Why the different Apollo RCS fuels?

[PhantomWolf]

Looking up the fuels I noticed that while the LM RCS used the same Aerozine 50/nitrogen tetroxide combination that the Descent and Ascent engines did, the CM RCS used pure UDMH as a fuel, rather than the 50/50 mix with Aerozine, and that the SM RCS used Monomethylhydrazine as its fuel. The SPS used the Aerozine 50/nitrogen tetroxide that the LM used.

Is there any obvious reason for the differences that I am missing? Why didn't they just use the Aerozine 50/nitrogen tetroxide combo for all the engines?

[ka9q]

I can't cite all the reasons offhand, but I do know that the different flavors of hydrazine (straight N2H4, MMH, UDMH) have very different densities, freezing and boiling points. Also, straight hydrazine is easily decomposed by a catalyst, a necessary property in a monopropellant engine but a drawback in bipropellant engines like all those on Apollo since it means less stability.

[ka9q]

N2H4: density 1.021; fp +2C; bp +114C
MMH: density 0.88; fp -52C; bp +87C
UDMH: density 0.793; fp -57C; bp +67C

Actually, there's a lot more on Bob B's site here: www.braeunig.us/space/propel.htm

[Bob B.]

In terms of ISP, the order of performance from best to worst is: N2H4, MMH and UDMH.

N2H4 has a high freezing point; therefore it’s not very desirable when cold temperatures are encountered. It’s an excellent monopropellant, however, and it widely used in that application. A RCS system generally doesn’t have a large volume of propellant, so heaters can be provided if necessary to prevent freezing.

When freezing point is an issue, we generally step down to the next best performing fuel, which is MMH. The problem with MMH, however, is that it lacks the thermal stability for use as a coolant in regenerative cooled engines. Most spacecraft systems are pressure-fed; therefore MMH is frequently used in spacecraft maneuvering and RCS systems, such as in the Space Shuttle.

UDMH is the least desirable in terms of performance, but it is stable enough for use as a coolant. It’s the obvious choice, therefore, for use in large regenerative cooled engines. This is why we often see UDMH used in launch vehicles.

Aerozine 50 was developed by Aerojet for the Titan 2. Aerojet discovered that by blending UDMH with N2H4 they got a propellant that was stable enough for regenerative cooling, had an adequate boiling point, and had a performance comparable to MMH.

Regarding Apollo…

From what I’ve observed in other applications, I would expect to see systems like those used in Apollo to use MMH as the fuel. I’m not sure why Aerozine 50 was selected. I know Aerojet built the SPS engine, so I think they likely used Aerozine 50 because of their extensive experience with it. The designers probably extended the use of Aerozine 50 to include the LM as well.

Clearly the use of Aerozine 50 in the LM RCS was because the APS and RCS systems were interconnected. The SM RCS was independent of the SPS system, thus there was no requirement to use the same fuel. The use of MMH would be pretty normal in that application. Regarding the CM RCS, I think MMH was used there as well, which again would be normal. (The OP says UDMH was used in the CM RCS, but I’ve seen other sources that say MMH.)

[ka9q]

Thanks Bob, you've confirmed some of what I'd thought and filled in the gaps. It makes sense that stability and ISP would be inversely correlated. N2H4 likes to fall apart with just a catalyst so it must also release more energy with an oxidizer than a stable fuel like UDMH. It also makes sense that the properties of MMH and Aerozine-50 would be similar as they have the same average number of methyl groups.

There's also UH-25, a 25-75 mixture of N2H4 hydrate and UDMH. The Ariane 1 launcher originally used straight UDMH (as you'd expect for a large rocket engine) but the failure of the early L02 mission due to a first stage combustion instability prompted an eventual switch. I remember that failure very well as the AMSAT Phase-IIIA spacecraft was aboard. Geez, it's already been 31 years.

[Bob B.]

Nov 14, 2011 16:07:32 GMT -4 ka9q said:

There's also UH-25, a 25-75 mixture of N2H4 hydrate and UDMH.

Yep, that's the only other hydrazine blend I'm aware of that's gotten a lot of use.

It should be noted that the freezing point of nitrogen tetroxide is not particularly good either at -9.3 C. Although you don't hear about it much, nitrogen tetroxide is often mixed with nitric oxide to bring down the freezing point. The resulting oxidizer is called MON (mixed oxides of nitrogen). It is usually followed by a number, e.g. MON-25, that indicates the percentage nitric oxide by weight. I suspect that when a document lists nitrogen tetroxide as the oxidizer, there's a good chance it's really some version of MON.

[JayUtah]

Straight hydrazine is generally unsuitable for large-scale spacecraft because of the warm freezing point. Hence for most practical purposes you either have to use one of the methylated hydrazines (which have physical and chemical properties that are more easily engineered around) or a mixture of hydrazine and methylated hydrazine.

The mixtures themselves are difficult to work with because hydrazine and its methylated counterparts do not mix well. Aerozine-50 (A-50) is a good fuel choice first because it's an excellent compromise between performance and handling, and second because Aerojet developed a process to manufacture it relatively cheaply on a large scale. If you know you're going to use a hydrazine mix, you'd better have a good reason for not using A-50.

The SPS was built by Aerojet also, and naturally used the company's A-50 product. Sometimes engineering decisions are heavily influenced by the company's proprietary interests and experience in working with certain technologies that it prefers.

But all the large-scale hypergolic engines used A-50, even those that Aerojet didn't make. It was simply the most popular and easily obtained hydrazine mixture. And because the LM APS used it, the RCS used it too because their fuel systems were crossfed.

Methylated hydrazines by themselves were used in the other RCS systems likely because their tanks lay close to the respective spacecraft skins and were thus more susceptible to thermal cycling than the more internally contained A-50 tanks. A hydrazine mix separates and stratifies when it approaches its freezing point, and once separated it is not generally possible to remix them.

So if you're going to use a derivative only, which one to choose? MMH is the highest-performing of the methylated hydrazines, and the fuel capacity of the SM RCS is limited. Hence you want the fuel that gives you the most bang for the tankage.

However, MMH is nearly three times more toxic than UDMH on the OSHA toxicity scale, so that's why UDMH is used in the CM -- a manned spacecraft. The CM can land with a substantial residual fuel load, and this poses a risk to flight and ground crews. The Apollo 16 safing crew was injured when a post-flight detanking and safing operation went awry.

So: A-50 in general, if you can keep it warm. Under cold temperatures: MMH if you need power, UDMH if you need safety.

[Bob B.]

Thanks, Jay. I did not know about the safety issues of MMH vs. UDMH.

[ka9q]

Nov 14, 2011 17:20:46 GMT -4 JayUtah said:

The CM can land with a substantial residual fuel load, and this poses a risk to flight and ground crews. The Apollo 16 safing crew was injured when a post-flight detanking and safing operation went awry.

I thought the CM thrusters were fired to depletion during final descent specifically to dump the residual propellants.

This was when the ASTP Apollo crew got into serious trouble. Some unreacted N2O4 got sucked into the cabin vent (it opens when the outside pressure exceeds 5 psi) and the entire crew was pretty sick by the time they hit the water; at least one (Vance Brand, I think) was unconscious. Fortunately, Tom Stafford managed to get an O2 mask onto him. The tape is pretty garbled but you can tell something is very wrong as they try to open the hatch.

They spent some time under observation in the ship hospital but there don't seem to have been any serious after effects.

Considering how each Apollo spacecraft carried tons of hypergolic propellants I'm thankful there weren't more incidents like this.

Did the practice of firing Apollo CM thrusters to depletion begin only after Apollo 16?

[ka9q]

Nov 14, 2011 17:27:53 GMT -4 Bob B. said:

Thanks, Jay. I did not know about the safety issues of MMH vs. UDMH.

Neither did I, but it seems almost like worrying about the relative toxicity of sarin vs tabun nerve gas. :-)

[Bob B.]

Nov 14, 2011 17:20:46 GMT -4 JayUtah said:

... and second because Aerojet developed a process to manufacture it relatively cheaply on a large scale.

Cheap at least in the 1960s. I've heard, and perhaps you can confirm, that over the years excessive government regulation kept driving the price up to the point that Aerozine 50 became cost prohibited, which was one of the factors leading to the retirement of the Titan family of launch vehicles. Do you know if there's any truth to that?

[chew]

Nov 15, 2011 4:27:31 GMT -4 ka9q said:

at least one (Vance Brand, I think) was unconscious. Fortunately, Tom Stafford managed to get an O2 mask onto him. The tape is pretty garbled but you can tell something is very wrong as they try to open the hatch.

It was Deke Slayton who lost consciousness. Stafford put his oxygen mask on then put an oxygen mask on Slayton. Good thing he listened to those safety briefs the airlines give about loss of cabin pressure.

[JayUtah]

Nov 15, 2011 4:36:18 GMT -4 ka9q said:

Nov 14, 2011 17:27:53 GMT -4 Bob B. said:

Thanks, Jay. I did not know about the safety issues of MMH vs. UDMH.

Neither did I, but it seems almost like worrying about the relative toxicity of sarin vs tabun nerve gas. :-)

Indeed, there is one form of toxicity which is the practical kind: if you get a good whiff of either one, you're in a world of pulmonary hurt. The Air Force has records of such accidents among the Titan workers.

There is another form embodied in OSHA "allowable exposure" restrictions. In terms of allowable concentrations in your work environment, you can have 2-3 times more UDMH there than MMH before it's considered an actionable hazard.

[JayUtah]

Nov 15, 2011 4:27:31 GMT -4 ka9q said:

I thought the CM thrusters were fired to depletion during final descent specifically to dump the residual propellants.

That was the procedure up through Apollo 15. Even so, a propellant burnoff and dump does not entirely remove all the material from the propellant feed system. The spacecraft is still considered hazardous until the propellant neutralizers are injected by the ground crew, after the CM is recovered.

Did the practice of firing Apollo CM thrusters to depletion begin only after Apollo 16?

Actually it was the norm for all manned Apollo flights, but ceased for Apollo 16 (and possibly 17 IIRC), then resumed with ASTP.

Because the RCS burnoff and fuel dump was implicated in the failure of Apollo 15's main parachute, those procedures were deleted for the following missions. Apollo 16 landed with approximately 200 lbs of unused NTO/MMH.

The relative inexperience of the safing crew to deal with this new procedure of post-landing safing of large amounts of hypergolic propellant was a contributing cause to the Apollo 16 safing accident and led to reconsidering the in-flight safing procedure.

[JayUtah]

Nov 15, 2011 9:44:35 GMT -4 Bob B. said:

Cheap at least in the 1960s. I've heard, and perhaps you can confirm, that over the years excessive government regulation kept driving the price up to the point that Aerozine 50 became cost prohibited, which was one of the factors leading to the retirement of the Titan family of launch vehicles. Do you know if there's any truth to that?

Yes to increased regulation being a major cost driver in launch vehicle operations, both for propellants and for other safety measures. Space is getting more expensive.

Yes to increased fuel costs putting the Titan out to pasture. Not just the costs of producing, but major handling costs at the launch site. RP-1 can be handled by people with no more certifications than for JP-x fuels, and with the same procedures -- Sheet 128, I believe, same as gasoline and diesel fuel.