Hypergolic Propellants in Extraterrestrial Situations

[shynung]

Rockets today generally use liquid hydrogen/kerosene and liquid oxygen as propellant, barring a few exceptions. Although efficient in terms of I_Sp, these fuel mixes, along with plenty others, have a common weakness: they cannot ignite themselves.

On Earth, it's not much of an issue; the ignition source can be supplied by the launching infrastructure (The Space Shuttle used external igniters). In space-borne environments, such luxuries are usually absent; the spacecraft must use internal means to start their engines, sometimes electric (spark plugs), but most often by injecting a small amount of hypergolic fuels.

Suppose we are in a small Lunar base, acting as a hub to a nearby mining operation, which produces, among other things: water, various metals (mostly aluminium), and some nuclear fuels (uranium, helium-3), but no complex organic compounds like those found in petroleum. Spacecrafts visiting the place generally use LH2/LOX engines, or Al/LOX hybrids, both of which isn't hypergolic on their own.

What would they use as a starting fuel, and how they would obtain them?

Edited July 12, 2014 by shynung

[Tex_NL]

How much would you need? If it's only a small amount each craft can easily carry some. And since you're talking about a mining operation you'll have ships shuttling the produce back and forth to earth. Those ships could re-supply the base with starter fuel.

Or

Your mining base, is it manned? If it's manned it's probably permanently manned and you'll have greenhouses growing food. Food consists for a large part out of amino acids and proteins: nitrogen. Still, without replacing them somehow you'll deplete the food chain of nitrogen. But your earth-moon produce shuttles can bring in fertilizers.

[shynung]

How much would you need? If it's only a small amount each craft can easily carry some. And since you're talking about a mining operation you'll have ships shuttling the produce back and forth to earth. Those ships could re-supply the base with starter fuel.

Or

Your mining base, is it manned? If it's manned it's probably permanently manned and you'll have greenhouses growing food. Food consists for a large part out of amino acids and proteins: nitrogen. Still, without replacing them somehow you'll deplete the food chain of nitrogen. But your earth-moon produce shuttles can bring in fertilizers.

Only a small amount is needed, just enough to ignite main fuel mix, plus a little more for ullage. Probably doubles as RCS.

The nitrogen is the weak part. If the base is close to Earth (in this case, it is), they indeed could send shipments of nitrogen, both for greenhouse needs and starter fuels.

Things would probably get a little difficult if we're a bit farther away (asteroid belt mining, say), or if complete independence from Earth-bound resupply is mandatory. That's what I was thinking initially; how do we get the nitrogen elsewhere?

Edited July 12, 2014 by shynung

[Tex_NL]

No matter the distance I believe the answer will basically be the same.

You're talking about a mining station. The main reason for mining off-planet is to get rare resources back to earth. To get those resources back you first need to send a ship from earth to the base. Use that ship to resupply the base. The ship probably uses the same hypergolic starter fuel. Should be pretty easy to give it an over sized tank.

[magnemoe]

No matter the distance I believe the answer will basically be the same.

You're talking about a mining station. The main reason for mining off-planet is to get rare resources back to earth. To get those resources back you first need to send a ship from earth to the base. Use that ship to resupply the base. The ship probably uses the same hypergolic starter fuel. Should be pretty easy to give it an over sized tank.

yes, its just another resource you need some of, very stupid to forget trough.

Did not know this was a common ignition method.

However I know about an weirder one: modern oil platforms use the touch on top of the tower only for emergencies like then you need to stop gas processing but you still have gas coming up.

To ignite the gas then they start to burn it one platform launch a firework rocket trough a special pipe up to the top of the tower where its flame ignite the gas.

The rockets is in a magazine and can be fired by the touch control system however I found it a weird solution.

Probably done so they don't have complex stuff up at the tower, rocket reach the top hit a bend and is tipped over to ignite the gas then fall down in a bucket after burned out.

[shynung]

However I know about an weirder one: modern oil platforms use the touch on top of the tower only for emergencies like then you need to stop gas processing but you still have gas coming up.

To ignite the gas then they start to burn it one platform launch a firework rocket trough a special pipe up to the top of the tower where its flame ignite the gas.

The rockets is in a magazine and can be fired by the touch control system however I found it a weird solution.

Probably done so they don't have complex stuff up at the tower, rocket reach the top hit a bend and is tipped over to ignite the gas then fall down in a bucket after burned out.

I find that method complicated. Petroleum gases are flammable by themselves (mostly methane), so a spark plug stuck into the pipe could set it off. Though, they probably don't like that solution pretty much; someone has to go up there to inspect/replace the spark plugs.

[SciMan]

As far as the LH2-LOX mix is concerned, I'm almost certain that you could design a spark ignition system that would work. I've heard that there's a rocket engine that uses some kind of spark-ignited LH2-LOX "blowtorch" type device that is designed to project a flame into the combustion chamber, and it's started before the main fuel flows are allowed into the combustion chamber.

On the J-2 rocket engine, it's called the Augmented Spark Igniter.

On the RL-10 engine, more modern versions have actually used a "dual direct ignition system" which is pretty much nothing more than a spark plug in the combustion chamber. It's mechanically simpler, but the engine control computer has to be more sophisticated to avoid a "hard start". A "hard start" is basically when an engine's ignition system functions "late", allowing too much fuel/ox mixture to build up in the combustion chamber. That can cause the combustion chamber to explode, which is obviously A Bad Thing.

For Al/Lox hybrid rockets, I imagine that a small amount of LH2 could be carried to supply a torch ignition system similar to the J-2's Augmented Spark Igniter. The same idea would probably work for other propellant combinations as well. Basic idea is to add small bottles of hydrogen/oxygen (probably supercritical instead of liquid) for the ignition system to use, probably located next to the (typically helium or nitrogen) tank pressurization bottles, and put a small spark-started H2-Ox burner in the combustion chamber. The H2-Ox burner would only need to be active during engine start, once the engine is running it could be turned off to conserve H2-Ox.

If it sounds a bit complicated, that's because it is complicated. There isn't really any way to simplify it without running into issues of one kind or another, anything from reduced performance to catastrophic failure and probably anything in-between.

[Camacha]

Hypergolic fuels are typically corrosive, toxic and otherwise dangerous. This is a major concern when used around humans, especially in places where you cannot go outside in case of a leak.

[magnemoe]

I find that method complicated. Petroleum gases are flammable by themselves (mostly methane), so a spark plug stuck into the pipe could set it off. Though, they probably don't like that solution pretty much; someone has to go up there to inspect/replace the spark plugs.

The problem with a spark plug is that you can easy get to high or too low gas concentration, you also have lot of wind so it might be hard to get an spark plug too ignite it, or you had to put the plug in a position where it would face the full intensity of the flame for days and it might be damaged.

You could probably light a pilot flame from a separate gas pipe with know pressure this way however they found it just as well to launch a rocket up the pipe.

[shynung]

I've heard that there's a rocket engine that uses some kind of spark-ignited LH2-LOX "blowtorch" type device that is designed to project a flame into the combustion chamber, and it's started before the main fuel flows are allowed into the combustion chamber.

I didn't know that. That certainly would solve the ignition problem on LH2-LOX engines. Also, the computers needed to run dual-direct systems is probably already in the market; modern Otto engines in current production cars need their spark plugs to fire at a precise moment, otherwise they'd run into problems with detonation (auto engineer's term for hard starts, where the fuel combusts too early in the chamber).

I suppose there will be a time in the future where we may not need nitrogen tetroxide anymore, and where hydrazine is strictly RCS. Thought, I'm not certain about it; even John D. Clark once thought that fluorine-fueled rockets would have had a future.

Edited July 12, 2014 by shynung

[Tex_NL]

Hypergolic fuels are typically corrosive, toxic and otherwise dangerous. This is a major concern when used around humans, especially in places where you cannot go outside in case of a leak.

That's why you should store them outside and you stay inside.

In case of a leak the nasty fumes dissipate into the vacuum of space and you'd still be sitting pretty.

[shynung]

Hypergolic fuels are typically corrosive, toxic and otherwise dangerous. This is a major concern when used around humans, especially in places where you cannot go outside in case of a leak.

Oxygen and hydrogen aren't necessarily better off in this context. Both are serious fire hazards and, in the case of hydrogen, explosion hazards as well.

[Kryten]

The most common hypergolic fuels have exposure limits in the parts-per-million range and have to be handled in sealed positive-pressure suits. Hydrogen is hardly in the same league.

[shynung]

It may not, but having concentrations of hydrogen anywhere between 4-74% in the crew compartment atmosphere is still a terrible situation. One careless spark is enough to bang the compartment to bits.

[cantab]

No matter the distance I believe the answer will basically be the same.

You're talking about a mining station. The main reason for mining off-planet is to get rare resources back to earth. To get those resources back you first need to send a ship from earth to the base. Use that ship to resupply the base. The ship probably uses the same hypergolic starter fuel. Should be pretty easy to give it an over sized tank.

This is a fair point. A space mining operation might well enjoy cheap freight shipments from Earth, because the freighters would otherwise be flying that trip empty. While you couldn't go overboard, adding any cargo takes fuel after all, I expect they'd receive no shortage of goodies.

[Camacha]

Oxygen and hydrogen aren't necessarily better off in this context. Both are serious fire hazards and, in the case of hydrogen, explosion hazards as well.

They are. Imagine those dangours, with the added risk of structural failure/damage due to the corrosive properties in case of a leak, and human illness or death in case your crew gets into contact with it. It is typically highly toxic stuff, so a leak is almost infinitely more serious than in case of hydrogen or oxygen. Unless everything blows up or concentrations are really high, people will be fine.

That's why you should store them outside and you stay inside.

In case of a leak the nasty fumes dissipate into the vacuum of space and you'd still be sitting pretty.

If you have to keep them seperated from the vehicle in which you need it you are not going to get much use out of it

[Tex_NL]

If you have to keep them seperated from the vehicle in which you need it you are not going to get much use out of it

I meant you'll be living in an airtight container. As long as you keep the dangerous stuff outside that you're 'safe'.

[Guest]

Well, unless your design is very cramped (like Apollo LM), then they can be kept apart only form the crew module, separated by vacuum. There's no reason for the propulsion system to be connected to the crew section by anything but structural elements. Indeed, there are many reasons for it not to be. Literally the only reason for the fuel lines to go on the crew compartment would be if you're using N2O or LOX and want to link the propulsion with your life support. In the former case, you can break down N2O into oxygen and nitrogen for an emergency air supply, in the latter case you can pipe the boiloff gas into life support tanks. Those two aren't actually that bad, as far as the fuels go. Pure oxygen is a fire hazard, but it's not too bad if there isn't too much of it. N2O is an anesthetic, and a leak might induce some irrational behavior in the crew (it's also known as "laughing gas" because of it's euphoric effects) which would probably be funny in most other situations, but it might be a cause for concern. That said, as long as they regain presence of mind to plug the leak, there should be no bigger problems even if they do fool around while life support gets rid of the N2O.

Also, hypergolics might be foul stuff, but they're actually not that much of a fire/explosion hazard. Even when they do explode, they do so in a pretty benign way. Gemini had no abort tower for precisely that reason. They're also incredibly compact and don't boil off, as any RSS player would probably know. With LOX/Hydrogen, you've got highly cryogenic fuels with very poor density, meaning you've got limited lifetime and huge tanks. None of those are viable features on a lander.

Edited July 12, 2014 by Guest

[Kryten]

Hypergolic fuels don't use N₂O, they use N₂O₄ (nitrogen tetroxide), a very powerful oxidiser. N₂O₄ entering the crew cabin nearly killed the astronauts on ASTP.

[Camacha]

I meant you'll be living in an airtight container. As long as you keep the dangerous stuff outside that you're 'safe'.

That was exactly the danger I was talking about. Airtight container alongside of a toxic substance. A minor leak can have major consequences. Or consider a leak that contaminates an airlock through which an astronaut enters the facility. The long story short is that a toxic substance plus very little option in case of danger is bad news. You might have some emergency procedure, but it is not like you can run outside.

Hypergolic fuels don't use N₂O, they use N₂O₄ (nitrogen tetroxide), a very powerful oxidiser. N₂O₄ entering the crew cabin nearly killed the astronauts on ASTP.

This. VERY TOXIC, according to EU classification. Much kill, very danger. I am not sure how else to explain it.

Edited July 12, 2014 by Camacha

[cpast]

That was exactly the danger I was talking about. Airtight container alongside of a toxic substance. A minor leak can have major consequences.

Yes, a minor leak would have major consequences. You see, a minor leak will rapidly lead to there being no air inside, which will kill the crew very very rapidly. This actually did kill the crew of Soyuz 11. However, given that the pressure inside the cabin is higher than the pressure outside, a leak will never result in the fuel coming in. You don't have the fuel tank right next to the pressurized compartment; most of the craft is unpressurized, and there's pretty much going to be hard vacuum in between the fuel tank and cabin. The *only* time it can enter is if the cabin is depressurized, at which point the crew is either in spacesuits or dead.

You know how, when handling toxic materials, you wear positive pressure suits, and how areas designed to keep out toxic substances are at positive pressure? What that means is that pressure inside is slightly higher than pressure outside; the reason this is done is that it means that even if the barrier is damaged, there will still be no contamination, because air will flow out, not in. Crew cabins (and spacesuits) are positive pressure environments - inside has air, outside is vacuum. There is zero enhanced risk to crew from hypergolics entering the cabin.

The Apollo leak happened during re-entry, where the spacecraft was taking in outside air, not in orbit.

@OP: Real spacecraft already often use hypergolics for when they need to be restartable; that's what the Apollo Service Module used, as well as the LM.

Edited July 13, 2014 by cpast

[shynung]

@OP: Real spacecraft already often use hypergolics for when they need to be restartable; that's what the Apollo Service Module used, as well as the LM.

They do, but the Apollo SM and LM engines used it as the primary fuel. Because of their modest delta-V requirements, in addition to being expendable, the use of hypergolics were deemed appropriate. The spacecrafts I envisioned are running off either LH2-LOX or AL-LOX, and are reusable designs used mainly as space trucks carrying material mined from Luna to a space station in MEO (so they don't need to reenter the atmosphere). The hypergolics' role in this case is to act as a starter fuel, as LH2-LOX / AL-LOX itself cannot ignite on their own.

As Tex said, the hypergolics in question can be brought from Earth (or scraped from a nearby planet with nitrogen in their atmosphere). Every spacecraft would carry some of these hypergols, to be used whenever they start a burn. Though, f there is a reliable spark ignition system that works with LH2-LOX, that'll put the nail in the coffin for the hypergolic starter fuels; the spacecrafts would probably carry these instead, as it is available on Luna.

[einsteiner]

On Earth, it's not much of an issue; the ignition source can be supplied by the launching infrastructure (The Space Shuttle used external igniters).

This is not true. The Space Shuttle engines had augmented spark ignitors inside the engine. The sparks you see in videos such as

are there to burn off any excess hydrogen that may be floating around. Similar ignitors were used in the Saturn 5 engines, where the second and third stages had to start in flight (twice for the third stage).