OK, here's a stupid question:For some launch vehicles with compartments, the air from these compartments is often replaced with nitrogen, to suppress any possible fires. Why don't they use helium, instead, which would make the launch vehicle lighter?
Quote from: Danderman on 02/21/2009 05:23 pmOK, here's a stupid question:For some launch vehicles with compartments, the air from these compartments is often replaced with nitrogen, to suppress any possible fires. Why don't they use helium, instead, which would make the launch vehicle lighter?Helium is one of those elements that we have enough of - for now - but that will not always be the case. Mission planners and engine designers are already aware of the decreasing availability of helium. It's not a problem - yet - but it will be, especially if we use it like you suggest and it just gets vented. There just is not an over abundant supply of it on earth and we need to be aware of our usage and not waste it.
Quote from: ugordan on 02/21/2009 06:13 pmQuote from: Antares on 02/21/2009 06:03 pmSome materials and electronics are sensitive to helium too.Sensitive to helium in what sense? It's supposed to be an inert gas. Diffusion problems?Dunno exactly. Helium can go through anything, apparently weakening some metals a tiny bit. Only a problem in very low margin areas. There are some spacecraft (guidance?) boxes that don't like helium. I don't know the mechanism, just that the providers ask to make sure that their boxes aren't around any helium sources.
Quote from: Antares on 02/21/2009 06:03 pmSome materials and electronics are sensitive to helium too.Sensitive to helium in what sense? It's supposed to be an inert gas. Diffusion problems?
Some materials and electronics are sensitive to helium too.
How does having a separate heat exchanger help with oxygen reactivity? When you're done, you still have a bunch of hot, high pressure oxygen.
There is also the added weight of the second exchanger
and the power required to move the coolant through it (which is much higher than you might imagine - thousands of horsepower for a mid-to-large rocket engine).
How about the weight of the fluid itself?Oxidizer tank would have to be built stronger to handle the increase in pressure, as the oxidixer absorbs heat the pressure would begin to rise immediately.Also adding complexity. Another fluid system, more pipes, more valves, tankage on the LV, infrastructure for servicing. Something else to fail.
Quote from: yinzer on 02/25/2009 05:54 amHow does having a separate heat exchanger help with oxygen reactivity? When you're done, you still have a bunch of hot, high pressure oxygen.I was thinking that the heat capacity and conductivity of the oxygen tank would be so enormous that the oxygen wouldn't get very hot at all, even locally.
QuoteThere is also the added weight of the second exchanger Ah, I can see how that would be a problem. Would the heat exchanger be more massive than an ablative nozzle?
Quoteand the power required to move the coolant through it (which is much higher than you might imagine - thousands of horsepower for a mid-to-large rocket engine).But is that any different from a normal regeneratively cooled engine?
In a normal regeneratively cooled engine, you have one heat exchanger operating at a large temperature differential. With your scheme, you have two heat exchangers each operating at a smaller temperature differential, which means more pressure drop and thus power.
Just an observation I had and wanted to pass along because I didn't quite hear it worded correctly and/or dumbed down until very recently.The way I heard it described (for those of us unsure about specific impulse (measured in seconds)), it's defined as how long a rocket will fire using a pound of prop/oxidizer to provide a pound of thrust. If this is inaccurate, please correct me.
Quote from: usn_skwerl on 02/27/2009 05:42 amJust an observation I had and wanted to pass along because I didn't quite hear it worded correctly and/or dumbed down until very recently.The way I heard it described (for those of us unsure about specific impulse (measured in seconds)), it's defined as how long a rocket will fire using a pound of prop/oxidizer to provide a pound of thrust. If this is inaccurate, please correct me.That is incorrect It is thrust per propellant flow rate or impulse (change in momentum) per unit of propellant.
Occasionally I see people mention air-started SSME's. I get the impression that this is either hard or dangerous. Somehow the J2X is supposed to be better in this regard. Is there a simple explanation of what the problem is and how J2X is better?
Now i need to ask... I guess I'm missing a key point on how rocket engines are started..is there a summary some where on how they start up rocket engines..I didn't realize a charge is needed to start the engine..jb
Quote from: mmeijeri on 02/28/2009 01:07 pmOccasionally I see people mention air-started SSME's. I get the impression that this is either hard or dangerous. Somehow the J2X is supposed to be better in this regard. Is there a simple explanation of what the problem is and how J2X is better?it is only hard for the SSME since it wasn't designed for it. It is head started which means the start box is small, the initial conditions are very critical. Also it depends on ground GSE for start and purges