Battery powered thermal rockets, where are they?

[nhnifong]

Everyone loves to cite the NERVA nuclear thermal rocket engine as the next step in high ISP propulsion. But the idea of a thermal rocket that heats up hydrogen propellant does not require any nuclear power. Why couldn't the propellant just be heated with microwaves powered by batteries. The battery power could be collected when it was available from solar panels.

This form of thermal rocket, if it could achieve the same temperatures as a NERVA would be *slightly more efficient* if the solar panels and batteries were lighter than the nuclear fuel, and *would have a longer useful lifespan* than a NERVA that runs out of nuclear fuel before propellant.

As far as I know, no such rocket has ever been put in orbit. Why? is there some fatal flaw, or just an aversion to risk?

[sal_vager]

It may require too much power, which adds mass, take for example laser propulsion where the laser and power supply are kept on the ground while the propellant (if any) is heated in the rim of the special mirror.

If a suitable power supply were small and light enough you could carry it with your laser and mirror, and be able to produce thrust in any atmosphere or in space using the fuel on-board.

You need a lot of heat to expand your fuel, batteries are just not good enough yet.

[Thaniel]

How about a couple of NTR's? Sure, the irony in getting power from a thermal nuclear powersource to run a heatcoil, but, with batteries, and run in periods instead of continuously, it could utilize any gas or fluid as fuel, atleast for interplanetary travel.

[AlexL]

Energy density would seem to be a big problem to me.

To store the equivalent energy of one kilogram of uranium would need 95,000 TONNES of rechargable lithium batteries. (source)

For long burns you'd need a lot of extra mass for batteries, giving a far less efficient craft. Alternatively you may only be capable of very short burns with long recharge times, their shortness would probably not be able to reach optimum temperature for the heating element, which would again reduce the efficiency of your engine.

relevant xkcd

EDIT:Thaniel, the problem with your suggestion is that you're converting energy multiple times. Each transfer reduces the available energy due to inefficiencies in the process, and still needs the extra mass of batteries.

Edited August 17, 2013 by AlexL

[nhnifong]

Ahh, ok. The old battery problem again.

[Rockhem]

what about instead of dodging uranium we actually use it, but the outcry from people would be horrendous, i am surprised they were able to slip by with RTGs on Curiosity.

[Bunsen]

That's pretty similar to some other forms of electrothermal propulsion, a few of which have been flown. Arcjets heat a fluid with an electric discharge. Pulsed plasma thrusters operate similarly, but ablate a solid propellant block instead of zapping a liquid or gas. VASIMR does its heating with microwaves, but adds the magnetic confinement to allow higher propellant temperatures without cooking the chamber walls.

They work fine, though they generally use a storable liquid rather than hydrogen. That's because high I_sp necessarily implies a lot of kinetic energy in the exhaust, and that means a lot of electricity for a little thrust. It generally takes a long time to gather that electricity, so you need a working fluid that can hang around a while.

[PakledHostage]

what about instead of dodging uranium we actually use it, but the outcry from people would be horrendous, i am surprised they were able to slip by with RTGs on Curiosity.

RTGs have been used on many missions, such as the Voyager probes, Cassini, New Horizons, etc. There was no "slipping by" involved in lobbing radioactive material into orbit along with those missions.

The publicly available documentation for the Curiosity mission talked about the contingencies and range safety considerations that were the direct result of RTGs being aboard. For example, the mission was required to launch early in the morning so that there would be sufficient daylight remaining in Africa that they could be recovered before nightfall if the RTGs came down there. The launch trajectory was also restricted from passing over major population centres in Africa.

Edited August 18, 2013 by PakledHostage

[sal_vager]

Even using RTG's is moving away from nhnifong's original inquiry which was about thermal rockets that didn't rely on nuclear power.

Batteries as we currently know them are out as they just don't pack enough power into a small enough package, you need to generate power another way, or more accurately you need to generate heat, you don't need the electricity.

I expect it's not easy to generate the incredible temperatures you need without using a chemical reaction, I don't know how you'd do it with electrical heating elements, so you'd be looking at a chemical rocket and fuel.

I guess you could use a high temperature reaction, thermite style, to drive the expansion of a cheaper propellant but I have no idea if that'd be worth doing.

[lajoswinkler]

Chemical batteries will never ever surpass NERVA-style engines because of laws of physics. Energy contained in chemical bonds is orders of magnitude lower than the energy in strong nuclear bonds inside atomic nuclei. Electron clouds vs. nucleons = 0:1

It's simply unbeatable.

As for the solar panels, it's even worse. ~1 kWh/m^2 around Earth is the theoretical maximum, and unit efficiency can be only <1, currently around 0.2 on average, I think. As you go farther from the Sun, energy density drops so you have to resort to things like huge solar sails to literally push you away.

There's no other solution than working with nuclear bonds at this moment.

[Fuzzy Dunlop]

Why not just cut the solar pannels, the microwaves and the battery and just focus the sun onto the propellent with a big curved mirror. AKA a solar thermal rocket.

Edited August 18, 2013 by Fuzzy Dunlop

[Thaniel]

Heating thru microwaves instead of directly heating coils. Still need the energy in the first place though.