Airbreathing SSTO

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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93143
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Airbreathing SSTO

Postby 93143 » Wed Feb 20, 2008 12:25 am

I've been reading an old paper about the QED engine spectrum, and apparently the spaceplane concept involved using conventional turbojets to get up to Mach 2 - 2.5, then QED rockets for the remainder of the flight. Total Isp was estimated to be similar to that of a turbojet (maybe up to 2500 s or so).

It seems to me that there might be benefit to using an airbreathing engine, despite the "copious ozone production". Take off with either electric QED turbojets (or conventional turbojets...) or an electromagnetic sled, and use a ram/scramjet-type QED engine to rapidly accelerate and climb to optimum thrust altitude. Once there, increase speed to very high Mach and go up and out, reaching orbital velocity or better by the time you run out of air. The only propellant required is for the circularization burn, yielding a ridiculously high Isp.

It may even be possible to design a combined-cycle engine that can go from ground start to ram/scram to rocket in one module, thus saving weight. This would be simplified by the EM-sled launch since a turbocompressor (and means of powering it) would not be required.

The disadvantage is that you'd have to design the airframe to fly at all speeds between Mach 0.8 and Mach 25. Probably not impossible even with current technology, but it sure sounds like fun.

However, I suspect Dr. Bussard's concept will need similar treatment, because lift is required to make up for the lack of thrust inherent in a high-Isp design, and thermal loads on the grid mean that reactors larger than maybe 6 GW (if we're really smart about cooling) start to scale marginally, so you can't just crank up the juice.

Besides, it doesn't have to cruise at that speed. It's like a re-entry, but in reverse.

Thoughts?

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Postby drmike » Wed Feb 20, 2008 1:35 am

I think a two stage system like Spaceship 1/2 is perfectly ok. Use air breathing for slow take off and get past the atmosphere before turning on rockets. You don't need that much fuel to get off the planet - you need a lot of fuel to punch thru the atmosphere.

The problem with a scram jet is that the initial boost will squash a human into a ball of goo. Most people don't like that.

Actually, I saw another idea for the X prize. They were going to use balloons to get the rocket above the atmosphere and then launch from high altitude. That way they save fuel and use only a single vehicle for carrying people - which saves money. Same basic idea though - soft take off to get past the atmosphere, then kick ass.

You can still use a Bussard rocket on both stages - one is air breathing, one isn't. Makes design a lot easier for each rocket since it does not have to do double duty.

Just my 2 cents!

saddogmobile
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Postby saddogmobile » Wed Feb 20, 2008 7:18 am

my own thoughts on how to acheive SSTO cappability center on the engine.

if you have a real interest in how a Polywell reactor will help make SSTO a Reality you might wqant to check out Reaction Engines they designed both the Scimitar Engine and Saber Engines for use in an advanced aerospace-plane project.

Air Breathing Engine is a misnomer for our use, since chemically fueled engines actually need to breath and our little electric friend should be fine for blasting anything into plasma.
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hanelyp
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Postby hanelyp » Wed Feb 20, 2008 4:20 pm

One major difference between going up and reentry: for reentry you want the airframe to act as a drag device, preferably heavy on form drag and light on skin drag.

One factor I've heard in the X-30 concept was cooling the airframe with the fuel going to the engine. Without such a store of liquid fuel as a heat sink things get much harder.

Drmike and I clearly haven't been reading the same material on scramjets. The impression I get is that they have rather modest acceleration, closer to other jet engines than to most rocket systems.

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Postby drmike » Wed Feb 20, 2008 6:34 pm

It has been a long time since I've read up on them. I think 10 to 20 g's was
the typical stuff back in the 70's.

Hmmmmm..... It could also be I'm remembering solid fuel boosters to get
the ram jets up to speed. Those things definitely give you a kick in the butt.


But it has been a long time too - I'm sure things have changed a lot.

93143
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Postby 93143 » Wed Feb 20, 2008 7:12 pm

Okay, so no one noticed...

Unless you want to use the aeroshell as a heat sink (you don't), you need to dump the hot hydrogen from the reactor cooling system somewhere, preferably into the engines. This is what Bussard called the ARC (All Regeneratively Cooled) design. Since you now have to dump tanked material overboard, your Isp goes down, but since the airflow in the engine dwarfs the hydrogen flow, you should still get very high thrust and Isp.

In addition, the hydrogen will burn, increasing thrust slightly and scavenging a bunch of the ozone Dr. B was worried about.

drmike wrote:You don't need that much fuel to get off the planet - you need a lot of fuel to punch thru the atmosphere.


My impression is that drag and gravity losses for a rocket are much smaller than orbital delta-V. SpaceShipOne didn't get anywhere near orbital velocity, and neither will SpaceShipTwo. For that matter, neither did Mercury-Redstone...

The problem with a scram jet is that the initial boost will squash a human into a ball of goo. Most people don't like that.


I don't like the idea of an engine that only works above Mach 4 either, hence the (turbo?)/ram/scram/rocket combined-cycle idea. Besides, you've got unlimited power onboard; boost time is limited only by cooling requirements. There's no need to rush...

Actually, I saw another idea for the X prize. They were going to use balloons to get the rocket above the atmosphere...


That was the Da Vinci project. I almost worked on it; I went to one meeting (free pizza, you see) but decided I was too busy that semester...

saddogmobile wrote:...you might want to check out Reaction Engines they designed both the Scimitar Engine and Saber Engines for use in an advanced aerospace-plane project.


I've been wondering for a while why no one's funding that thing. It's a lovely design, and the payload mass fraction is higher than that of the shuttle stack.

It's also one of the reasons I think a combined-cycle engine is feasible. Morphing ramp technology (used so far in only one multi-speed wind tunnel as far as I know) could give you ram/scram capability, and for rocket mode you just make like SABRE and shut the front... A morphing nozzle could give you optimum performance at any altitude.

I recently came up with a concept for an ion turbine engine that used the accelerating ionized air (and electron counterflow) as a magnet to run electric motors to turn a turbocompressor. It had full-spectrum capability (ground start, ramjet, rocket, VASIMR/DS-1), but to get the required current you had to step down the entire 6 GW of reactor power by a voltage ratio of 100:1 or more, resulting in stupidly heavy power handling equipment...

hanleyp wrote:One major difference between going up and reentry: for reentry you want the airframe to act as a drag device, preferably heavy on form drag and light on skin drag.


True. I just meant that the flight plan doesn't involve cruising at top speed at any given altitude, so the skin temperature doesn't get as high. It's still a tougher problem because of the required shape.

If a spaceplane capable of Mach 25 is too expensive, you can simply switch to rocket mode and leave early. You don't HAVE to do the whole boost in the atmosphere...

One factor I've heard in the X-30 concept was cooling the airframe with the fuel going to the engine. Without such a store of liquid fuel as a heat sink things get much harder.


See top of post; I think I've solved this. You have to dump hydrogen from the reactor cooling system into the engines anyway; a cooling loop for the hot areas of the skin would add weight, but it could help with the thermal protection problem.

Drmike and I clearly haven't been reading the same material on scramjets. The impression I get is that they have rather modest acceleration, closer to other jet engines than to most rocket systems.


Supersonic combustion ramjets have pathetic thrust per unit fuel flow. The airstream is so fast already that adding extra energy (which is tough at that speed) doesn't result in much momentum transfer. The high thrust he's talking about is probably the rocket start.

I would expect a QED scramjet to have a similar problem with thrust. However, since energy introduction is easier (no supersonic combustion to deal with), I would expect it to be mitigated somewhat.


I haven't done a design study (obviously), but it seems to me that the enormous Isp you get from airbreathing operation, even with coolant dumping, is something you'd want to take every possible advantage of. Dr. Bussard's spaceplane has a mass fraction to orbit not much better than Skylon, although it does have the advantage of having lofted a very high-power deep space drive into the bargain. Personally, I'd like to be able to start in LEO with the tanks still almost full; it makes for a more versatile spacecraft...

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Re: Airbreathing SSTO

Postby Roger » Wed Feb 20, 2008 7:24 pm

93143 wrote: you'd have to design the airframe to fly at all speeds between Mach 0.8 and Mach 25. Probably not impossible even with current technology,


I'm reminded of the X-15, circa early 1960's. 100,000 ft+ Mach 7, IIRC.

Was it 4 yrs ago that the X-43a dropped off a B-52 wing and hit mach 9.6 ?

The Mach 10 research vehicle separated from the booster and flew under its own power and preprogrammed control. It was separated from the booster rocket by two small pistons. Shortly after separation, its scramjet engine operated for about ten seconds obtaining large amounts of unique flight data for an airframe-integrated scramjet. The engine thrust was very close to its design value in each flight - sufficient to accelerate the vehicle during the Mach 7 flight and to allow the vehicle to cruise at constant velocity in the Mach 10 flight.
in each case, when the scramjet engine test was complete, the vehicle went into a high-speed maneuvering glide and collected nearly ten minutes of hypersonic aerodynamic data while flying to a mission completion point, hundreds of miles due west (450 miles at Mach 7, 850 miles at Mach 10) in the Naval Air Warfare Center Weapons Division Sea Range off the southern coast of California. Each vehicle splashed into the ocean, as planned, and was not recovered

NASA


Seems doable to me.
I like the p-B11 resonance peak at 50 KV acceleration. In2 years we'll know.

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Postby drmike » Thu Feb 21, 2008 2:12 pm

I got another idea for you to work on. Start out with your fuel tanks empty, and use the BFR to propel the atmosphere. As you climb, you compress more air than you use and fill the tanks, so that by the time you get to high altitude you have full tanks and can proceed to launch into orbit. Never need to carry fuel for the rockets, just propellent. And that way, you can pick up propellent on any planet with an atmosphere. So long as the tanks can handle sulphuric acid atmosphere :D

Another idea for skin heat is not mine - it is from an old aerospace engineer. He called it "the hairy airplane". The idea was to use really small fibers on the outside skin which would be long enough to flow over each other and short enough to quickly disapate heat. It does two things for you: 1) it breaks up the boundary layer so you have more turbulent flow (same as a golf ball, less friction) and 2) it has a changing surface so the heat is mixed around more material so the inner portions see less heat overall.

In his day it wasn't possible because there were no materials that could do the job. I bet we've got more choices now and nano-fibers of all kinds might do the trick.

Fun stuff to think about anyway!

93143
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Postby 93143 » Fri Feb 22, 2008 7:01 pm

Funny; I was thinking something similar for my ion turbine concept - my chosen propellant was liquid nitrogen. Ease of fueling, you see...

I wouldn't start out with empty tanks, though. If the refueling system failed in transit you'd be out of luck.

Since the propellant isn't hydrogen any more, the reactor would probably need closed-loop hydrogen cooling on a secondary refrigeration cycle with very hot radiators (1800 deg C gets you ~1 MW/m^2, so maybe a couple of square patches 80 feet on a side to cool one 6 GW core, or half that area if you boost to 2200 deg C). This will serve you well in space, where you may want to operate at high power; this arrangement means you've already got the radiators, and since they can handle hypersonic flight, you don't need to shut down the reactor and retract them if you want high-thrust rocket operation for some reason.

This is not an easy arrangement to design, and if you're using hydrogen as fuel it's probably not desirable. However, it's not intrinsically impossible, and it makes your spaceplane glow nicely when it's under power...

Also, the fueling system would have to be fairly versatile, since the atmospheres of different planets have different properties (which is also a reason not to try designing a reactor cooling system around the propellant) - LNG for Titan, high-pressure gaseous CO2 for Mars (it doesn't liquefy below 5.1 bar, and dry ice in the pumps is something you want to avoid), etc. My thoughts on Venus were essentially "well, no one will want to go there anyway", but I suppose you could rig it to withstand the acid if you really wanted to...

Naturally this sort of thing would not be the very first BFR-powered spacecraft you'd build...

As for the "hairy airplane", it's an interesting idea, and the tech may exist. I don't find it aesthetically appealing, but maybe that's just me... The golf ball effect isn't really an issue at hypersonic speed; the flow is basically turbulent instantly, and any structure small enough to exhibit laminar flow separation would probably be too weak structurally to stand the drag. At altitudes high enough for this to not be true anymore, drag is probably minimal, and the concept of a boundary layer starts to fall apart anyway...

On an unrelated note, I'm not sure I like the EM-sled launcher. I think Reaction Engines is on to something with their idea of making Skylon as much like an airplane as possible. An airbreathing SSTO similar to what I've described should be able to loft a complete boronated-water shield around the reactor, not just a shadow shield, making it safe for spaceports. I don't like the idea of using conventional turbojets, though. I'd like to power the ground-start drive with the reactor if at all possible; I suppose you could just use rocket mode to get up to speed, open the front for ramjet mode, and start replacing the propellant you just used...

...or you could use an electric turbojet. If you can safely bleed off enough reactor power at low voltage to run the cooling system without counterproductive inefficiencies, you can probably get enough to run a turbocompressor on an electric motor with superconducting magnets. The trouble with this idea is that I can't see a way to give it a scramjet mode, so it's limited to whatever speed you can safely hit in ramjet mode with LN2 injection cooling. In addition, it's probably heavier and more complicated...

One more issue: Why did Bussard use turbojets to get up to speed? Was it just to conserve propellant? Or does a QED drive generate enough bremsstrahlung to be dangerous to a launch crew?

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Postby 93143 » Sat Feb 23, 2008 12:05 am

Maybe I'm looking at this thing sideways. How hot can the hydrogen in the cooling loop possibly get? There's going to have to be a refrigeration loop in order to get the regeneratively-cooled propellant mass flow rate down to a reasonable level, isn't there?

Of course, gaseous CO2 probably doesn't cut quite as much mustard as liquid hydrogen in terms of acting as a heat sink, leading to a very large, heavy refrigeration unit if you design for ARC in the worst-case propellant scenario. So you will probably still need a radiator.

Is it worth it to try to do something similar to the GCNT design for high-thrust mode? Realizing that if we do this, it will be useless in deep space and we'll need to have special deep space radiators anyway? Probably not, considering the extra plumbing that would be required...

So we're back to a complex, expensive single-stage-to-anywhere spaceplane that glows when under power, can land and take off without servicing any number of times on any reasonably-sized body with an atmosphere and at least twice on any relatively small body without an atmosphere (such as, for instance, the moon), and which can even scoop fuel from an atmosphere without actually landing. (Stay away from Jupiter - 6 GW is not going to do it...)

Obviously the first thing to do is to try out an ARC/LH2 design instead, like Bussard's turbojet/QED rocket combo, or maybe even my combined-cycle all-QED scramjet-to-orbit idea...

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Postby drmike » Sat Feb 23, 2008 2:09 am

Once I get a BFR to work, turning it into a rocket engine will be the first thing I do!
:D

93143
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Postby 93143 » Mon Feb 25, 2008 1:19 am

Hmm... One of the problems is that the optimal shape for a QED rocket is a cylinder because of the magnetic shielding requirement, whereas the optimal shape for a ramscram is probably a movable three-sided box shape protruding from the airframe. It's really difficult to do structural morphing with a cylindrical wall; you'd need overlapping plates, which complicates things enormously.

Maybe the air-breather and the rocket should be separate engines. Or maybe it's worth it to try to build a cylindrical ramscram just to get the high-Isp rocket performance without having two engine systems...

Or maybe I should shut up and pay attention to the heat transfer problem I'm supposed to have solved for the meeting tomorrow...

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Postby drmike » Mon Feb 25, 2008 2:39 am

Yeah, this is fun, but reality has to come first.
:D

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Postby 93143 » Mon Feb 25, 2008 11:17 pm

Hmm... maybe I'm not as clear as I thought on the general configuration requirements for a scramjet. There's a poster on the wall that I walk past every day that shows a scramjet design with a shock spike.

It's not difficult to build a rocket into the back of a shock spike. This results in an ejector jet, which is significantly better for takeoff than the rocket would be by itself. Whether said rocket could be used as a main vacuum drive...? It might be big enough, and maybe the translating (and morphing?) forebody could be translated all the way to the back. Or maybe it doesn't need to be...

All right, I admit it. I haven't studied hypersonic flight OR rockets in enough depth to pretend any longer that I have any idea what I'm talking about. I'll respond to constructive replies, but I think I better get off my soapbox for now...

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Postby djolds1 » Tue Mar 04, 2008 7:15 am

93143 wrote:Supersonic combustion ramjets have pathetic thrust per unit fuel flow. The airstream is so fast already that adding extra energy (which is tough at that speed) doesn't result in much momentum transfer. The high thrust he's talking about is probably the rocket start.

I would expect a QED scramjet to have a similar problem with thrust. However, since energy introduction is easier (no supersonic combustion to deal with), I would expect it to be mitigated somewhat.


I haven't done a design study (obviously), but it seems to me that the enormous Isp you get from airbreathing operation, even with coolant dumping, is something you'd want to take every possible advantage of. Dr. Bussard's spaceplane has a mass fraction to orbit not much better than Skylon, although it does have the advantage of having lofted a very high-power deep space drive into the bargain. Personally, I'd like to be able to start in LEO with the tanks still almost full; it makes for a more versatile spacecraft...


IIRC, Dr. Bussard's first "airbreathing QED" concept was to raster scan a relativistic electron beam across the undercut/partial nozzle present on all hypersonic airframe designs. Superheat the air, which then acts as the reaction mass. No fuel necessary until switchover to rocket for final injection to orbit.

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