beam powered scramjet

[Idobox]

I've been thinking a lot about beam powered crafts, SSTOs and related stuff recently, and I just had an idea.

Part of the reason scramjets are so difficult to design is that they have to burn fuel. You need to inject fuel in a supersonic stream, mix it with air and burn it in the very short time it takes for air to go through the combustion chamber.

If you have several MW of microwave beamed power available, you could simply use an electric arc to heat the air, without trying to burn anything. That way, you don't create drag by injecting fuel, you don't need a long combustion chamber, and the whole thing is much more stable in regard to speed.

As a bonus, the antenna and power electronics can be used for a MPD thruster when outside the atmosphere.

Of course, it doesn't solve heating issues (actually, it makes them worse since you can't use fuel as coolant), or how to reach initial velocity (although it could probably work as a ramjet too), and although you get rid of the fuel (not such a big mass in a space plane anyway), you have to put a big antenna and power electronics, as well as a massive emitter on Earth.

A simpler alternative would be to build a thermal microwave scramjet. You get rid of the antenna and power electronics, put a big slab of silicon carbide on the underside of your ship that will turn microwave into heat. Force your supersonic airstream to get in contact with it, and it will heat. The same slab can also be used as heat shield for reentry. The main issue I see with that you could not use it for thermal protection while accelerating.

Obviously, designing a beam powered scramjet would be difficult and expensive, since neither scramjets nor beam powered are mature technologies, but if we get to build emitters powerful enough for beam powered thermal test rockets (we have emitters of comparable power in particle accelerators), we might as well try to use them for scramjets too.

Objections?

[Ralathon]

The main objection is that microwave beamed power doesn't work like it does in KSP interstellar at all. To collect microwave energy you need a receiver, the larger the more power you can leech. If you try to pump enough energy for a scramjet through a small receiver you'll just end up melting the hull and frying the electronics. But on the other hand, large receivers aren't very aerodynamically favorable in hypersonic airstreams.

[Idobox]

I used to design antennas, and I can tell you it's not that difficult to build a receiver antenna.

You can have up to several MW/m² and efficiency in excess of 90%, leaving several hundreds of kW of waste heat, difficult but manageable.

If you go for microwave thermal, it's even easier. Silicon carbide, for example, can be tuned to a specific resonance frequency and serve as a very efficient converter, as well as a very decent heatshield.

If you have a small craft, for example the size of the X43, you can get a few MWe. Not sure how much thrust you could get, but if you get even just a little more than drag, you will accelerate, and you can do it for hours since you don't depend on fuel.

[shynung]

Very decent heatshields are heavy, and will probably weigh down the entire craft significantly. Also, 'just a little more than drag' probably won't get you very far in the thick part of the atmosphere; it'd take ages to go to orbit. Most likely, it'd get too far from the emitter before it ever reaches to orbital speed.

Edited May 11, 2014 by shynung

[whynot]

Scramjets only work once you are moving at a few Mach, so you still need a separate propulsion system to get up to speed. But heck, magnetic rail launch, perhaps?

Then you have to deal with the fact that transmitting highly focused energy through the atmosphere will generate a lot of waste heat in the air, contributing to global warming as well as degrading the performance of your system. But as far as I know this can be overcome with 'MOAR LAZORZ'.

I think the real show-stopper is the thermodynamics of scramjet engines. The scramjet has a very low pressure ratio, which means that even though they may be fuel-efficient, they still produce very low thrust per unit of air that flows through them. This boils down to a need for a very large air intake which is a heavy and drag-producing structure, which must be made from exotic materials and probably also be actively cooled. If you somehow manage to wirelessly transfer megawatts of power to a launch vehicle, it is probably easier and more efficient to push all that power into some form of more conventional thermal rocket. Just my two cents.

[Idobox]

There are designs that work both as ram and scramjet. Also, if you don't have to inject and burn fuel, it's probably easier to extend operation speed of the engine.

The heating provided by a few GW for minutes is absolutely nothing, even without considering that most of this energy will not heat the atmosphere but just go through it. For comparison, 1GW is roughly what 1Km² of land receives in solar power at noon.

Thermal microwave focus on rockets rather than jets, so the specialists must agree with you. It still seems to me that accelerating gas to proper exhaust velocity is easier if it already moves fast, and that you save on weight if you don't have to carry it around.

From what I understand, the main performance issue with both ram and scramjets is that you need to slow the air flow down to have time to combust properly, and that induces heat, drag and poor pressure factors. But if you don't chemical combustion, I'm not sure you need to slow the flow down in a significant way, as a result you should be able to improve performance.

[whynot]

1. The combined ram- and scramjets I've seen all have moving ramps and intakes which are heavy and complex propositions, or inefficient compromises where only a portion of the internal flow goes supersonic. Maybe there are others? Like ablative combustion chambers? Also the need for heat resistant structure does not come from the combustion of fuels per se but from the need to rise the energy of the gas in order to produce thrust. If it ain't running hot, it really ain't running at all. (see #3:)

2. If you take that GW/km² and concentrate it on the cross-sectional area of your vehicle, the atmosphere will heat up considerably along the path of your beam and you will have transmission losses.

3. The ammount of thrust you get is really dependant on how much you accelerate the reaction mass, not the absolute magnitude of the exhaust velocity. So increasing the speed of the airflow from mach 5 to mach 25 is not the same thing as accelerating the same mass of propellant from 0 to mach 25. To accelerate the air through your engine you will need to raise it's pressure and temperature at the same time, raising the temperature alone will not produce any thrust because heat is just random motion. In order to extract thrust from this random motion you must direct as much of it as possible rearwards. That's what rocket nozzles do, and when a gas is accelerated like that it's temperature and pressure will fall. If the air flow was at atmospheric pressure to begin with, you will end up with a partial vacuum behind your engine and that's just gonna slow you down.

As I understand it the point of having a scramjet is that you can do without some of the oxidizer needed to get to orbit because you can hog some from the atmosphere. Taking the fuel out as well, really what you have is a lot of chemical potential energy (oxygen in the air) flowing through the engine that is just wasted. Cheers.