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So, I've been reading a bit about thermal rocketry, from NTRs to beamed propulsion ideas. Now the basic concept is to squeeze more Isp out of traditional propellants such as liquid hydrogen. Specifically, if you heat H2 to around 2700K (depending on pressure), it breaks up into quantities of H1, which gives a big boost in exhaust velocity and thus Isp.

I'm sure I'm not the first person to think of this, but instead of elaborate schemes to beam energy to a moving craft, why not just use an energy source while on the ground to create heat, perhaps a dense plasma like in fusion experiments, and then use that to heat the propellant that will lift the craft into orbit? Basically plug the rocket in. You only need around ~10 minutes to get into orbit, so this system would only have to be viable for a short time. You could use reflective barriers to minimize radiative heat loses and channel them to the heat exchanger.

I guess this is more of a thought experiment than a serious proposal. What are the drawbacks of this sort of design?

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This is a helpful thought experiment that appears to be a natural question for one learning rocketry. Only, the speculation about the energy source seem to distract from the question. If you're talking about a literal electrical cord, then the power source is the grid, and that won't be limiting. We don't care what power plant produces the energy. Otherwise, we're going in circles about whether it's beamed power or not. –  AlanSE Feb 27 at 19:59
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After reading more, it looks like I might have not understood what's being asked either. From the title (in particular), I thought you were suggesting an electrical connection maintained as it ascends, which would be obviously limited to an abbreviated first stage. But from the comments it looks like you're interested in an exotic matter state as a means of thermal energy storage. I can't really say that's clear from the question. –  AlanSE Feb 27 at 20:07

3 Answers 3

The problem would be enormous expansion in volume and/or increase in pressure on the tank walls, so you can't simply preheat all the volume of the cryogenic propellants, you want to store them at high density but at still controllable pressure, then preheat them as they enter the expansion chamber to improve their exhaust velocity. Another way to think of this is also that the stored propellant density in relation to its density once expelled (thermal expansion) is stored energy that the rocket uses to propel itself. This is what we refer to as reaction mass. If you preheated all of it, even if you managed to somehow stably store it without adding much to rocket's weight, you'd be throwing a lot of this potential energy away. Either that, or the pressure on the tank walls would be too high and we're again back at the tank weight and volume problems.

If you only superheated some fraction of these propellants, to use that later on as your source of thermal energy to preheat the rest of it as it's fed into the engine, the problem becomes one of insulating this high temperature mass from the rest of your propellants to keep them stable. Which would again require more insulation, stronger structure to contain it, and needlessly add weight and complexity to the rocket. It would also have limited thermal potential, a bit like mixing hot water from the boiler with cold water, and you'd never be able to make use of all of the potential of thermal expansion. So it's simply easier and more economical to preheat propellants on demand and reuse as much of the stored thermochemical energy they provide for it as they burn in the combustion chamber.

For what it's worth, rockets actually are plugged into the wall with umbilicals until minutes before launch when they are switched to internal power. So they can use all the juice they require from the grid for their electrical power needs right up until the very last few minutes (usually around T-3 minutes) before flight, and so conserve their onboard power sources.

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To clarify, I wasn't thinking about preheating the propellant before launch, I was thinking more along the lines of your second example - but using a dense plasma as the heat source. It would be much hotter than the temperature that you want to heat the propellant to, tens of thousands of degrees most likely, and the radiative heat would then be distributed through a heat exchange layer. I don't know how feasible such a system is, though. –  I'ts Me Feb 27 at 18:12
    
As to the weight problem, the advantage of a thermal rocket is that you're not using LOX or anything else to mix with your propellant to create a chemical reaction, so there are major weight savings there. A heavy chamber used to contain reaction mass might still come out way ahead of having many tons of liquid oxygen on board, depending on the chamber's size, even considering that the LOX is progressively used up and doesn't contribute to the final mass. –  I'ts Me Feb 27 at 18:32
    
Yeah I have to admit I had to re-read your question and that's why I added that second example afterwards. The problem would still remain, you'd only have thermal expansion as your stored energy potential and part of it (the one you'd superheat) would actually have a negative potential. So you'd be losing a great deal of it, and still have to find ways to insulate masses of two distinct thermal characteristics. Nuclear thermal rocket would be a better option. Still a heavy generator block, but at least you're not losing energy potential of your reaction mass. –  TildalWave Feb 27 at 18:42
    
Could you expand on your comment a bit? I really don't know much of anything about thermodynamics. What does it mean to have negative energy potential? Is there a formula for calculating the speed at which two substances of differing heats will reach an equilibrium? –  I'ts Me Feb 27 at 18:50
    
The superheated part would already have its maximum potential energy per its storage volume of such a system before the mixing. It wouldn't expand more as it mixes with the cold part of your reaction mass, it would actually occupy less of its volume at the same pressure as it's cooled by the cold part of the reaction mass. That's that negative potential. And there's another problem that I didn't yet touch, that the superheated part's stored thermal potential would decrease as you start consuming it. So performance would also rapidly drop as the hot part cools down. –  TildalWave Feb 27 at 18:59

I'm not a physicist, and certainly can't do the exact maths. But let's have a look at the numbers involved.

Very (!!!) roughly, a first-stage rocket engine consumes over 1000 L / second of fuel for about five minutes. That's 300 tons of fuel, roundabout, that would need to be heated by about 2700 K.

You can get a plasma as hot as 10^7 K in the lab... but those are minute amounts, which means that mixing them with even as little as 1 L of LH2 won't give you more than a puff of H2. Realistically, you will have to settle for much lower heat.

If you allow for ten tons of "heat storage", even assuming 100% efficiency would require the "storage" to be heated to 81,000 K. (Picture it in your mind. Ten tons of something 25 times as hot as the boiling point of iron.)

Now imagine the engineering problems involved in getting it that hot in the first place, then keeping it insulated, and then transferring the heat to the fuel in the required amounts in the required time.

Then remember you don't get 100% efficiency, not with heat as an energy form. You either need yet more weight, or yet more K in the storage.

And that's using the most optimistic numbers, which I am sure any true scientist will wince at and point out that I overdid it by several orders of magnitude.

Heat as energy storage? No chance.

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Generically this is known as Beamed Power Propulsion.

Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that uses energy beamed to the spacecraft from a remote power plant to provide energy. Most designs are thermal rockets where the energy is provided by the beam, and is used to superheat propellant that then provides propulsion, although some obtain propulsion directly from light pressure acting on a light sail structure, and at low altitude heating air gives extra thrust.

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I'm thinking more along the lines of supplying all of the energy on the ground. At liftoff the rocket would have all of the energy it needed to reach orbit as heat that would be tranfered to the propellant, and no more energy would be given to the system. –  I'ts Me Feb 27 at 18:20
    
That seems like a problematic issue. Energy storage is 'hard'. –  geoffc Feb 27 at 19:13

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