External microwave propulsion technology for single-stage-to-orbit space flight [pdf] | Hacker News Reader

External microwave propulsion technology for single-stage-to-orbit space flight [pdf]

escapedynamics.com

51 pointscurtis12 comments

This is similar to an early proposal on the Skylon and other systems. Generally works much better in dense air, and it makes an interesting beam weapon when you aren't launching things with it. I read an earlier proposal which I can now not find which used the same sort of laser aggregation that the NIF was planning on using for fusion containment but as a field of smallish (10kW) lasers which could all be trained on the same target to input a bunch of heat which the target would use to convert a liquid gas into the necessary reaction mass to lift into orbit.

They all seem to peter out at 25 km or so. The goal with lasers was to pick a wavelength that the atmosphere was relatively transparent too but could be easily converted into heat through absorption on the receiving end. Microwaves would work to if you are careful about your wavelengths and don't point them at satellites.

I have an extended family member who has been working on this for 30 years. Much of it is self funded but at times he's worked with NASA and most recently the Brazilian air force. Similarly to Elon Musk his goal is to lower the cost to space.

[1] http://science.nasa.gov/science-news/science-at-nasa/1999/pr...

[2] https://youtu.be/KtH-SxqdtaA?t=1m

[3] http://www.space.com/7067-laser-propulsion-wild-idea-finally...

One consideration is formation of air plasma in the beam (dielectric breakdown) between the transmitting array and the vehicle. If that happens, the plasma will absorb power, blocking it from the launch vehicle. Kind of the microwave analog of LIBS (https://en.wikipedia.org/wiki/Laser-induced_breakdown_spectr...).

The breakdown field strength of gases decreases as pressure declines until a minimum of a few Torr (see Paschen's Law: https://en.wikipedia.org/wiki/Paschen%27s_law).

The power density in a beam focussed on a launch vehicle would be pretty high. Anybody know if air can sustain beam propagation at the E-field gradients that would be needed for this to work?

Will it kill everything in between microwave transmitter and target aircraft (birds, insects)?

Looks like there's a proof of concept, at least for something really light for a very short distance. The longest distance is 127 meters, according to this article:

http://www.technologyreview.com/view/420700/microwave-powere...

Escape Dynamics is also developing a next generation system, which relies on the direct heating of plasmas flowing through aresonant cavity with a weight similar to that of the weight of the heat exchanger in a thermal thruster. The goal of thisdevelopment is to allow operation with specific impulse above 1,500 seconds, leading to propellant mass fraction below 50%,which is comparable to airplanes.

Interesting. But since your power requirements are basically your specific impulse times your thrust that'll up the power they need. Still, electricity is cheaper than rockets.

How feasible is it to use compressed air instead of hydrogen as a working fluid? Would make the craft extremely light.

Edit: compressed air taken from the atmosphere in-flight

That would be very draggy. Given enough power it could work, but I suspect any mass savings would be eaten up by the drag. And since you're tied to substantial ground infrastructure anyhow, the work to source and compress the hydrogen prior to launch pretty much comes free.

That's an interesting idea, replace the burners in a scam jet with a electo-magnetic plasma doohickey. Basically an electric ram-jet.

Edit: My dad mentioned in school doing some scut work for a group designing an external combustion engine. Use the shock waves off the body to compress air, add fuel, zoom. Not sure if you could make that work with with an external heat source.

Side note: My understanding why people want to use hydrogen is it's a light particle, making it easy to accelerate it to high speeds. The higher exhaust velocity the better the ratio of payload to fuel becomes.

There is an engine design that uses a small unshielded nuclear reactor to heat atmospheric air as a working fluid in a RAM jet, giving several months of flight time before the fuel runs out: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...

There are some significant drawbacks to this approach (assuming you managed to overcome the technical limitation of making everything work at the temperature and radiation involved) such as massive radiation leakage, which is why this was intended to be part of a weapon system that could drop nuclear bombs and then just spend its time flying around your enemies cities irradiating them until they shoot it down or it runs out of fuel, causing yet another nuclear disaster to deal with as it crashes.

The successful test run shows the design can work, so an external microwave heat source could do the same if you could transfer enough power.

The specific impulse of the fuel is going to be inversely proportional to the square root of the molecular weight of the propellant, which is why they use hydrogen. You can theoretically use air from the atmosphere but the machinery to do that at the sort of speeds required to get into orbit is very heavy, very experimental, produces drag, and doesn't work when you start getting higher. It makes sense for Skylon because the oxygen in a hydrogen-oxygen rocket is most of the weight and the combustion temperature of that mixture is a lot higher than you can currently do with beamed microwaves.

It's a good idea but it has proven very difficult to implement in practice. The benefit of Escape Dynamics' (ours because I'm a Co-Founder) is that we keep the most complex parts of the system on the ground where servicing is cheap. Check out: https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

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