments:"This type of propellant has a long history going back to the first rockets..." Given that for 600 years all chemical rockets used solid propellants, that comment seems peculiar, or rather conspicuously false. Or am I missing something?Magneticlifeform (talk) 15:26, 2 November 2010 (UTC)[reply]
