Is my math correct? LM descent engine force at nozzle

[cjameshuff]

I Wonder how a similar fueled engine could be kept viable for longer duration missions (Mars). How to put pressure in the tanks without the restraint of keeping helium from heating up and eventually have to be vented. Big plastic bags inside the tanks supplying mechanical pressure? 

A common solution is a gas generator, which could be a pyrotechnic device or something more complex decomposing a liquid fuel.

Cryo-fuels are fine when you have a big industial complex to supply fresh fuel, but in Space, that's not an option - I think. Can a gas be kept cool enough for a long time? Actively cooled?

Cryogenic propellants certainly are an option in space. LOX and CH4 would be relatively easy to keep liquid in the vicinity of Earth or Mars orbit, even with passive cooling. (The WISE instruments reached equilibrium at about 30 K after the cryogen ran out...it took nearly a year for the block of frozen hydrogen it used to sublime away.)

[ka9q]

The LM descent engine had this shelf lifetime problem only because they decided to store the pressurizing helium as a supercritical fluid to reduce mass. You can easily make such a system last indefinitely by just keeping the helium at ambient temperature, and that's exactly what's done on spacecraft that need to produce large delta-Vs after long periods of time, e.g., interplanetary spacecraft arriving at another planet.

The bigger problem is that the important hypergolic propellants freeze at fairly high temperatures. The main problem is N₂O₄, which freezes at -11.2 C. Straight hydrazine (N₂H₄) freezes at an even higher temperature, +2 C, which is a challenge for spacecraft that use it as a monopropellant. Bipropellant engines usually use a lower-freezing hydrazine like UDMH (-57 C) or MMH (-52 C), either pure or mixed with straight hydrazine (e.g., Aerozine-50, a 50-50 mix of UDMH and straight hydrazine). I don't know much about its low temperature properties but I've heard it will separate out and cause a problem.

[Allan F]

I see. So the choice to pressurize with supercritical helium was because this system was more compact and lightweight than ambient-temperature storage?

The temperature problem is of course important, but can be managed with solar energy - heat exchangers - maybe even using the tanks as heatsinks for electronics. There is plenty of sunlight (in the inner parts of the solar system) to supply electrical energy.

( I hope you all don't mind me asking all these questions. I have absolutlely no real world use of them, but I enjoy talking to people with skill and information, and I - even at 44 (tomorrow 45) years of age enjoy to get my curiosity satisfied. Even when each answer leads me to ask even more questions )

[ka9q]

That's right, I don't know the details but I did read the supercritical He storage was strictly a mass issue. I guess the lower pressure meant less tank weight. There is also an ambient He tank in the DPS that gets things started but most of it comes from the supercritical tank.

[VQ]

The LM's descent and ascent engines (and the SPS) were pressure-fed, with supercritical helium supplying the pressure. I understand this system was limited in it's longivity, because heat seeped into the helium tank, raising it's pressure. And when the burst disk blew, the engine was dead.

I Wonder how a similar fueled engine could be kept viable for longer duration missions (Mars). How to put pressure in the tanks without the restraint of keeping helium from heating up and eventually have to be vented. Big plastic bags inside the tanks supplying mechanical pressure? 

Cryo-fuels are fine when you have a big industial complex to supply fresh fuel, but in Space, that's not an option - I think. Can a gas be kept cool enough for a long time? Actively cooled?

Probably the same way long-duration satellite missions keep their thrusters pressurized. Come to think of it, I am pretty ignorant of how these systems stay pressurized for very long periods of time. No need to use a bladder though - one option would just be to use pressurized helium gas instead of supercritical. Much less space and weight efficient, though.

[Allan F]

Do you need ullage with pressurized tanks? That's not necessary with a bladder system. Helium is such an obnoxius gas - it creeps through the tiniest cracks, and you risk losing your pressure. It is the smallest "particle" of gas, since it is a singular atom, not like hydrogen or oxygen which bond together two by two. If you try to inflate a tire with it, it bleeds right through.

[ka9q]

As I understand it, while He is a very "slippery" gas the main reasons for using bladders is to keep it from dissolving into the propellants and to avoid having it get into the propellant lines in zero-G.

[Noldi400]

As I understand it, while He is a very "slippery" gas the main reasons for using bladders is to keep it from dissolving into the propellants and to avoid having it get into the propellant lines in zero-G.

Just as a side comment, when looking at how things were done on the LM, it's good to keep in mind that (as you know, Bob...) the bug was very much purpose built - every design feature was oriented to the single purpose of getting two men to the surface and back to orbit (with some exploration & such in between) with as high a probability of success as possible.  The ascent stage, in particular, was designed with as few things to go wrong as possible.  Armstrong had even proposed putting manual valves in the propellant lines so that if the circuits failed, they could fire the engine "by hand".  He says in First Man that "management didn't think that was up to NASA's standards of sophistication". 

[ka9q]

Manual propellant valves seem simple enough, but there are more than just two of them. Besides the valves that actually fire the engine, the tanks are pressurized with helium through one-way valves to keep the propellants from mixing in the manifold. The APS is cross-connected with the RCS (two separate systems) to allow propellant sharing.

Then you have all the other things that have to happen within a split second of ascent, such as firing the guillotine cutters and bolts between the stages.

[Zach]

Manual propellant valves seem simple enough, but there are more than just two of them. Besides the valves that actually fire the engine, the tanks are pressurized with helium through one-way valves to keep the propellants from mixing in the manifold. The APS is cross-connected with the RCS (two separate systems) to allow propellant sharing.

Then you have all the other things that have to happen within a split second of ascent, such as firing the guillotine cutters and bolts between the stages.

It would have been feasible as a last-chance option. A "manual valve" doesn't have to literally open or close a single line, it could be a series of valve functions incorporated into a device that operated with one manual motion. And as far as I'm aware, there isn't any show-stopper that would have prevented firing the guillotines and stage separation devices a few seconds early. The AS would remain in a stable position on the DS until the AS engine fired. Not sophisticated, to be sure, but elegant in its simplicity, especially if it's the difference between being entombed 1/4 million miles from home or being able to see your family again. The decision to not include such an option probably came down to probabilities. If enough stuff had failed to where you're down to that as an option, what is the likelihood that manually firing the engine would result in a successful rescue?

[ka9q]

I always wondered if the ascent stage really would remain stable without the attach bolts, cables and piping. Some of the LMs landed at decidedly non-level attitudes.

Whether it makes sense to implement something like manual APS valve control really depends on an understanding of the various failure mechanisms and probabilities. Our intuition about such things is often wrong, and it doesn't make sense to spend effort (and more importantly, mass and volume) on backups for things unlikely to fail when other fatal failures are much more likely.

[Noldi400]

I always wondered if the ascent stage really would remain stable without the attach bolts, cables and piping. Some of the LMs landed at decidedly non-level attitudes.

Whether it makes sense to implement something like manual APS valve control really depends on an understanding of the various failure mechanisms and probabilities. Our intuition about such things is often wrong, and it doesn't make sense to spend effort (and more importantly, mass and volume) on backups for things unlikely to fail when other fatal failures are much more likely.

True enough, but the idea belonged to Armstrong, who should know those things as well or better than anyone. The statement may have been influenced by the fact that is was made around 2003 (to James R. Hansen), and with Neil's dry wit he may have been halfway joking. The actual passage reads:

"We did have various means of controlling the circuitry to the valves - opening the flow of propellants to the engine. So that was an alternative. I had proposed many months earlier - maybe even years earlier - that we just put a big manual valve in there to open those propellant valves rather than, or in addition to, having all the electronic circuitry. But management didn't think that was up to NASA's standards of sophistication.  So I really knew that circuitry very well. But it wasn't really a problem, because if we fired the engine and it didn't fire, we weren't out of time. We had a lot of time to think about the problem to figure out what else we could do. When pilots get really worried is when they run out of options and out of time simultaneously.*"
-FIRST MAN, James R. Hansen

* Like, say, trying to miss a boulder field and a giant crater while your fuel runs low and your computer acts up?