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Comment by hliyan | Hacker News Reader

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

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.

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.

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.

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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