
External microwave propulsion technology for single-stage-to-orbit space flight [pdf] - curtis
http://escapedynamics.com/wp-content/uploads/2015/05/Escape-Dynamics-External-Propulsion-WP.pdf
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ChuckMcM
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.

~~~
nickfromseattle
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...](http://science.nasa.gov/science-news/science-at-
nasa/1999/prop16apr99_1/)

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

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

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ridgeguy
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...](https://en.wikipedia.org/wiki/Laser-
induced_breakdown_spectroscopy)).

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](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?

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

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stretchwithme
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...](http://www.technologyreview.com/view/420700/microwave-powered-
rocket-ascends-without-fuel/)

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Symmetry
_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.

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hliyan
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_

~~~
DrStalker
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...](https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile)

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.

