Alternative Methods Of Heating Hydrogen For High Efficiency Rockets

[Skye]

Are there other ways you could heat hydrogen to 2-3000 K for use in a high-efficiency rocket without the use of a nuclear reactor? The idea here is to have a rocket as efficient as an NTR, but having it be able to be used in the atmosphere, being able to land and be reused, and to be not deadly radioactive. I think that having it be able to do these things could make it worth using (unlike NTRs, which are kinda useless, just refuel). They could potentially be used on first stages with Methalox side boosters? (Picture SLS but replace solids with Methalox and replace Hydrolox with LH2 and these rockets (HVRs? - Hydrogen Vapour Rockets?) and also on second stages, and large kick stages, potentially.

My dad jokingly suggested filling a tank with hundreds of gerbils, soaking them in liquid hydrogen, igniting them, and then using the heat to warm the hydrogen! 

[Jim]

Are there other ways you could heat hydrogen to 2-3000 K for use in a high-efficiency rocket

yes, just burn it.  You aren't going to find a better solution.

[Skye]

Are there other ways you could heat hydrogen to 2-3000 K for use in a high-efficiency rocket

yes, just burn it.  You aren't going to find a better solution.

I choose to remain hopeful, because burning won’t leave pure H2 as exhaust 

[Proponent]

Solar-thermal propulsion with hydrogen as a working fluid was proposed long ago for in-space propulsion, but it does not seem to pencil out very well for any particular application. For one thing, the thrust-to-weight ratio is terrible, and keeping hydrogen liquid for long periods isn't fun either.

https://forum.nasaspaceflight.com/index.php?topic=52349.msg2156478#msg2156478
https://forum.nasaspaceflight.com/index.php?topic=16081.msg370859#msg370859
https://forum.nasaspaceflight.com/index.php?topic=55131.msg2305917#msg2305917

One amusing idea is to use LiH as a propellant. It's a solid at room temperature but dissociates at a a few hundred degrees Celsius, IIRC. Maybe you could dissociate it into Li and H₂ (gaining some energy from H + H -> H₂) and heat the gas. Li is not quite as light as H₂, but it's still lighter than typical chemical exhaust products, and it has the advantage of being monatomic. Maybe it would even be worthwhile refining the Li to the fairly abundant ⁶Li isotope. Somewhere there is a thread about this, in connection with a peri-solar pass for an outer-solar-system mission, IIRC. This all good fun, but again, it doubt it works out well when you look into the details.

[edzieba]

I would recommend looking through Atomic Rockets' Engine List. There are a lot of propulsion architecture both studied and proposed already, and not much reason to retread the same ground without something new to add.

[Paul451]

There are proposals to use ground-based lasers to heat the propellant in a launch vehicle. Presumably something like that is the only non-nuclear answer to your suggestion. But the practicalities of both generating sufficient heat density onto the vehicle via the laser and efficiently moving that heat to the propellant while retaining that heat density seems like a show-stopper to me.

In practice, you don't need especially high Isp for the first stage, and certainly not for boosters, you need high thrust-density propellants. And chemical fuels are already nearly perfect. It's why solid propellant boosters are so useful, even though their Isp is garbage.

(For upper stages, higher Isp helps, especially for BLEO. Likewise for interorbital manoeuvres. But that wasn't your question.)

[Skye]

There are proposals to use ground-based lasers to heat the propellant in a launch vehicle. Presumably something like that is the only non-nuclear answer to your suggestion. But the practicalities of both generating sufficient heat density onto the vehicle via the laser and efficiently moving that heat to the propellant while retaining that heat density seems like a show-stopper to me.

In practice, you don't need especially high Isp for the first stage, and certainly not for boosters, you need high thrust-density propellants. And chemical fuels are already nearly perfect. It's why solid propellant boosters are so useful, even though their Isp is garbage.

(For upper stages, higher Isp helps, especially for BLEO. Likewise for interorbital manoeuvres. But that wasn't your question.)

What about using mirrors to reflect the sun?

[edzieba]

There are proposals to use ground-based lasers to heat the propellant in a launch vehicle. Presumably something like that is the only non-nuclear answer to your suggestion. But the practicalities of both generating sufficient heat density onto the vehicle via the laser and efficiently moving that heat to the propellant while retaining that heat density seems like a show-stopper to me.

In practice, you don't need especially high Isp for the first stage, and certainly not for boosters, you need high thrust-density propellants. And chemical fuels are already nearly perfect. It's why solid propellant boosters are so useful, even though their Isp is garbage.

(For upper stages, higher Isp helps, especially for BLEO. Likewise for interorbital manoeuvres. But that wasn't your question.)

What about using mirrors to reflect the sun?

Solar Thermal and Laser thermal are known propulsion architectures.

[Jim]

There are proposals to use ground-based lasers to heat the propellant in a launch vehicle. Presumably something like that is the only non-nuclear answer to your suggestion. But the practicalities of both generating sufficient heat density onto the vehicle via the laser and efficiently moving that heat to the propellant while retaining that heat density seems like a show-stopper to me.

In practice, you don't need especially high Isp for the first stage, and certainly not for boosters, you need high thrust-density propellants. And chemical fuels are already nearly perfect. It's why solid propellant boosters are so useful, even though their Isp is garbage.

(For upper stages, higher Isp helps, especially for BLEO. Likewise for interorbital manoeuvres. But that wasn't your question.)

What about using mirrors to reflect the sun?

not at outer planets.

[Jim]

Are there other ways you could heat hydrogen to 2-3000 K for use in a high-efficiency rocket

yes, just burn it.  You aren't going to find a better solution.

I choose to remain hopeful, because burning won’t leave pure H2 as exhaust 

false hope
 there is nothing new under the sun when comes to chemical and thermal propulsion. 

[ShadowAndFlame]

Keep a little anti-hydrogen onboard and feed slowly into the chamber?

Please ignore the glaring issue of containment.

I would recommend looking through Atomic Rockets' Engine List. There are a lot of propulsion architecture both studied and proposed already, and not much reason to retread the same ground without something new to add.

Oh look it's on the list! https://projectrho.com/public_html/rocket/enginelist3.php#amplasma

[Vultur]

Yeah, many of these things have been considered.

But for first stages hydrogen is not a great idea (yeah, SLS, but it's kind of a Frankenstein design). Even if you didn't care about radiation, NTR first stages would be bad (even with an Isp of 800 or so) because the dry mass is so huge.

Laser launch might work since the "engine" mass has basically been offloaded to the ground, but that's a ton of development and cost needed. You can't really start with a "Falcon 1" for that.

[Skye]

What about using mirrors to reflect the sun?

not at outer planets.
[/quote]

You’re right. Damnit!

[Twark_Main]

For solar thermal, see also the TransAstra Omnivore thruster and architecture.

[lamontagne]

Are there other ways you could heat hydrogen to 2-3000 K for use in a high-efficiency rocket without the use of a nuclear reactor? The idea here is to have a rocket as efficient as an NTR, but having it be able to be used in the atmosphere, being able to land and be reused, and to be not deadly radioactive. I think that having it be able to do these things could make it worth using (unlike NTRs, which are kinda useless, just refuel). They could potentially be used on first stages with Methalox side boosters? (Picture SLS but replace solids with Methalox and replace Hydrolox with LH2 and these rockets (HVRs? - Hydrogen Vapour Rockets?) and also on second stages, and large kick stages, potentially.

My dad jokingly suggested filling a tank with hundreds of gerbils, soaking them in liquid hydrogen, igniting them, and then using the heat to warm the hydrogen! 

Have you looked at the lunar oxygen rocket, LANTR?  It uses hot hydrogen from a nuclear reactor to react with oxygen to get higher ISP.  Same result, inverse process. 

[tenkendojo]

Are there other ways you could heat hydrogen to 2-3000 K for use in a high-efficiency rocket without the use of a nuclear reactor? The idea here is to have a rocket as efficient as an NTR, but having it be able to be used in the atmosphere, being able to land and be reused, and to be not deadly radioactive. I think that having it be able to do these things could make it worth using (unlike NTRs, which are kinda useless, just refuel). They could potentially be used on first stages with Methalox side boosters? (Picture SLS but replace solids with Methalox and replace Hydrolox with LH2 and these rockets (HVRs? - Hydrogen Vapour Rockets?) and also on second stages, and large kick stages, potentially.

My dad jokingly suggested filling a tank with hundreds of gerbils, soaking them in liquid hydrogen, igniting them, and then using the heat to warm the hydrogen! 

Yes. There is a paper from a few years back about using novel conventional explosives to trigger pure fusion pulses for rocket propulsion, the fuel would be lithium deuteride of course.

If you want to get more speculative, I guess theoretically you can use Antihydrogen as antimatter fuel, just 1 gram should get you  43 kilotons of output, as close to E = MC^2 efficiency as you can get. That said, we are only able to produce trace amount of antihydrogen with those huge particles colliders right now.

[edzieba]

In between fusion-heats-remass and antimatter-photon rockets would be 'fusion fragment' rockets, where you try and corral the high energy fusion products into an exhaust stream directly without needing to thermalise them and heat additional reaction mass. May be viable for charged fusion products (e.g. p-B and its trip of emitted Alphas), not much good for Neutrons or gamma emission.

[Paul451]

My dad jokingly suggested filling a tank with hundreds of gerbils, soaking them in liquid hydrogen, igniting them, and then using the heat to warm the hydrogen!

Gerbils are a complex hydrocarbon, they won't burn in a reducing atmosphere like hydrogen. You still need an oxidiser.

[Skye]

You could probably shoot them out through the nozzle, spraying them with LOX & rapidly disintegrating them on the way 

Not that it”d be highly efficient - or efficient at all, really

[lamontagne]

You could probably shoot them out through the nozzle, spraying them with LOX & rapidly disintegrating them on the way 

Not that it”d be highly efficient - or efficient at all, really

The Gerbils are, after all, mostly water.  So masswise the combustion would be inefficient.  It's something of an inefficient variation of the LANTR.