X-37 / X-51 article in Aerospace Testing International.

[chrismb]

FYI, as I know you guys and gals like this kinda stuff (I presume/hope this is a general access URL, rather than subscription only);

http://viewer.zmags.com/publication/17f ... 7fdb9ec/22

[GW Johnson]

X-37 was a program for a two-man spaceplane NASA abandoned. Left as the smaller drone prototype, USAF is doing things with it in orbit that they do not reveal. Best guess is eval-as-spy-platform stuff, but really, I dunno.

X-51 flew about 3 minutes at Mach 5 cruise the other day, after being boosted there by a rocket. The breakthroughs or records are not speed, but scramjet duration (3 minutes vs about 2 sec before) and fuel (first time kerosene). The control of waste heat and friction as an energy flow within the overall engine system , to enable a long burn, is the successful piece of technology here. That has never been done before.

The NASA scramjet was X-43 which flew twice back about 2004, on hydrogen as fuel. Both were around 2 sec burns only, after being boosted there by rockets. The first successful flight reached pretty close to Mach 7, breaking the old airbreathing speed record of Mach 6. The second flight essentially reached Mach 10. The records there were speeds and first successful flights with any kind of scramjet burn at all.

There was a scramjet article flown in the 60's on the X-15 rocket plane, but it was not propulsive at all, just a cold flow device to verify shock structures and flow paths. That flight did severe airframe damage to the X-15 from shock impingement heating, at about Mach 6.67. But this was a rocket-powered flight, not an airbreather.

The previous airbreathing speed record of Mach 6 was set about 1980 by a plain (subsonic-combustion) ramjet test article called ASALM-PTV (which I worked on, in a small way). It was a test prototype for a supersonic cruise missile never fielded, fueled by the synthetic kerosene RJ-5. The speed record was set accidentally on the first test flight, and down near 20,000 feet altitude, to boot. It wasn't even supposed to be able to fly that fast, being designed only for cruise at Mach 4, and that at 80,000 feet. ASALM flew 7 times, the other 6 flights were letter perfect, including the design mission test.

The speed record test was supposed to be a nice sedate racetrack pattern at M2.5 around the Eglin test range. An assembly error in the fuel control caused it to scurry away from us like a scalded dog. The sense of the last telemetry we had from the bird was "I'm still accelerating, my speed is Mach 6, my skin is melting, and I just ran out of fuel. Bye". We found it four days later in a farmer's field, some 10 miles off the base. You have to understand, this was a 20-inch diameter, 14-foot long steel dart sticking up in that farmer's field. Very impressive sight.

That record stood from 1980 to the M7 X-43 test in 2004.

[Tom Ligon]

I use a slide of the X-51 in my fusion talks this year. Previously, with only a couple of seconds of burn, the air-breathing Polywell-powered space plane Dr. Bussard proposed was, in my opinion, the one with the biggest technological hurdles to overcome. Besides relying on a power source that was unproven, the propulsion system looked nearly impossible if the life was just a couple of seconds.

A 200 second flight earlier this year and now a full 5 minutes dropped the spaceplane concept from fantasy to an exciting possibility.

[GW Johnson]

It was a real technological problem in scramjet design to balance out and utilize friction and waste heat as part of the engine combustion system.

I do not know what USAF did to solve that problem. I do know about the problem, and that its solution is very difficult. That part has been a bugaboo since the 60's with scramjet.

I presume it would be the same bugaboo, even substituting nuclear heating for combustion heating. This is a separate issue from substituting heat transfer and mixing for flame stabilization and mixing, all in a supersonic flow.

[GW Johnson]

I'd be careful thinking scramjet is the key to SSTO spaceplanes. The faster you fly in the air, the bigger the drag integral you have to pay for with your fuel. About the minimum ram takeover speed with scramjet is Mach 4. So, 0-4000 fps of your orbital delta-vee requirement of 26,000 fps is the scramjet's booster, right off the bat. Let's say you fly to M10-12 before the air gets too thin for the airbreather to be accelerative.

So, you added about 6-8000 fps with the high Isp airbreather. Except, above M8, its Isp isn't so very high anyway. Plus, your drag integral is very, very high. And, you still need 12-14,000 fps more from a rocket to reach orbit. That's 16-18,000 fps rocket vs 6-8000 fps airbreather, at an extreme drag cost and with serious thin-air frontal thrust density limits. You can get 16-18,000 fps from a chemical rocket, but only as a separate stage. The higher drag integral makes that closer to an ideal delta-vee of 20-22,000 fps, though.

So, you are really looking at TSTO, not SSTO. And the frontal thrust density and drag integral problems are precisely what killed the X-30 / Orient Express scramjet SSTO long ago.

Now, consider instead plain subsonic combustion ramjet instead, as part of a TSTO system. About the limit for the airbreather is Mach 6. It takes over quite effectively at Mach 1.5. You need 0-1500 fps from a booster off the deck, you get 4500 fps from a demonstrably-high Isp airbreather, and the remaining 20,000 fps are easily within reach of a conventional one-shot throwaway second stage at decent payload.

Your drag integral is far smaller at only M6 in the atmosphere vs M10-12. Plus, all the technologies to support it have been operational in missile work since the late 1960's. Some of it dates all the way back to the mid-40's. No new component technology developments. Big, big advantage.

The only trick with doing a ramjet-based TSTO, is doing it before the last of us "real ramjet guys" dies. Look around, you will find hardly any of us truly full-capability guys left.

[Tom Ligon]

You make the presumption about the air becoming "too thin" that may miss the mark.

Years ago I ran a fairly simple model (on a Radio Shack Model III, back in the last millenium when we still used Roman numerals for computers). While I did not have a rigorous hypersonic drag model, the basic principle of the model should work with some adjustment regardless of the exact drag model.

Basically, if you are encountering enough air to produce drag, you are encountering enough air to make thrust, providing you have sufficient power and have a way to effectively couple it to the mass flow stream. My model assumed a craft about the size and power of a particular fighter jet, which hits maximum dynamic pressure at Mach 3 and 80,000 ft (based on the SR-71), with an electric propulsion system that ionizes incoming air and imparts energy via a wiggler field. At a power level in the range of 40-60 MW I was able to show acceleration up to 70 miles. At that point the problem was not high drag, but just the opposite. There was not enough air left for either acceleration or drag. Orbital velocity was reached at about 3/4 of the way around Earth at the equator. A short burst of water into the engine cleared the atmosphere and circularized the orbit.

The principle is to continue to climb as the speed increases, maintaining a fairly constant dynamic pressure as long as there is usable air. Eventually the dynamic pressure (and with it both drag and thrust) starts to fall off.

The craft in question is the "Sunfire" I first included in two stories from before I met Dr. Bussard, and he was impressed that I had run the numbers on it. The craft is illustrated in the internal pictures of the January 1986 story, and actually made the cover in its second appearance in November 1993.

[ladajo]

And just for fun, Reaction Engines are at it again...

http://www.foxnews.com/scitech/2010/09/ ... e-tourism/

[rjaypeters]

And just for fun, Reaction Engines are at it again...

Speed the day!

[IntLibber]

It was a real technological problem in scramjet design to balance out and utilize friction and waste heat as part of the engine combustion system.

I do not know what USAF did to solve that problem. I do know about the problem, and that its solution is very difficult. That part has been a bugaboo since the 60's with scramjet.

I presume it would be the same bugaboo, even substituting nuclear heating for combustion heating. This is a separate issue from substituting heat transfer and mixing for flame stabilization and mixing, all in a supersonic flow.

They use the fuel as airframe coolant, and use the airframe cooling system as a device with which to crack the hydrocarbon into hydrogen and carbon, injecting the cracked gasses into the combustion chamber. One of the problems the X-51 has though is they need an additive that prevents the carbon from coking up the cooling lines, and in particular, they're using common jet fuels, which according to rocket experts, has too much sulphur in it, which contributes to coking problems. RP-1 has little to no sulphur in it for this reason, in comparison to regular jet fuels. This is why I believe the X-51 didn't complete its full mission envelope.

The advantage of the electron gun or other heating from polywell power source is that the energy can be focused in the center of the airstream, leaving the air flow at the surface boundary to receive very little energy and would thus be useful to preheat the hydrogen flowing into the polywell via a airframe skin heat exchange system similar to that used in the X-51 to crack its hydrocarbon fuel.

[IntLibber]

I'd be careful thinking scramjet is the key to SSTO spaceplanes. The faster you fly in the air, the bigger the drag integral you have to pay for with your fuel. About the minimum ram takeover speed with scramjet is Mach 4. So, 0-4000 fps of your orbital delta-vee requirement of 26,000 fps is the scramjet's booster, right off the bat. Let's say you fly to M10-12 before the air gets too thin for the airbreather to be accelerative.

So, you added about 6-8000 fps with the high Isp airbreather. Except, above M8, its Isp isn't so very high anyway. Plus, your drag integral is very, very high. And, you still need 12-14,000 fps more from a rocket to reach orbit. That's 16-18,000 fps rocket vs 6-8000 fps airbreather, at an extreme drag cost and with serious thin-air frontal thrust density limits. You can get 16-18,000 fps from a chemical rocket, but only as a separate stage. The higher drag integral makes that closer to an ideal delta-vee of 20-22,000 fps, though.

So, you are really looking at TSTO, not SSTO. And the frontal thrust density and drag integral problems are precisely what killed the X-30 / Orient Express scramjet SSTO long ago.

Now, consider instead plain subsonic combustion ramjet instead, as part of a TSTO system. About the limit for the airbreather is Mach 6. It takes over quite effectively at Mach 1.5. You need 0-1500 fps from a booster off the deck, you get 4500 fps from a demonstrably-high Isp airbreather, and the remaining 20,000 fps are easily within reach of a conventional one-shot throwaway second stage at decent payload.

Your drag integral is far smaller at only M6 in the atmosphere vs M10-12. Plus, all the technologies to support it have been operational in missile work since the late 1960's. Some of it dates all the way back to the mid-40's. No new component technology developments. Big, big advantage.

The only trick with doing a ramjet-based TSTO, is doing it before the last of us "real ramjet guys" dies. Look around, you will find hardly any of us truly full-capability guys left.

I agree, I'm generally a skeptic toward the utility of scramjet combustion. Flying with just ramjet combustion to mach 6 and then zooming thence on rocket power into space is sufficient to increase the trip average Isp that the required fuel mass fraction to reach orbit drops to about 0.78, which is entirely reasonable for aircraft.

[DeltaV]

They use the fuel as airframe coolant, and use the airframe cooling system as a device with which to crack the hydrocarbon into hydrogen and carbon, injecting the cracked gasses into the combustion chamber. One of the problems the X-51 has though is they need an additive that prevents the carbon from coking up the cooling lines, and in particular, they're using common jet fuels, which according to rocket experts, has too much sulphur in it, which contributes to coking problems. RP-1 has little to no sulphur in it for this reason, in comparison to regular jet fuels. This is why I believe the X-51 didn't complete its full mission envelope.

The reduced X-51A performance is believed to be due to the failure of a seal between the engine and exhaust duct, per Aviation Week (unfortunately not in the online subset of their articles, or I'd link it).

[GW Johnson]

Don't get me wrong, I think scramjet has its place: missiles, but only those where M6+ is called for. In particular, the scenario I am thinking of is naval battle group defense against incoming bombers carrying supersonic (ramjet) anti-shipping missiles (SS-N-22 Sunburn, for one). The only thing I can imagine that could average Mach 8 over 200 nmi, and fit in a vertical launch bay, is a scramjet.

For space launch, I think scramjet is much less practical. Ramjet has a positive T-D margin at M4-to-6 conditions, even up to 100 kft. Trouble is, the thrust and drag numbers are small, yet the weights are not. Actual vehicle acceleration potential is quite low, way under 0.1 gee typically. It is no different with scramjet. It is very hard to fly level at conditions like that, much less accelerate or climb.

Flying to higher speeds in the air, staying deep enough so that (T-D)/W >> 0.1, increases your total drag integral drastically, eating up the gains you made in Isp. And at M8+, even scramjet Isp is not all that great. At M6, ramjet and scramjet are about the same, at Isp around 800 sec. The faster you try to fly, the lower Isp drops. Simpler rocket starts looking a lot better when that happens, plus you can get T-D/W high enough to do some good with the rocket.

Those of you who went over to "exrocketman" and looked at the ramjet performance data I posted there, you already know what I am talking about. For a hypersonic-airplane launch of a second-stage rocket, I actually decided that M6 release at 100 kft was too high in the thin air. It would have worked far better releasing at M6, 60 kft. You can see it in the acceleration-potential curve with altitude.