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Second Hypersonic X43 Scramjet Ready for Testing
Dan writes "I am sure most of you remember how NASA was forced destroy their first hypersonic X43 seconds in it's maiden flight, which was a big setback for the american hypersonic scramjet program. Well NASA just finished one of the final tests and is preparing to launch it as early as February 21! I wish them the best."
Been there done that.
Basically, yes.
The thing about getting to orbit isn't so much the vertical velocity required, its your horizontal velocity. Rockets going to orbit don't go straight up; if they did they would end up coming straight back down... The trick is getting enough horizontal velocity so that as gravity pulls you down towards the earth you are moving fowards fast enough that you are continually "falling over the edge" of the horizon.
With a scramjet you only need half the fuel of a traditional rocket, as you burn oxygen from the atmosphere instead of carrying it all with you. Yes, a traditional rocket IS needed to get you out of the atmosphere, but using a scramjet for the initial acceleration would end up saving a lot of fuel, and hence weight.
So, why carry the oxygen, why not get oxygen from the air? For LH2-LO2, that eliminates most of the mass and solves the mass fraction problem right away. The 1960's Aerospaceplane project originally considered liquifying the O2 from the air -- careful tweaking can be enriched on LO2 over LN2 on account of boiling point differences. You used (boiled off) some of your LH2 to get the coolant.
The trouble with LACE (liquid air cycle engine) is that you have to slow down the air rushing into the inlet (or speed it up to your rushing vehicle). If you are going fast enough relative to orbital velocity, slowing the O2 down in the inlet will heat it so much that you cannot burn it with H2 and get any energy -- the stagnation temperature of the shock front gets higher than your flame temperature. Hey, if this were not the case, orbital velocity would be low compared to rocket exhaust velocity and mass fraction would not be a problem.
Ah, the scramjet, and scramjet was also considered for Aerospaceplane. It is literally the taking a drink from a fire hose. You only slow down the inlet air stream a little bit so you get some compression, and burn H2 in that hypersonic air blast and 1) hope that the flame doesn't blow out and 2) hope that you get any positive net thrust out of the works.
If you could get any single-stage-to-orbit vehicle built that had reasonable engineering margins, you could fly it like an airplane, and even if it had a very small payload, you could fly it often enough to make a profit. NASA blew a wad in the late 80's, early 90's with National Aero Space Plane (NASP) and pulled the plug. But forget the scramjet -- if you could build a rocket out of composite materials, you could get the mass fraction. NASA blew a wad in the late 90's on the X-33 and then pulled the plug.
Jerry Pournelle states that the Strategic Defense Office (which needed a way to loft Star Wars into orbit) could have done the job -- the DC-X demonstrated the control of vertical-takeoff vertical-landing (lands tail first on rocket flames just like in Buck Rogers -- maybe not so wasteful of fuel because reentry is mainly aerobraking and landing is to last applying the brakes on a mainly empty vehicle), and he talks about a program called Have Region (don't know the source of Air Force code names, although NASA these days seems to have projects code named Have Boner) that proved that the mass fraction target was achievable and one didn't need scramjets.
This is only relevant for scramjets that use hydrogen as a fuel. If there were a scramjet which used jet fuel B, then that type of savings would be much smaller.
However, the X-43A vehicle does indeed use hydrogen for its fuel. (Perhaps for that very reason?)
Whoever modded this as interesting knows even less about physics and aerospace technology than did the writer. The heat generated by friction at high speed is an issue that must be addressed, but while there will be drag it's not going to rip anything apart unless it's not designed properly in the first place. That's one of the things wind tunnels and computer modeling help deal with long before a model is test-flown.
The SR-71's fusalage expanded from heat, true. The material is going to have to deal with heat, true. The NASA shuttle deals with the heat of mach 25 on re-entry, and it is not torn apart by drag unless something goes wrong, but the same happens when a commecial airliner gets seriously out of shape in-flight. Like the one that lost its rudder over Long Island Sound a couple years ago.
The stealth bomber (B-2) is subsonic. Carbon fiber is used due to its strength-to-weight and radio-frequency transparency, not heat resistance. I would be looking at exotic metal alloys, metal composites, ceramics (which is what the space shuttle tiles are) and use of circulating fuel for cooling of critical areas. The flight profile for a long duration hypersonic craft would probably involve extended flight at altitudes where drag is less of an issue, further reducing friction heating.
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That's not entirely correct. The O2 is a third of the mass. Keep in mind that in addition to eliminating the weight of the 02, scramjets push such an amazing amount of air out the back that they are far more efficient than rocket engines.
The main problem with space launches is the initial climb and acceleration, when you are pushing forward all of the craft's stages and fuel. By eliminating the 02, it translates into vastly, vastly smaller requirements.
Better to simply make the fuel and oxidizer tanks bigger (because fuel and oxidizer is -so- much a -tiny- part of a launch cost) and stick bigger engines on it.
Scramjets are far simpler than rocket engines. It would be much cheaper to build boosters that use a scramjet as a first stage as opposed to a rocket engine. The fuel savings, the increased payload, and the cheaper cost all make the scramjet a superior option.
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