The Next Generation of Human Spaceflight

[93143]

Recombination. Hmmmm. I'm not very clear on your concept, but we hit smack into recombination problems pushing subsonic ramjet to Mach 6-ish. In fact, that is the subsonic combustion ramjet speed limit mechanism.

The nozzle converts thermal energy to kinetic energy at the expense of a pressure drop, but it does not convert recombination energy to kinetic energy. If the chamber temperature gets much over 4500 F (near 2000K) in the airbreather, we got serious recombination penalties. I think in the rocket scenario, this is probably nearer chamber temperatures around 6000F (near 3000 K), but I could be wrong.

Do you mean charged-particle recombination (which is what I meant) or chemical recombination?

With an initial air temperature of 1775 K (2735 F), a pressure of 1250 psi, and a slightly lean fuel/air ratio of 5% by mass, CEA is giving me a free-electron mole fraction of about 1e-9 and an NO+ fraction of about twice that, plus a few negative ions in the 1e-10 range. Chemistry-wise, the mole fractions of hydroxyl, carbon monoxide, and nitric oxide are all around a percent or two, with atomic oxygen and atomic hydrogen looking like they might get interesting soon as well. (If you're interested, the final chamber temperature was 4850 F.)

Chemical nonequilibrium is addressed by my methodology (very roughly) in that I average the Isp values obtained by assuming local equilibrium concentration during expansion with those obtained by assuming chamber composition during expansion (frozen) in a ratio of 2:1 respectively. This is a very crude model, but it roughly matches what I get from trying to simulate real hydrolox rocket engines.

Charged particle recombination is untracked by my method, since CEA doesn't know that the heat was added by a REB and assumes chemical equilibrium in the chamber, which involves virtually no ions even at quite high temperatures...

For what you describe, I think you probably need a process to extract usable energy from the EB-heated plasma until it recombines, then put it through the nozzle, and burn with LOX if you like. Use the extracted energy in some way, like preheating the working fluid, so that this energy is not lost.

So, regenerative cooling of wherever the relaxation is taking place?

The timescale is what concerns me. If you have to retain the plasma at maximum temperature and pressure for a significant length of time, the engine could become extremely large and heavy. Perhaps I should look into this in detail some time and see if I can't figure out whether this is buildable or not...

Fortunately, with airbreathing the chemical reaction isn't quite so critical; there's plenty of spare reaction mass...

[GW Johnson]

Yes - I was talking of ionization - recombination effects. At the short residence times of missile-sized rocket and ramjet engines, it can be significant.

What happens in a (subsonic combustion) ramjet, regardless of fuel or geometry, at flight speeds much above Mach 5, the air in the inlet is already very hot. When you burn, that energy release goes more into ionization than it does temperature increase. You see it modeled as changes in molecular weight and specific heat ratio from thermochem codes that try to account for this.

Trouble is, nozzles turn thermal energy into kinetic energy quite efficiently by means of pressure drop. They do not transform ionization recombination energy into kinetic energy. Thrust and impulse start failing to reflect the fuel you burn. It's getting pretty significant around Mach 6.

That's why so many have tried so hard, so long, to make scramjet (supersonic combustion ramjet) work. The basic idea is not to decelerate the air to subsonic speeds (relative to the airframe), so that its static temperature is far below is stagnation temperature. Then you have much more "margin" on gas temperatures between incoming static and the ionization point (not really a point, rather ill-defined actually).

Some of the same things can happen in rocket engines, although up to now it has not been that much of a problem, even for LH2-LOX at 6000 F. It certainly was a problem for the old "Project Pluto" nuclear ramjet, although basic materials overheat problems were far worse, just as in the solid-core nuclear reactor rocket devices. The early ones, like the Phoebus series, shed very radioactive plumes from core erosion that was mostly "fixed" in the later NERVA units.

For nuclear power heat as the driving source for some sort of rocket or ramjet or combined-cycle engine, those two problems were, are, and likely always will be, the "bugaboos": overheated materials, and unintended ionization that detracts from nozzle recovery of performance potential.

[DeltaV]

USAF is leaning towards airbreathing TSTO for smaller payloads: Air Force Study Points To Reusable Orbiter



I think my Polywell space hopper will also use dorsal surface intakes. Unless Mach-Woodward Effect succeeds sooner than expected.

[GIThruster]

USAF is leaning towards airbreathing TSTO for smaller payloads: Air Force Study Points To Reusable Orbiter



I think my Polywell space hopper will also use dorsal surface intakes. Unless Mach-Woodward Effect succeeds sooner than expected.

"Although the study held out some optimism for RBCC proponents in the long run, its findings could be a telling blow for supporters of air-breathing turbine-based combined-cycle (TBCC) hypersonic space launch concepts. Study officials say even though the TBCC performance indicates greater specific impulse than rockets, the reusable booster turned out to be smaller, lighter and less costly."

Looks like rubbish to me. "Less costly" than what? Certainly not less costly than Falcon. maybe cheaper than Atlas if you plan to distribute development costs over 500 launches.

I don't believe the report, as much as I'd like to see a reusable vehicle.

[GIThruster]

USAF is leaning towards airbreathing TSTO for smaller payloads: Air Force Study Points To Reusable Orbiter



I think my Polywell space hopper will also use dorsal surface intakes. Unless Mach-Woodward Effect succeeds sooner than expected.

Jim will be back in the lab in the next couple weeks and for the first time in a decade he'll have grad student support. We can afford to hope for some speedy results but the way these things go. . .there's no telling what will happen. Even if he gets obvious amounts of thrust, he's not working in the wormhole regime where one expects he'd need to in order to enable human spaceflight. Of course, given the right response to the right results, that could happen in very short order.

Either way, it'd be a joy to see what you have cooked up so far. Ventral. . .dorsal. .to the best of my knowledge, there's never been an hypersonic separation event between spacecraft--the question that killed the Sanger, IIRC.

[rjaypeters]

...to the best of my knowledge, there's never been an hypersonic separation event between spacecraft--the question that killed the Sanger, IIRC.

A nit: In what flow regime is Space Shuttle separation from the tank? If hypersonic, I'm sure the dynamic pressure is low. Also, I'd bet the Solid Rocket Boosters probably separate at merely low supersonic speeds.

SR-71s had a piggy-back drone (D-21) that didn't work out well. Those separations were merely low supersonic also.

[GW Johnson]

Dorsal intakes tend not to work very well with turbines, and not at all with ramjets, on lifting vehicles. The intake is "shadowed" in the wake of the body. No ram air pressure.

If it won't work for ramjet, it won't work with scramjet, either, which is just a ramjet with supersonic flow in its combustor and less deceleration in its inlet. Same goes for any combined-cycle concepts using these airbreathers.

Supersonic / hypersonic separation depends very greatly upon both dynamic pressure and geometry. You can do pretty much anything you want if the dynamic pressure is so low the aero forces are less than the weights.

On the other hand, you can pretty much stage off anything behind you, no matter what the dynamic pressure is. It is the side-by-side stuff that gets so very intractable.

On a blunt object, "hypersonic" is usually considered to be Mach 3. On a slender object, it's usually considered to be Mach 5. It's kinda fuzzy and arbitrary.

[93143]

Space Shuttle tank sep is at the end of the burn to orbit, so something like 200 km or so. My standard atmosphere calculator gives me digital zeros for the density and pressure at that altitude.

Booster sep is at 44.5 km and Mach... 4? I can't find a reference that says. Anyway, it's sort of hypersonic, but the dynamic pressure is a couple hundred Pa to a few kPa at most (anyone with a better number can speak up). If I'm right about the dynamic pressure, the drag on an SRB at zero angle of attack would be a few percent of its weight or less. They tumble, of course, after separation, but by that time they're already well clear of the rest of the stack by virtue of the separation motors.

[DeltaV]

Jim will be back in the lab in the next couple weeks and for the first time in a decade he'll have grad student support. We can afford to hope for some speedy results but the way these things go. . .there's no telling what will happen. Even if he gets obvious amounts of thrust, he's not working in the wormhole regime where one expects he'd need to in order to enable human spaceflight. Of course, given the right response to the right results, that could happen in very short order.

May it work out so. Even a fraction of a newton (indisputably out of the 'noise') would be awesome.

Either way, it'd be a joy to see what you have cooked up so far. Ventral. . .dorsal. .to the best of my knowledge, there's never been an hypersonic separation event between spacecraft--the question that killed the Sanger, IIRC.

My basic scheme has always been SSTO. TSTO would require more ground support and infrastructure, and probably a second pilot, and would take up too much of my back yard.

Previously, I was leaning towards dedicated, embedded, electric lift fans for VTOL (HTOL optional for heavier payloads), + dedicated, embedded, multistage, electric ducted fans for low-medium cruise (to M2.5), + Bussard's REB-driven QED-ARC for orbit boost (> M2.5), all modes strictly using air as propellant, except for the REB, when the atmosphere thins to the point where onboard reaction mass must be used.

Considering how much of the vehicle's volume the embedded lift fans would consume, I'm now leaning towards a more volumetrically-efficient design: parallel, embedded, multistage, electric ducted fans, with F-35 style variable-geometry intakes and exhausts for VTOL (one or the other end having variable geometry for VTOL, not both, depending on whether that duct is dedicated to forward or aft lift). Spin direction for the forward lift ducts being electrically reversible for VTOL. Lower efficiency in reverse, of course, due to the optimum forward-flight blade shapes, but maybe good enough for brief VTOL periods. Otherwise, pivot the entire fan portion of the duct by 180 deg, like a big spherical valve, so that intake becomes exhaust and vice versa.

All duct geometry (and rotor spin direction) transitions to normal for low-medium cruise (to M2.5). Then, QED-ARC for orbit boost (> M2.5).

[DeltaV]

Dorsal intakes tend not to work very well with turbines, and not at all with ramjets, on lifting vehicles. The intake is "shadowed" in the wake of the body. No ram air pressure.

If it won't work for ramjet, it won't work with scramjet, either, which is just a ramjet with supersonic flow in its combustor and less deceleration in its inlet. Same goes for any combined-cycle concepts using these airbreathers.

My rationale for dorsal intakes was to avoid sealing problems with ventral intakes for reentry. Come to think of it though, NASP/X-30 would have had a similar issue with its ventral intakes.

Could roll inverted for reentry, in which case the rudders would have to be on the 'down' side in normal atmospheric flight.

[GW Johnson]

Here's an oddball idea. Put the intakes and the rudders on the same side for ascent, as ventral. During re-entry, roll them over to dorsal location. Protects both intakes and rudders during re-entry, but sticks them both into "clean" air on ascent when they are most needed. (The rudders stick out far enough to work either ventrally or dorsally.)

[DeltaV]

That's what I said.

[krenshala]

That's what I said.

I think GW is suggesting to roll just the rudders and intakes, not the entire vehicle, which is why he (rightly, in my opinion ) called it an oddball idea.

[93143]

Given fixed rudders and intakes, would it be plausible to have the cockpit on the bottom too? Perhaps the interior could be rotated or reconfigured for reentry (using the same windows), so the crew doesn't have to take negative gees...

Alternately, figure out a good way to seal the intakes for reentry. It can't be that hard; REL's Skylon is pretty well along as a design, and it relies on the engines' shock cones making an adequate seal when they're moved forward for reentry...

The upside-down idea does have the advantage that you don't need to make any penetrations in the heat shield - no landing gear, no ventral cargo bay doors, nothing. You could plumb the whole top side as a high-temperature radiator for high-Isp deep space work...

[DeltaV]

Given fixed rudders and intakes, would it be plausible to have the cockpit on the bottom too? Perhaps the interior could be rotated or reconfigured for reentry (using the same windows), so the crew doesn't have to take negative gees...

I don't see a big problem with the cockpit being on the bottom at takeoff (probably better for VTOL anyway) as long as there is adequate visibility of the surroundings. Visibility augmentation using cameras and video screens is a good idea regardless (I've always wanted a full 360x360 view using electronics). Rotating the interior wouldn't be that hard. The crew cabin could maintain roll attitude while the rest of the vehicle performs an automated roll sequence. Just make sure the flight control system takes pilot inputs with a grain of salt while roll inversion is in progress.

Alternately, figure out a good way to seal the intakes for reentry. It can't be that hard; REL's Skylon is pretty well along as a design, and it relies on the engines' shock cones making an adequate seal when they're moved forward for reentry......

A drop down ventral intake wouldn't be that hard, true. I'd want video, electrical and pressure confirmation of seal integrity before reentry. Maybe use the drop-down scoop for the REB and leading edge intakes for the ducted fans (only needed below M2.5), with boost/reentry covers that conform to and slide along the leading edge. VTOL variable geometry complicates things, but that's why they invented SolidWorks and people like tombo.

The upside-down idea does have the advantage that you don't need to make any penetrations in the heat shield - no landing gear, no ventral cargo bay doors, nothing. You could plumb the whole top side as a high-temperature radiator for high-Isp deep space work...

SSTA.