Molten Metal as Liquid Rocket Fuel.... Has Anyone Tried It And Possible Uses

[Spacescifi]

I was thinking of a dense fuel that would not require massive storage tanks like LH and it dawned on me that molten metals could work.

Advantages: If you can manage a chemical reaction with some other element then you get smaller storage tanks making your vessel lighter and overall increasing thrust.

 

Disadvantages: Molten metal is hot and in space you may need radiator fins... hopefully not as heavy the massive tanks you would have had if you used LH instead.

 

[kerbiloid]

Aluminium fuel rockets. KSPI-E has this.

Also alcali metals powders have been tried as an admixture to the hypergolics.
Legends say, the results  weren't good, but who knows what actually was there.

Edited November 12, 2022 by kerbiloid

[Nazalassa]

And when you don't have enough fuel, you can fuel your engine with your empty stages. Very interesting.

[Kerbart]

As FUEL? What would be the oxidator? And whatever that is, how do you prevent it from reacting with the rest of the machinery and plumbing?  As propellant maybe though, for some kind of nuclear engine.

[JadeOfMaar]

2 hours ago, Spacescifi said:

Advantages: If you can manage a chemical reaction with some other element then you get smaller storage tanks making your vessel lighter and overall increasing thrust.

Umm. No. That's not how the universe works. If you have to super-heat a metal to use it as fuel then you lost the fight before it started:

Spoiler

1. You're going to waste your ship's mass fraction with the systems needed to heat (or keep heated) the metal. You're better off using normal RCS for main engines.
2. You want to generate heat quickly. You want to lose heat quickly. You do not want to hold onto sheer volumes of it, otherwise the ship will unavoidably become a furnace.
3. You will have a huge tank of hot substance because in the real world, anything that's dense will suck for exhaust velocity, and anything that relies on chemical combustion will suck for exhaust velocity.
4. The oxidizer is usually a cryo-substance. You do not want to have a huge tank of super hot fuel and super cold oxidizer on the same ship. This is sacrilage to proper engineering practice.
5. Your combustion process becomes endothermic. You do not want this. If you're going to give energy to your propellant then the engine type must be anything but chemical. (See: electro-dynamic (ions), magneto-dynamic (mass drivers), solar thermal, or nuclear thermal.)
6. There are already several very nice (and very nasty) fuel mixes and in many of them the fuel is kept at room temperature. Where the fuel is metal it's always an alkali metal, the lightest and most reactive metal in whatever row in the Periodic Table. Every other metal is the opposite.

If you want to use molten metal in your rocket then you're looking at liquid core nuclear. Forget about having a lightweight or compact ship. Forget about "oxidizer."

Edited November 12, 2022 by JadeOfMaar

[kerbiloid]

1 hour ago, Kerbart said:

As FUEL? What would be the oxidator? And whatever that is, how do you prevent it from reacting with the rest of the machinery and plumbing?  As propellant maybe though, for some kind of nuclear engine.

https://en.wikipedia.org/wiki/ALICE_(propellant)
or
LOx

Actually, aluminium powder IS the Shuttle/SLS SRB main fuel, but together with solid oxidizer.

P.S.
Several (five? six?) years ago here was a hot discussion about the Al+LOx pair as a perspective ISRU fuel  for the Moon.

(I even imagined whole mountain ridges of alumina along the rocket paths.)

[sgt_flyer]

It was tested once by rocketdyne within a tripropellant rocket engine, with an ISP of 542 (record with chemical engine)

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

they went with gaseous H2, Fluorine and liquid lithium.

outside of the absolute madness of that combo :

Lithium : in case of a leak, the lithium at these temps would end up catching fire by itself and be very hard to extinguish

Liquid Fluorine : 'nuff said

Afterwards you need to store those fuels in a rocket without thermal transfers between your h2 and your liquid lithium (180°c) . (Imagine between liquid hydrogen and liquid lithium

naturally liquid metal at ambient temperature you likely don't want to handle that stuff (mercury).

Edited November 12, 2022 by sgt_flyer

[darthgently]

Does the metal exhaust eventually cool and solidify in orbit?  Depending on size of solids that could be an interesting Kessler issue.  I imagine it would be on the scale of molecules but don't know

[kerbiloid]

It's molecular, and ~3 km/s fast. So, it's a quickly expanding gas (and probably microdust) cloud, retrogradely decelerated down to 5..6 km/s, so falling back to the Earth.
 

P.S.
Interesting fact. If get burned enough much aluminium-fueled rockets, the exhaust self-gravity can form an aluminium planetary system.

[Nuke]

there is feep which is a liquid metal engine, supposidly its in use on some telescopes for its precision thrust capabilities and high isp, it is however terrible in the thrust department, so it usually gets relegated to fine attitude control. 

Edited November 12, 2022 by Nuke

[JoeSchmuckatelli]

Honestly, this line of thought does not seem that far off from my 'can we get a burn rather than a bang from nuclear fission' thread - the purpose being to harness great energy from dense material.  As explained to me, nuclear reactions work vastly different from chemical - but I still respect the asking about unique ways to get propulsion and perhaps break our current reliance on the chemical fuels currently in use. 

[insert_name]

Maybe an open cycle nuclear rocket, that way the metal heats itself

https://en.wikipedia.org/wiki/Gas_core_reactor_rocket

[wumpus]

And once again we are back at the rocket equation.  But oddly enough, the rocket equation that KSP teaches you won't help.  Often the critical variable used is v_e (exhaust velocity) instead of Isp (specific impulse) as used in KSP.  The key issue of exhaust velocity is you want the lightest metals possible.  So Al works (standard SRB fuel), you might even get Na to work as well (nasty rocket fuel).  But something like iron or even mercury is right out.

[JadeOfMaar]

8 hours ago, darthgently said:

Does the metal exhaust eventually cool and solidify in orbit?  Depending on size of solids that could be an interesting Kessler issue.  I imagine it would be on the scale of molecules but don't know

8 hours ago, kerbiloid said:

It's molecular, and ~3 km/s fast. So, it's a quickly expanding gas (and probably microdust) cloud, retrogradely decelerated down to 5..6 km/s, so falling back to the Earth.
 

P.S.
Interesting fact. If get burned enough much aluminium-fueled rockets, the exhaust self-gravity can form an aluminium planetary system.

Tiny metal particles should (I suppose) cool faster than large particles so it's possible. It's more likely to cause Kessler around the Moon (assuming they wouldn't deorbit if released there. idk Moon orbit velocities), given kerbiloid's mention of the particles being deorbited do to retrograde vector. In any case, the more immediate threat is that the particles may have enough mass to perforate whatever they hit.

 

9 hours ago, kerbiloid said:

https://en.wikipedia.org/wiki/ALICE_(propellant)
or
LOx

Actually, aluminium powder IS the Shuttle/SLS SRB main fuel, but together with solid oxidizer.

P.S.
Several (five? six?) years ago here was a hot discussion about the Al+LOx pair as a perspective ISRU fuel  for the Moon.

(I even imagined whole mountain ridges of alumina along the rocket paths.)

CRP provides Alumina in-game, and I've taken a swing at configuring parts to take advantage. But I've only known of the hybrid solid style of fueling: Aluminium cake  with LOX pumped over it. TIL that "Al-ICE" is a thing.

6 hours ago, JoeSchmuckatelli said:

Honestly, this line of thought does not seem that far off from my 'can we get a burn rather than a bang from nuclear fission' thread - the purpose being to harness great energy from dense material.  As explained to me, nuclear reactions work vastly different from chemical - but I still respect the asking about unique ways to get propulsion and perhaps break our current reliance on the chemical fuels currently in use. 

Yeah, nuclear is a world apart from chemical. Breaking our reliance on chem rockets requires solutions that are tied to a can (a very big can) of worms.

6 hours ago, insert_name said:

Maybe an open cycle nuclear rocket, that way the metal heats itself

https://en.wikipedia.org/wiki/Gas_core_reactor_rocket

Heh. In my spoiler (6 reasons why molten metal is bad for a checm rocket fuel) I suggested liquid core and specifically not gas core because the metal isn't molten anymore here -- it's in the gas phase.

[KSK]

This seems like an opportune moment to recommend ‘Ignition’ by John Clark.

For those that don’t already know, it’s an informal history of rocket propellants written (mostly) for a lay audience, and covers a pretty good range (I don’t know enough to say ‘all’) of the weird, wonderful and outright what propellant combinations that have been tried.

See, for example, his sage words on chlorine trifluoride as quoted in this article.

https://www.science.org/content/blog-post/sand-won-t-save-you-time

 

[Nuke]

i think chemical engines will be the go to for earth to orbit fort some time.  using hydro-lox is 100% renewable. and fi you are using nuclear power on the ground to generate the fuel from water. it will always be cleaner and less dangerous and more efficient than flying the reactor on the rocket.

reactor as payload is another matter.  those reactors can be fueled on orbit. launching fuel rods is a lot safer than launching a live reactor. we launch rtgs all the time with very little hubbub.

Edited November 13, 2022 by Nuke

[Spacescifi]

How about doping metal beads into liquid propellant for an extra kick?

Something very reactive with LOX?

I am throwing stuff against the wall here to see what sticks.

 

As far as I can tell the extra mass shot out the back would deliver extra thrust.

 

Or we could just go crazy and put enriched uranium beads in liquid methane and light it up with LOX.

Probably blow up the rocket lol.

 

I am not sure if using a reactor and liquid propellant doped with metal beads is a good idea... but who knows?

Edited November 14, 2022 by Spacescifi

[kerbiloid]

dense = high atomic number = high atomic mass  = low heat speed = low ISP

[Spacescifi]

3 minutes ago, kerbiloid said:

dense = high atomic number = high atomic mass  = low heat speed = low ISP

 

But more thrust right? That is the trade off?

[kerbiloid]

Thrust = ISP * consumption rate

Power = force * speed

Energy per time = thrust * ISP * g

No difference for the thrust. Just very non-dense fuels need bigger and thus heavier nozzles and are affected from the air pressure, so their thrust is limited.

The thrust can be made a little (2 times or so) higher, but this doesn't solve any problem.

[KSK]

Doping propellant with metal beads sounds like one of those ideas that might be useful in some circumstances but any benefit obtained is probably going to be outweighed by the difficulty of getting the engine to work.

My understanding is that the metal will increase thrust at the expense of ISP.  For chemical propulsion that’s not a great trade-off - chemical rockets usually have pretty good thrust already but relatively low ISP.

This might be useful for ‘changing gear’ on a high ISP / low thrust engine (such as a fusion rocket) where a temporary boost to thrust might be situationally useful and the reduction in ISP is acceptable.

However,  I can think of a number of drawbacks to metal doped propellant. In no particular order:

Keeping the propellant homogenous and avoiding having the beads settle out on the bottom of the propellant tank or any internal tank structures (slosh baffles for example)

Wear and tear on the turbopumps. Those things are spinning at ungodly speeds and shot blasting them with metal beads sounds like a quick way to a broken engine.

Beads clogging the injector plate and being a general pain to move through the engine plumbing at any sort of speed.

Unwanted reactions between propellant and beads although that will be highly dependant on which propellant and which metal you choose.

Crudding up the engine with metal deposits.

There are probably more issues that I haven’t thought of, not being an actual rocket engineer (or indeed any sort of engineer!)

 

 

[Kerbart]

On 11/12/2022 at 12:48 PM, kerbiloid said:

Actually, aluminium powder IS the Shuttle/SLS SRB main fuel, but together with solid oxidizer.

Yes, powder, but we're talking about molten metal here.

[wumpus]

3 hours ago, Kerbart said:

Yes, powder, but we're talking about molten metal here.

Chemically, there's typically some matrix holding the whole thing together which you can now lose.

Mechanically, do you really want to pump molten aluminum through a turbopump?  I guess you could use pressure fed engines, but only as a lower stage (I'd assume that they are going to have some significant dry mass in leftover fuel).  So the answer is NO when the inevitable "will this work with SSTO" followup comment comes up.

[kerbiloid]

6 hours ago, Kerbart said:

Yes, powder, but we're talking about molten metal here.

The polybutadiene acrylonitrile (PBAN) which is the SRB binder and another fuel component is hardly solid at that temperature.

It can be treated as a viscous liquid with aluminium suspension.

Edited November 14, 2022 by kerbiloid