BillKerman123

Using solid methane, oxygen, etc as a rocket propellent?

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So a while ago there was a lot of talk about solid hydrogen, but what about other propellents/oxidizers? Octane (the closest thing to RP-1 I could find) boils at 398K and melts at 216K. Methane boils at 110K and melts at 90K. Oxygen boils at 90K and melts at 54K. Hydrogen boils at 20K and melts at 14K (with extreme pressure). So, if you can freeze your propellent enough to make liquid hydrogen, it should be no problem to make solid RP-1, methane, oxygen, etc. So why don't people do this?

I've seen people talk about solid oxygen before, but all of the problems they found in it are easily dismissed by just heating it on its way to the combustion chamber, maybe using overlapping magnetic fields to cause internal friction (superchilled oxygen is magnetic), or maybe by wrapping the propellant lines in electrical heaters. Or better still, put heating elements inside the propellant tanks, so as the propellents heat up they expand and are forced through the propellant lines, negating the need for a pressure feed system (at least early on in the flight). And for interplanetary craft, it's even better, as it negates boil-off. So, why isn't it done? 

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They at least have tested slosh-like oxygen (i.e. liquid oxygen with snow-like oxygen floating in it) as oxidizer, and proposed slosh-like hydrogen as fuel.

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I think that the excitement about metallic hydrogen it that if you can form it is is believed to be metastable so you have an extremely high density form of hydrogen that can be stored without cryogenic tanks.

Other gasses when cooled aren't going to form metals so all you get is a slightly denser form of the substance that has to be kept extra cold.

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Posted (edited)

Slush hydrogen has also been tested, but dismissed as not worth the effort.

Look, feeding gels and powders into the inferno of a combustion chamber has been a nightmarish task - many have tried. You need a positive displacement system because pressurizing gas just wants to blow a tunnel from the top of the tank to the propellant intake. Solidified propellants don’t atomize like sprayed liquids, and have serious trouble combusting. You have to worry about phase shifts in propellant lines (gels had a nasty tendency to dry up and cake up everything)... baaically, everyone’s given up unless it’s a heterogenous monopropellant (aluminium dust in lOx or water ice).

Edited by DDE

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One key point: solidification doesn't actually solve the boil off problem. It just delays the problem until after it's all melted. You still have heat creeping into a cryogenic tank that must be removed for long term storage. 

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53 minutes ago, Starman4308 said:

One key point: solidification doesn't actually solve the boil off problem. It just delays the problem until after it's all melted. You still have heat creeping into a cryogenic tank that must be removed for long term storage. 

Except with metallic hydrogen(in theory), which is why there is so much interest in it.

Hydrogen is especially difficult to store, so if it is stable at higher temperatures and lower pressures after being made metallic, that would be a huge win.

(not to mention the much higher energy per unit if you start with monoatomic hydrogen over H2, or the much higher density of metallic hydrogen)

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3 hours ago, DDE said:

Slush hydrogen has also been tested, but dismissed as not worth the effort.

Look, feeding gels and powders into the inferno of a combustion chamber has been a nightmarish task - many have tried. You need a positive displacement system because pressurizing gas just wants to blow a tunnel from the top of the tank to the propellant intake. Solidified propellants don’t atomize like sprayed liquids, and have serious trouble combusting. You have to worry about phase shifts in propellant lines (gels had a nasty tendency to dry up and cake up everything)... baaically, everyone’s given up unless it’s a heterogenous monopropellant (aluminium dust in lOx or water ice).

Hybrids seem to be pretty popular (solid fuel + liquid oxidizer), especially among amature and low end [budgetwise] rocket scientists.  I think Spaceship One even used it.  Most of the benefit appears to be that it requires a single system to deliver the liquid oxidizer to the combustion chamber (typically pressure fed N2O).  Perhaps an electric turbopump pumping LOX would provide most of the Isp of liquid rockets, but only require half the batteries and similar technical difficulties (while avoiding most of the safety issues involved in building a monoprop solid rocket).  If anything, such a design appears to be the most straightforward means to throttling I can think of (that doesn't involve venting pressure and wasting thrust).

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13 hours ago, wumpus said:

Hybrids seem to be pretty popular (solid fuel + liquid oxidizer), especially among amature and low end [budgetwise] rocket scientists.  I think Spaceship One even used it.  Most of the benefit appears to be that it requires a single system to deliver the liquid oxidizer to the combustion chamber (typically pressure fed N2O).  Perhaps an electric turbopump pumping LOX would provide most of the Isp of liquid rockets, but only require half the batteries and similar technical difficulties (while avoiding most of the safety issues involved in building a monoprop solid rocket).  If anything, such a design appears to be the most straightforward means to throttling I can think of (that doesn't involve venting pressure and wasting thrust).

The problem is that you need to keep the solid fuel cryogenic at all time after you form it. Liquid can be stored in an refrigerated tank and pumped into the rocket just before launch. 
Hybrids has the benefits of being simple while letting you control trust. 

 

18 hours ago, tomf said:

I think that the excitement about metallic hydrogen it that if you can form it is is believed to be metastable so you have an extremely high density form of hydrogen that can be stored without cryogenic tanks.

Other gasses when cooled aren't going to form metals so all you get is a slightly denser form of the substance that has to be kept extra cold.

Metallic hydrogen will also release lots of energy then converted to gas. If it is meta stable it would be an very high isp fuel 

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14 minutes ago, magnemoe said:

Metallic hydrogen will also release lots of energy then converted to gas. If it is meta stable it would be an very high isp fuel 

So high-ISP that you’d have serious chamber cooling issues.

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1 hour ago, DDE said:

So high-ISP that you’d have serious chamber cooling issues.

Unknown but that could well be an problem, on the other hand it would be an expensive fuel who you would only use in small engines who is easier to cool. 

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3 hours ago, DDE said:

So high-ISP that you’d have serious chamber cooling issues.

So you bring along extra LH2 to be injected as a coolant.

Solid methane is giving me ideas, 

Like running LOX over solid methane like a hybrid motor.

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41 minutes ago, Xd the great said:

So you bring along extra LH2 to be injected as a coolant.

Solid methane is giving me ideas, 

Like running LOX over solid methane like a hybrid motor.

Who require that the SRB has been cooled below −188 °C at all time also unsure if frozen methane would work well, perhaps it to fragile and break up easy. 

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47 minutes ago, Xd the great said:

So you bring along extra LH2 to be injected as a coolant.

Solid methane is giving me ideas, 

Like running LOX over solid methane like a hybrid motor.

The chamber temperature of you solid methane hybrid engine is going to be over 3000K. Your methane isn't going to be solid for long.

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1 hour ago, tomf said:

The chamber temperature of you solid methane hybrid engine is going to be over 3000K. Your methane isn't going to be solid for long.

Useful for abort motors...

When you need insane thrust in a short time and it needs to automatically prevent misfiring after some time...

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2 hours ago, Xd the great said:

So you bring along extra LH2 to be injected as a coolant.

Solid methane is giving me ideas, 

Like running LOX over solid methane like a hybrid motor.

Run LOX or LF2 through a metallic hydrogen slug for the ultimate hybrid motor...

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Posted (edited)
1 hour ago, StrandedonEarth said:

Run LOX or LF2 through a metallic hydrogen slug for the ultimate hybrid motor...

Dubious. Recombination of likely monomolecular metallic hydrogen requires no additional pollutants.

Such as oxidizers, which would reduce ISP.

1 hour ago, Xd the great said:

Useful for abort motors...

When you need insane thrust in a short time and it needs to automatically prevent misfiring after some time...

But will it melt fast enough to burn?

3 hours ago, Xd the great said:

So you bring along extra LH2 to be injected as a coolant.

At which point your performance takes such a hit it’s not clear why you bothered in the first place.

Edited by DDE

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The reason I asked in the first place is because I'm trying to figure out how hard it would be for an amateur rocketry group to put a payload into orbit, and I thought having solid methane/oxygen as propellants on small rockets might be doable. I considered we could have tanks with solid methane and solid oxygen, and if the tanks are heated then the propellents will be a) liquified, and b) pushed into the propellant lines as they expand. I never considered doing a hybrid rocket style design, but in retrospect, that could work as well.

Which one would be more efficient though? If you have the solid methane in the combustion chamber, then as it is heated it will melt, and the resulting expansion will rapidly increase the burn rate. This may be a good thing or a bad thing. In a conventional design, nothing very interesting will happen since the propellant will be gasified by the time it reaches the combustion chamber.

I'm leaning towards the hybrid style design because the fuel being turned from solid to gas with the resulting expansion and increase in propellant burn rate should provide a lot of thrust, since as the propellant is liquefied and gasified the burnable surface area goes up very fast. 

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Posted (edited)
3 hours ago, BillKerman123 said:

I'm leaning towards the hybrid style design because the fuel being turned from solid to gas with the resulting expansion and increase in propellant burn rate should provide a lot of thrust, since as the propellant is liquefied and gasified the burnable surface area goes up very fast. 

You don't want uncontrolled thrust, you want thrust that can be controlled.

pushing liquid O2 past frozen CH4 and lighting it sounds like it would momentarily become a thermobaric bomb as the CH4 melts/expands/evaporates and bursts open the containment, then after it mixes with atmospheric O2, the flame shooting out of the nozzle will light the entire cloud.

Or, if your containment vessel is strong enough to contain it, it will be much too heavy to lift off the ground.

(you are dealing with the same sorts of forces Frackers use to shatter stone, so thin walled containers are not going to hold it very well)

Edited by Terwin

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4 hours ago, BillKerman123 said:

The reason I asked in the first place is because I'm trying to figure out how hard it would be for an amateur rocketry group to put a payload into orbit, and I thought having solid methane/oxygen as propellants on small rockets might be doable.

It’s probably nowhere worth the fuss. The gain in density would be probably quite abysmal, and that’s assuming an amateur team can even handle cryogens. Heck, I’m not sure cryogens are easier than hypergols at that scale.

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2 hours ago, Terwin said:

You don't want uncontrolled thrust, you want thrust that can be controlled.

pushing liquid O2 past frozen CH4 and lighting it sounds like it would momentarily become a thermobaric bomb as the CH4 melts/expands/evaporates and bursts open the containment, then after it mixes with atmospheric O2, the flame shooting out of the nozzle will light the entire cloud.

Or, if your containment vessel is strong enough to contain it, it will be much too heavy to lift off the ground.

(you are dealing with the same sorts of forces Frackers use to shatter stone, so thin walled containers are not going to hold it very well)

A simple 2 stage solids will do. No need for such solid combination of propellants.

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Okay, what about something like this: 

e5eH12E.jpg

Would that solve the problem? The only surface of the SCH4 burning is small and controlled. As the SCH4 burns, the heat melts the SCH4 behind it, which flows downwards and burns, and the heat rises up and melts more SCH4. 

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Posted (edited)
Spoiler

CH4 + 1.5 O2 → CO + 2H2O

16 kg of methane : 48 kg of oxygen, 1:3

density: 0.4 : 1.2

volume: ~ 1:1

Why not one above another?

(got it, methane is solid)

What is strength limit of the solid methane?

And how will it stay solid near the flame?

Edited by kerbiloid

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2 hours ago, kerbiloid said:
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CH4 + 1.5 O2 → CO + 2H2O

16 kg of methane : 48 kg of oxygen, 1:3

density: 0.4 : 1.2

volume: ~ 1:1

Why not one above another?

(got it, methane is solid)

What is strength limit of the solid methane?

And how will it stay solid near the flame?

It's not supposed to stay solid. The idea is it liquifies (or gasifies) in a controlled manner. 

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23 hours ago, BillKerman123 said:

in a controlled manner

That might be problematic. Even purpose-chosen ablators or solid motor grains can be tricky.

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It costs energy to convert a solid to a liquid and this energy will not be available for thrust. 

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