Why Thermal Combustion Rockets On Par With Chemical Or Solid Rocket Thrust Will Be Fiction For An Indefinite Time...

[Spacescifi]

Unless we figure it out that is.

Chemical and solid combustion are both chemical reactions that release lots of energy, the same kind of thing that powers automobiles. The main reason electric can even compete with gas cars is because they have a road to push off of, but a rocket has nothing but it's own propellant to push off of.

To me the requirements for a thermal combustion rocket that uses heat rather than chemical reactions to burn the propellant hot enough to give equivalent thrust os not impossible.... just wildly inefficient.

Since chemical reactions give more energy per pound than the massive batteries that would be required to give the same energy for thrust.

To do that we would need batteries that are compact and not massive that can discharge chemical level power discharges of energy.

If we could do that then we could actually have thermal combustion rockets that could compete with chemical and solid rockets.

Question: Is it not true that even if we had compact super batteries, the discharge of electrical energy needed to combust propellant on par with a chemical or solud rocket would cause X-rays?

It's a guess, since I read somewhere online that in Back to the Future the Time Machine car is really a death mobile, since the sheer number of watts it is putting out would give off X-rays and kill the passengers.

Now if this is true for a thermal electric combustion rocket on par with chemical or solid rockets for thrust, the obvious solution would be to use reflectors to reflect and concentrate the X-rays into the propellant exhaust to heat it for extra thrust.

But nothing is free..

The mirrors can be made of glass, ceramic, or metal foil, coated by a reflective layer. The most commonly used reflective materials for X-ray mirrors are gold and iridium. Even with these the critical reflection angle is energy dependent.

Mirrors add weight, and that decreases overall thrust because of extra mass.

So I am not saying thermal electric combustion engines will never rival chemical or solid rockets for thrust.

What I am saying is that some fusion rocket or some NSWR is actually easier to pull off than a pure electric thermal combustion rocket, since we do not have to make a super dense energy storage medium, we are simply unlocking it and directing it out the back with fusion.

[Shpaget]

Combustion is necessarily a chemical reaction, so I don't see how electricity play into your proposal. Are you proposing an electrical heater to heat a some inert fluid and achieve thrust that way, without a chemical reaction? Something line NERVA, but electrical? But in that case, there is no combustion.

In cars, planes, ships, etc. we use chemical reactions instead of batteries because they are much more energy dense. Nuclear fission is even more dense than energy you can get out of chemical reactions.

3 hours ago, Spacescifi said:

Question: Is it not true that even if we had compact super batteries, the discharge of electrical energy needed to combust propellant on par with a chemical or solud rocket would cause X-rays?

It's a guess, since I read somewhere online that in Back to the Future the Time Machine car is really a death mobile, since the sheer number of watts it is putting out would give off X-rays and kill the passengers.

First time I hear this. Production of X rays is tied to the method of energy release, not magnitude. A simple forest fire releases a tremendous amount of energy, but no X rays. As for DeLorean, it will produce as much X rays as the plot requires.

[mikegarrison]

Agreed. "Combustion" is a chemical reaction.

[sh1pman]

What you described already exists, and it’s called electric propulsion. Good news: it doesn’t produce X-rays. 

[JoeSchmuckatelli]

I am kinda with the OP here - I'd like to see some kind of peer reviewed paper suggesting an alternate to just combustion lift on the horizon. 

I thought I might have been onto something with my idea of mitigating (slowing down) a fission reaction - to which folks were kind enough to point out that atomic reactions are not conceptually similar to chemical reactions.  (The closest thing to what I was hoping for is some variation of the nuclear salt water rocket.) 

It's really hard to turn handwavium into practical application! 

 

[farmerben]

It depends on how battery power is supplied.  If we use a grid based heavy ion thruster it creates no x-rays.  A cyclotron to accelerate alpha particles to .9c does create x-rays.  As has been said before, x-rays can be reflected or shielded.   

[Spacescifi]

15 minutes ago, farmerben said:

It depends on how battery power is supplied.  If we use a grid based heavy ion thruster it creates no x-rays.  A cyclotron to accelerate alpha particles to .9c does create x-rays.  As has been said before, x-rays can be reflected or shielded.   

So you are saying with a super battery you could make an ion thruster as thrusty as a chemical or solid rocket? With the same inefficiency? Meaning you would still burn through your propellant in minutes at good thrust.

[farmerben]

I said no such thing.  

[kerbiloid]

About the X-rays.

They probably mean that the energy input would be so high, that the car would be heated up to the temperature when it emits X-rays, like a nuke does.

[Nuke]

i still think we are better off sticking to chemical propulsion when dealing with earth-leo operations. for long term sustainability lh2/lox are optimal fuel mix, they get the job done and have fairly good isp (for chemical) and a sustainable fuel cycle (provided the electricity supply is also sustainable). it will always be less dangerous (keep the reactor on the ground) than any exotic propulsion means for the application of boosting payloads surface to orbit. once in space start relying on solar arrays and plasma drives for the inner solar system, with nuclear reactors being developed when we transition to the outer system. by that time we should have shipyards on the moon and can launch from there (and with a hot drive). but you are still going to need a way to get passengers and cargo to and from the earth's surface in order to supplement these activities. 

[Spacescifi]

6 hours ago, Nuke said:

i still think we are better off sticking to chemical propulsion when dealing with earth-leo operations. for long term sustainability lh2/lox are optimal fuel mix, they get the job done and have fairly good isp (for chemical) and a sustainable fuel cycle (provided the electricity supply is also sustainable). it will always be less dangerous (keep the reactor on the ground) than any exotic propulsion means for the application of boosting payloads surface to orbit. once in space start relying on solar arrays and plasma drives for the inner solar system, with nuclear reactors being developed when we transition to the outer system. by that time we should have shipyards on the moon and can launch from there (and with a hot drive). but you are still going to need a way to get passengers and cargo to and from the earth's surface in order to supplement these activities. 

 

For superheavy SSTO launch it is theoretically possible to use spin launch on external pulse propulsion SSTOs with pusher plates at the rear, dropping advanced pure fusion bombs for propulsion that can be scaled up or down in yield.

 

A lighter version of the craft could even SSTO on it's own off an Earth world, at the expense of fusion bombing the launch site.

 

[magnemoe]

1 hour ago, Spacescifi said:

For superheavy SSTO launch it is theoretically possible to use spin launch on external pulse propulsion SSTOs with pusher plates at the rear, dropping advanced pure fusion bombs for propulsion that can be scaled up or down in yield.

A lighter version of the craft could even SSTO on it's own off an Earth world, at the expense of fusion bombing the launch site.

I would use an good fusion engine, in atmosphere you heat air to generate trust, higher up you use reaction mass, in space you simply use the reaction waste who will go relativistic as your only reaction mass.  
Probably an spaceplane as your TWR will still be pretty limited unless you kill you ISP. 
Only problem is that most fusion engine designs are much more suited for space as they are an framework of magnets to better get rid of radiation and neutrons. 

[sevenperforce]

On 9/10/2023 at 12:41 PM, Spacescifi said:

So you are saying with a super battery you could make an ion thruster as thrusty

That's not remotely what @farmerben said, but it should be noted that batteries themselves are powered by (reversible) chemical reactions.

Using a chemical reaction in a battery to generate electrical current to heat a propellant to produce thrust makes no sense. Just use the chemical reaction to heat up the chemicals themselves and shove them out the end AS the propellant. 

On 9/10/2023 at 12:41 PM, Spacescifi said:

as a chemical or solid rocket?

SOLID ROCKETS ARE CHEMICAL ROCKETS

On 9/10/2023 at 12:41 PM, Spacescifi said:

With the same inefficiency? Meaning you would still burn through your propellant in minutes at good thrust.

Inefficiency is NOT a factor of burn time.

A single Raptor engine burning all the propellant in a Starship would take 31 minutes to complete the burn, while a DEC Centaur will take 453 seconds to complete its full burn, even though the Centaur is more propellant-efficient.

[farmerben]

Batteries are heavy.  You want something light such as a fission power plant   If you use that power thermally you get about 800 ISP and good thrust for orbital transfers.  Ion propulsion is good for 2000-3000 s of ISP but low thrust.  You would need even greater ISP than that for an interstellar mission which is why I think a cyclotron hurling helium ions at .9c is a promising technology.

[KerikBalm]

On 9/10/2023 at 5:46 PM, Shpaget said:

.Are you proposing an electrical heater to heat a some inert fluid and achieve thrust that way, without a chemical reaction? Something line NERVA, but electrical? But in that case, there is no combustion.

Ignoring his lack of understanding of what combustion is - you are, and I guess he is, describing a resistojet rocket

https://en.m.wikipedia.org/wiki/Resistojet_rocket

They are pretty bad, but better than cold gas thrusters.

I wouldn't use them for more than RCS - or maybe station keeping on a small satellite 

[magnemoe]

On 9/18/2023 at 8:58 PM, farmerben said:

Batteries are heavy.  You want something light such as a fission power plant   If you use that power thermally you get about 800 ISP and good thrust for orbital transfers.  Ion propulsion is good for 2000-3000 s of ISP but low thrust.  You would need even greater ISP than that for an interstellar mission which is why I think a cyclotron hurling helium ions at .9c is a promising technology.

The better batteries get the more they equal high explosives, we are getting into the black powder level. Making fast helium you want an good fusion engine any other way and you create more waste than your trust so your real isp is lower. 
Or you could use laser pumped solar sails and if crazy use anti matter for braking as in the Avatar movies. However that would put you into the Kardashev 1 bracket. 
Its very obvious you can only use an fraction of that down at earth  so its not an lack of power.