What state?

It seems that the use of liquid fuel isn't particularly favoured for military use because they can't be left in the rocket for any extended period. Solid rockets are better, because they can be left sitting ready on the launch pad, prepared for action. David Reynolds (Apollo: The Incredible Journey to the Moon) cites the fact that the V-2 was liquid fueled as evidence that Von Braun and his team weren't developing rockets for warfare but for space applications. However, rockets like the Redstone and the Atlas were originally ICBMs and were liquid fueled.

My question: Was Reynolds correct in his assessment or was liquid fueled rockets the only feasible option early on in the history of rocket development?

Hypergolic fuel?

The SM and CM RCS used MMH. The Space Shuttle RCS and OMS use MMH. The LM RCS however uses Aerozine 50. This is understandable since the LM APS used Aerozine 50. Both systems could be run off the single fuel tank. The SPS used Aerozine 50 as well. The Titan II used Aerozine 50.

My question: Why is it that MMH seems to be preferred for low thrust engines such as the RCS, while Aerozine 50 seems to be preferred for higher thrust engines.

Also, it is clear that the advantage of hypergolic fuel in systems such as the LM are useful because they don't need an ignitor, so that is one less thing that can fail. But aren't hypergolic propellants generally toxic? What's the advantage in using them in rockets like the Titan II?

Was Reynolds correct in his assessment or was liquid fueled rockets the only feasible option early on in the history of rocket development?

If by "early rocketry" we mean the the 1930s and 1940s, the issue there was control. Solid-fueled rockets in the form of fireworks were well understood. Unfortunately solid-fueled rockets of the time typically had very short burn times, very erratic thrust forces, and no ability to shut them off. This makes it difficult to use them in cases where accuracy is important, such as orbital delivery and long-range ballistic missiles.

This persisted in ICBMs fueled with cryogenic and then hypergolic fuels. The degree of accuracy needed for these applications could only be satisfied with liquid-fueled rockets. Nevertheless the storability problem didn't go away, so that was the impetus for research into longer-burning solid fuels and ways of packing it in rocket casings that would ensure steady thrust.

My question: Why is it that MMH seems to be preferred for low thrust engines such as the RCS, while Aerozine 50 seems to be preferred for higher thrust engines.

MMH is denser and slightly more powerful than UDMH, which means you can put it in geometrically smaller tanks to get the same specific impulse. Ultimately the choice of fuel is based on several engineering factors and can't really be tied to any one feature.

But aren't hypergolic propellants generally toxic?

Yes. The chemical properties that make them hypergolic makes them want to bond with a variety of compounds, including the linings of human lungs.

What's the advantage in using them in rockets like the Titan II?

Storability. You can't store LOX in the rocket, which means you need an hour to fuel the rocket before you can fire it. That's an eternity in nuclear brinksmanship, so a rocket that could be fired at the push of a button -- and had the desirable accuracy of liquid fuel -- is a big win. And yes, Air Force people died in the silos in accidents that ended in fuel leaks. Not many, thankfully. Hypergols still represent the best way to achieve both storability and control.

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Thanks.

I saw on a recent science show/channel that some types of ICBMs had to be checked on a regular basis. The silos were in Montana I believe. The show said that the propellent was solid. Anything that was said did not surprise me. But, one thing that really struck me was that when the missile (and all the components such as the solid rocket propellent) were checked, this was done by a sergeant of some type. Is there no officer in the U.S. airforce available to supervise? Is the fate of mankind in the hands of NCOs (rightfully so); or, are all U.S. officers too busy doing , what I saw in my army, nothing?

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In the U.S. Army, the rank of "Technical Sargeant" is required and sufficient for many highly-skilled and highly-specialized jobs. It's doubtful that the same individual would also be qualifed to repair a tank or tune a radio transmitter.

As for the officers, they were too busy giving orders to handle such mundane tasks as ICBM periodic maintenance... :wink:

Officers tend to do the paperwork and management. NCOs tend to be the technicians, they are highly trained in their role.

My experience is with the british forces, Warships are run by Officers and Engineering officers are in charge and have a deep understanding of mechanical engineering, day to day maint and running of actual 'Plant' is done by the Petty Officers and ratings, same for weapons and electronics.

In the civilian world it is the same, In an IT department, the IT director or department manager doesn't actualy build servers or install software.

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I was an Air Force Senior NCO, now retired after 20-plus years. Officers typically do not perform maintenance duties. These functions are performed by junior enlisted personnel and NCOs with extensive training in various specialties. NCOs also provide leadership in the way of first-level supervision and middle management. Rest assured that the sergeant in question was supervised...but likely by a higher ranking NCO, who in turn, was supervised by a senior NCO, who reported to an officer.

For example, when I was a Technical Sergeant working on fighters in an aircraft maintenance unit (AMU), I supervised maintenance operations for my specialty. I reported to the Specialist Flight Supervisor who was in charge of specialized maintenance for the unit. In turn, he reported to a senior NCO, the Production Superintendant, who oversaw all maintenance and launch operations for our unit. The "Prod Super" reported to the unit Maintenance Officer (MO) who had overall responsibility for unit operations. The AMU had over 180 people assigned but had only three officers on staff...the MO, the assistant MO, and a weapons MO.

On an additional note, the Air Force differs a bit from the Army in the utilization of NCOs and officers. It's not at all unusual to find AF senior NCOs in positions that, in the Army, would be staffed by junior officers. The bulk of AF officers are pilots (of course), middle and senior management/leadership, and professionals such as doctors, nurses, and such. I would imagine in a missle complex, maintenance operations would be structured roughly similar to the example above, with a small group of officers assigned to manage overall operations. The officers that are really in the hot seat, though, are the launch control officers.

Cheers...Loreto

Gramma lopeto: Thank you for your thoughts on how this is. I have been in many types of situations where my trained ability is ,at best, better than my officers ability was. As a SGT in my own army I have had grave responsibility as well (but never as great as running a nuclear silo). My point was that with my training and maturity, I do not know if I would have what it takes to do this. However, there are , in this world, only a few that do this eh?

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As an aside, would it have been practical to use liquid hydrogen in the LM to power the descent stage? Or would too much have boiled off on the way to the moon?

Another drawback is that it's not very dense, so it would take up a lot of space.

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Just look how big the ET is to see how much space cryogenics take up. Their advantage is specific impulse, not storability or volume.

And to answer the other question, yes, too much of it would boil off. To give an idea of how fast it boils off, when a Shuttle launch is scrubbed it is actually simpler to dump all the fuel in the External Tank and retank later than to keep topping off the tanks until the next launch opportunity -- even if the next window is only 23 hours away.