Author Topic: The LM maneuvres pre docking  (Read 71013 times)

Offline JayUtah

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Re: The LM maneuvres pre docking
« Reply #45 on: October 03, 2014, 11:01:40 AM »
Spark plugs don't work very well in a vacuum.  ;D

So yes, large (pump-fed) liquid- and solid-fueled rockets use hypergolic igniters or similar.  I've had two separate visitors to the state in the past four weeks, and that has meant two trips up to the ATK rocket park where you can see the length of the grains in some of these large SRM casings.  The igniter typically has to produce a flame that travels the length of the grain's internal cavity within a few milliseconds.

I need to take a picture of the shuttle SRM segment without its grain.  (With a banana for scale.)

I've spent substantial time around jet engines hearing them start up.  I'm quite familiar with the sound of the igniters, but that's the first time I've heard the equipment called a "banger box."  I'll add that to my Jay-is-cool vocabulary.
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Offline JayUtah

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Re: The LM maneuvres pre docking
« Reply #46 on: October 03, 2014, 11:07:34 AM »
I normally don't gush, but anyone interested in jet propulsion should check out AgentJayZ's YouTube channel.  He's a jet engine mechanic in Canada, and he takes the time to really get into the (literal) nuts and bolts of how jet engines are built.

https://www.youtube.com/channel/UCh57rwk3ySElDpzgCDLh9KA
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Offline cjameshuff

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Re: The LM maneuvres pre docking
« Reply #47 on: October 03, 2014, 01:46:16 PM »
It depends on mission duration, but I suspect that you need solar panels to do better than fuel cells; even lithium primary batteries probably wouldn't cut it alone for more than a very short mission.

A metal fuel cell of some variety (something like an aluminum or liquid alkali metal fuel cell) might perform better by avoiding the requirement for high pressure gaseous hydrogen or deeply cryogenic liquid hydrogen tanks. It would definitely be easier to use for long mission durations.


NASA has sponsored development of replacement propellants for some time, and most of the promising combinations use nitrous oxide, N2O as oxidizer. Despite being made of the same elements as nitrogen tetroxide, nitrous oxide is vastly less toxic and corrosive; in fact, it's widely used as an anesthetic. But it's usually not hypergolic, so any engines that use it will need an ignition system. Its big advantage over liquid oxygen is that, like nitrogen tetroxide, it can be stored as a liquid at room temperature but under considerably more pressure.

Though as Virgin Galactic found out, high pressure nitrous oxide can detonate and is shock sensitive. This can be mitigated by mixing in other gases such as nitrogen, but this further dilutes what was already a less effective oxidizer.

One other promising propellant is an ionic liquid based on hydroxylammonium nitrate, which is toxic, corrosive, and may be carcinogenic (though probably not as bad as hydrazine or N2O4), but has a high enough vapor pressure that there isn't a major fume hazard and is a monopropellant that gives better performance than hydrazine monopropellant.

Offline Bob B.

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Re: The LM maneuvres pre docking
« Reply #48 on: October 03, 2014, 02:17:39 PM »
Adding to what ka9q said about hypergols:

After WW2 it was believed that ballistic missiles would be a big part of future warfare, so research was conducted to identify the most likely propellant combinations.

For long range missile everybody agreed that LOX was the optimum oxidizer.  For the fuel, liquid hydrogen was obviously the best performing, but it was hard to come by, hard to handle, and had an extremely low density.  Alcohol, gasoline and kerosene all worked pretty well and were easier to deal with.  Kerosene would win out, but new clean-burning formulations had to be developed.  And, in time, the US perfected the use of liquid hydrogen.

For uses such as JATO and short-range tactic missiles, storable propellants were desired.  Available oxidizers were nitric acid, hydrogen peroxide, and nitrogen tetroxide.  Hydrogen peroxide and nitrogen tetroxide both froze at temperatures likely to be experienced at high altitude or in Siberia, thus nitric acid was the leading candidate.  Which fuel to use was much less clean-cut.  There were many candidates that all had similar performance.  Only hydrazine stood out - it was hypergolic with the prospective oxidizers, it had a high density, and its performance was better.  But its freezing point (1.5o C) was higher than water.

Prior US experience was with aniline and RFNA (red fuming nitric acid).  This stuff was extremely nasty and dangerous.  RFNA was so corrosive that is had to be loaded just before firing.  It produces dangerous and painful burns, and simply pouring it produces dense clouds of highly poisonous nitrogen dioxide.  Aniline was almost as bad.  Everyone agreed a better alternative was needed.

Considerable research and experimentation ensued.  The first breakthrough came in 1951 when contracts were granted to Metallectro and Aerojet to synthesize certain derivatives of hydrazine and to determine their suitability as rocket propellants.  The three derivatives were monomethylhydrazine, symmetrical dimethyl hydrazine, and unsymmetrical dimethyl hydrazine.  The hope was that a very slight alteration to the structure might give it a reasonable freezing point without changing the energetics enough to matter.

Symmetrical dimethyl hydrazine turned out to have too high a freezing point, but the freezing points of monomethylhydrazine (MMH) and unsymmetrical dimethyl hydrazine (UDMH) were -52.4o C and -57.2o C.  After determining their thermodynamic properties, MMH and UDMH were both found to be very good fuels.  MMH was a little denser than UDMH and had slightly higher performance.  But UDMH was less liable to catalytic decomposition and had such good thermal stability that it could be used for regenerative cooling.  UDMH was also more soluble and would tolerate a large percentage of water in the fuel.  Both were hypergolic with nitric acid.  The final decision to concentrate on UDMH was made on economic grounds.  Military specifications for UDMH were first published in 1955.

RFNA was largely hated by all who used it.  Not only for the reasons previously given, but also in cold weather it would react with its aluminum storage drum to produce a slimy white precipitate that would settle to the bottom of the drum.  If this sludge got into a rocket engine it would plug up the injectors.  If stored in a stainless steel drum the results were even worse - the corrosion was faster and the acid's performance was seriously degraded.

Mixed acids and WFNA (white fuming nitric acid) were tried, but they weren't much better than RFNA, though at least they didn't produce the deadly clouds of nitrogen dioxide.

The fourth possibility was nitrogen tetroxide.  Although it is poisonous, if you can avoid handling it in the field, this doesn't matter much.  As long as nitrogen tetroxide is kept out of water it is practically noncorrosive to most metals.  In fact, ordinary mild steel would do.  Thus the tanks of a missile could be filled at the factory and the operators would never have to see, smell, or breathe the nitrogen tetroxide.  And it was perfectly stable in storage and didn't build up any pressure.  The only problem was that it froze at -9.3o C, which the military would not accept.

A major breakthrough came in 1951 when a chemist named Eric Rau thought that a coating of fluoride on stainless steel might protect it from the acid.  It was found that adding 0.5% hydrofluoric acid to WFNA reduced the corrosion rate of steel by a factor of 10 or more.  As it turned out, HF was even more effective at inhibiting the corrosion of aluminum, and it was just as good with RFNA as with WFNA.

With the corrosion problem solved, the military now had an oxidizer that could be loaded at the factory and had a low enough freezing point to meet their requirements.  Military specifications were written up in 1954.  The terms RFNA and WFNA were thrown out and the designation "Nitric Acid, Type I, II, III, and IV" were adopted.  These contained 0%, 7%, 14% and 21% nitrogen tetroxide respectively.  HF inhibited acid was designated Type I-A, II-A, III-A, and IV-A, and contained 0.6% HF.

It was type III-A that gradually edged out all the others and became the nitric acid oxidizer.  Engineers came to call it IRFNA, or Inhibited Red Fuming Nitric Acid.

IRFNA was widely used in tactical missile, where freezing point of the propellant matters.  For strategic missiles, which are fired from hardened and heated sites, nitrogen tetroxide, with its somewhat greater performance, is the oxidizer used.  Nitrogen tetroxide is typically mixed with nitrogen oxide to depress the freezing point.  The resulting oxidizer in called MON (mixed oxides of nitrogen) and is given a number (e.g. MON-10, MON-25) designating the percentage of NO in the mix.  When "nitrogen tetroxide" is listed as the oxidizer it is likely actually some form of MON.

For the Titan missile, Aerojet developed Aerozine 50, a 50-50 hydrazine-UDMH mixture.  Aerozine 50 has better performance than straight UDMH but is still stable enough to be used for regenerative cooling.  Aerozine 50 was also the fuel of choice for Apollo.

For anyone wanting to read more, I recommend the book Ignition, An Informal History of Liquid Rocket Propellants by John D. Clark.
« Last Edit: October 03, 2014, 02:30:44 PM by Bob B. »

Offline Bob B.

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Re: The LM maneuvres pre docking
« Reply #49 on: October 03, 2014, 03:24:37 PM »
Hypergolic fuels aren't much used in launch vehicles anymore (the Proton is one of the last, and I think the North Koreans are using them)

The Chinese still use them as well; Chang Zheng 2, 3 & 4 all used nitrogen tetroxide and UDMH in its main stages.  It's my understanding that their next generation launch vehicle, Chang Zheng 5, will use LOX and kerosene.

Offline Bob B.

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Re: The LM maneuvres pre docking
« Reply #50 on: October 03, 2014, 03:35:13 PM »
NASA has sponsored development of replacement propellants for some time, and most of the promising combinations use nitrous oxide, N2O as oxidizer. Despite being made of the same elements as nitrogen tetroxide, nitrous oxide is vastly less toxic and corrosive; in fact, it's widely used as an anesthetic. But it's usually not hypergolic, so any engines that use it will need an ignition system. Its big advantage over liquid oxygen is that, like nitrogen tetroxide, it can be stored as a liquid at room temperature but under considerably more pressure.

N2O is also the poorest performing oxidizer that I've run computations on.  All that extra nitrogen drives up the mean molecular weight of the exhaust, thereby hurting performance.

Offline ka9q

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Re: The LM maneuvres pre docking
« Reply #51 on: October 04, 2014, 04:45:21 PM »
N2O is also the poorest performing oxidizer that I've run computations on.
Firestar Technologies is claiming an Isp of 300 s for their nitrous oxide fuel blend (NOFBX), a monopropellant consisting of an emulsified mixture of N2O and ethane, ethene or acetylene. It sure sounds explosive to me, but apparently it's pretty stable.

Offline ka9q

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Re: The LM maneuvres pre docking
« Reply #52 on: October 04, 2014, 04:48:06 PM »
N2O is also the poorest performing oxidizer that I've run computations on.  All that extra nitrogen drives up the mean molecular weight of the exhaust, thereby hurting performance.
But there's plenty of N2 in the exhaust of any hypergolic rocket using a hydrazine and/or N2O4.

Offline Luke Pemberton

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Re: The LM maneuvres pre docking
« Reply #53 on: October 04, 2014, 06:16:14 PM »
It sure sounds explosive to me, but apparently it's pretty stable.

Sure does sound pretty explosive. All those pi electrons screaming out for attention from the electrophilic nitrogen. Ethene and ethyne are strong Lewis bases and one would expect such a mix to be slightly temperamental. I was quietly surprised that the mix needs a catalyst bed to initiate the reaction.

I was also quite intrigued that N2O mixes were tested by German scientists prior to World War 2, but with ammonia rather than ethene or ethyne.
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Offline Bob B.

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Re: The LM maneuvres pre docking
« Reply #54 on: October 04, 2014, 07:55:47 PM »
But there's plenty of N2 in the exhaust of any hypergolic rocket using a hydrazine and/or N2O4.

There's plenty of N2 but not in the proportion that N2O produces.  Consider for instance N2O4 vs. N2O when burned with hydrazine.

N2O4 + 2 N2H4  --> 4 H2O + 3 N2

2 N2O + N2H4  --> 2 H2O + 3 N2

The mean molecular weight of the exhaust in the first example is (4*18+3*28)/7 = 22.3, and in the second case it's (2*18+3*28)/5 = 24.0.  That's enough to make about a 4% difference in exhaust velocity, i.e. (22.3/24)1/2 = 0.964.  Granted, that's not a big difference but it's there nonetheless.  Every sample computation I've made in which N2O was compared against another oxidizer with the same fuel, N2O has had the worst performance.
« Last Edit: October 04, 2014, 07:58:27 PM by Bob B. »

Offline ka9q

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Re: The LM maneuvres pre docking
« Reply #55 on: October 04, 2014, 08:04:05 PM »
Every sample computation I've made in which N2O was compared against another oxidizer with the same fuel, N2O has had the worst performance.
Oh, I can believe that. But 300 s for a monopropellant (or a stable mixture of propellants) is pretty impressive, if true.

It's interesting that the relatively stable N2O has such a high enthalpy of formation: +82.05 kJ/mol vs +50.63 kJ/mol for straight hydrazine (yes, I know their molecular weights aren't the same). Even N2O4 is only +9.16 kJ/mol.

N2O does have the advantage of not producing ammonia when it decomposes. Ammonia has an enthalpy of formation of -46 kJ/mol, so I can see why it's preferentially produced over the elements when hydrazine is decomposed.

Offline Bob B.

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Re: The LM maneuvres pre docking
« Reply #56 on: October 04, 2014, 09:06:46 PM »
But 300 s for a monopropellant (or a stable mixture of propellants) is pretty impressive, if true.

300 s for a monopropellant sounds awfully high.  I'd be interested to know exactly what concoction they say will give that.  A few years ago I did some calculations for straight N2O and came up with only about 170 s.  Hydrazine as a monopropellant yields about 230-240 s.

Offline ka9q

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Re: The LM maneuvres pre docking
« Reply #57 on: October 04, 2014, 10:38:06 PM »
But 300 s for a monopropellant (or a stable mixture of propellants) is pretty impressive, if true.

300 s for a monopropellant sounds awfully high.  I'd be interested to know exactly what concoction they say will give that.
It's a claim by Freestar, and it sounded awfully high to me too. I also had trouble believing that their mixture wouldn't be dangerously explosive. Still, with the right fuel N2O (bipropellant or mixed monopropellant) could provide pretty good performance. How about acetylene dissolved in acetone or acetonitrile or some other solvent that would get its volume density acceptably high without pressurizing it too much?

Offline ka9q

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Re: The LM maneuvres pre docking
« Reply #58 on: October 04, 2014, 10:48:39 PM »
Bob, as a baseline how do propane (or LPG) and N2O perform as bipropellants? I've always thought they would be a good choice for amateur high-power rocketry since both are readily available, liquids at room temperature under reasonable pressure that can be used instead of pumps, and essentially stable and nontoxic.

Offline Bob B.

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Re: The LM maneuvres pre docking
« Reply #59 on: October 04, 2014, 10:49:24 PM »
... a monopropellant consisting of an emulsified mixture of N2O and ethane, ethene or acetylene.

I'm not sure that's really a monopropellant.  It might be stored in one tank, but both an oxidizer and a fuel are present.  N2O is an oxidizer and ethane, ethene and acetylene are fuels.  Surely combustion will occur as if we had a bipropellant system.  This would explain the high specific impulse.