The LM maneuvres pre docking

[JayUtah]

Indeed, a motor that produces a strong oxidizer as an exhaust is not necessarily good in confined spaces.

[Luke Pemberton]

Indeed, a motor that produces a strong oxidizer as an exhaust is not necessarily good in confined spaces.

It happened aboard HMS Sidon.

[JayUtah]

And perhaps USS Scorpion.

[Luke Pemberton]

And perhaps USS Scorpion.

Interesting reading about the Scorpion. I always found the Indianapolis story interesting, despite the terrible consequences that followed its sinking and the cargo it carried to Tinian.

[JayUtah]

My good friend's grandfather was a USS Indianapolis survivor.  The only story he ever told us about that vessel was about having his wisdom teeth extracted by the ship's doctor.  He has passed away now, but about three years ago I read a book about the tragedy and came away thinking what a stupid way to run a navy.

[Sus_pilot]

Even so, you can never fully empty a propellant feed system simply by running it to fuel "depletion" because the limitations of tanking and plumbing mean some residue always remains.  And one of the post-landing CM safing crews suffered injury from just the very little bit of hypergols remaining in the CM RCS.  That stuff is just plain evil.  I think USAF lost a missile crew or two due to propellant leaks or tanking/detanking mishaps.

Finding propellant formulations that are energetic without being viciously toxic, and which let us have simple, reliable engines is why rocket science is still a going concern.

A good book on the subject is Command and Control by Eric Schlosser.  Story of a real bad day at an Arkansas Titan base.

[Bob B.]

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.

I tested liquid nitrous oxide (N₂O) with liquid propane (C₃H₈).  It seems to work moderately well if we can run the engine at a fairly high pressure.  For example, I selected 34 atm (500 psi) and got a specific impulse of 235.8 s (sea level) with the nozzle expanded to 1 atm pressure.  Of course that's a very high pressure to run at for a pressure-fed system.  If we run at a pressure of 7 atm (which seems to be typical for many pressure-fed systems), we get an I_sp of only about 185 s.  That's pretty poor; we'd be better off sticking with solid propellants.

On the other hand, N₂O and C₃H₈ seems viable for a low-pressure, high-expansion ratio engine.  I selected a combustion chamber pressure of 7 atm and a expansion ratio of 50:1.  Under these conditions I compute a specific impulse of 290.5 s (vacuum).  However, I doubt an amateur would ever need an engine with a 50:1 expansion ratio.

Firestar Technologies is claiming an I_sp 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.

I tested all of these combinations, however I couldn't easily find the thermodynamics properties for liquid phase.  Instead I ran the computations assuming gaseous reactants, which results in higher specific impulse.  Again I used a pressure of 7 atm and a 50:1 expansion ratio.  I first ran the computations for gaseous N₂O and C₃H₈ so we can see how it compares to the liquid phase computations.

N₂O + C₃H₈ ---> 297.6 s (vacuum)

We see that the liquid phase is 2.4% lower.  We can probably assume about the same for the other propellants.  Under identical conditions, here's what I get for the other combinations:

N₂O + C₂H₆ ---> 299.0 s (vacuum)

N₂O + C₂H₄ ---> 301.8 s (vacuum)

N₂O + C₂H₂ ---> 312.1 s (vacuum)

So we can see that 300 s is definitely feasible.

[Allan F]

If they store acetylene and nitrous oxide already pre-mixed, how do they keep the combustion from running back up the line and into the tank?

[cjameshuff]

You use silver with moderate concentration H₂O₂ solutions, such as for naval torpedo turbine motors.  With high-concentration propellant, you need platinum because it has a higher melting point than silver and isn't contaminated by the stabilizers you have to put into high-concentration propellants.  Rocketry generally favors high concentrations due to mass budgets and the ability to delay tanking until just prior to launch.

IIRC, Armadillo Aerospace's early experiments were with HTP using silver-plated ball bearings for the catalyst. They always seemed to be having trouble getting good flow, decomposition rate, and durability. I think the difficulty of getting reliable performance and the difficulty of obtaining high-concentration hydrogen peroxide drove them to liquid oxygen for their later stuff.

I really wonder about the stability of nitrous oxide fuel blend. Nitrous oxide is widely considered "pretty stable", but is capable of explosive decomposition. The dilution might stabilize it, but diluting it with fuel makes me a bit nervous. There's presumably some degree of insult that'll set it off...so how much does it take? Also, what substances catalyze the reaction? HTP is tricky to handle because of the extreme cleanliness requirements required to keep it from decomposing into water and oxygen...potentially in a dangerous runaway reaction. I also wonder how stable the emulsion is...is this something you have to use just after you mix it?

[Bob B.]

Here's some information:

Surrey Research on Nitrous Oxide Catalytic Decomposition for Space Applications

[ka9q]

Correct.  When hydrogen peroxide comes in contact with a catalyst it will decompose into hydrogen and steam and will release heat.

Hydrogen peroxide scares me. Almost anything will catalyze its decomposition, whether you want it to or not. And since heat is also a good catalyst, and the decomposition of the pure stuff liberates enough heat to turn the resulting water into superheated steam... You see the problem. How many submarines has this stuff sunk?

In some ways I'd almost prefer to handle hypergols. They're reasonably stable (or at least N₂O₄ and the organic hydrazines are) so they aren't likely to blow up as long as you keep them apart. So you can protect yourself pretty well with scape suits. There's probably nothing you could wear to protect you against a large tank of high test H₂O₂ that decides to go up.

At least H₂O₂ is more environmentally benign. You can hose down a spill with plenty of water, and the stuff is unstable enough that it will probably decompose into harmless water and oxygen by the time it gets out of your fueling facility.  Hypergol spills are not usually welcomed by the nearby communities. Or the EPA.

[JayUtah]

Hydrogen peroxide scares me. Almost anything will catalyze its decomposition...

Such as shaking the bottle.  A marked hidden risk for launches where acoustic loading is the design driver (and often overlooked until the shake table).

[ka9q]

N₂O + C₂H₂ ---> 312.1 s (vacuum)

My eyebrows went up when I saw this as one of Firestar's mixtures. Acetylene would make a great rocket fuel if not for its habit of exploding at pressures above 2 bar absolute. Even if you stored it at low pressure, which would mean a very big tank, you couldn't pump it into an engine. This instability is why tanks of it actually dissolve it in a solvent like acetone.

It would be ironic (and surprising) indeed if dissolving acetylene in an oxidizer like N₂O actually stabilized it, given that N₂O has a vapor pressure of 51.5 bar at 20C, far above the explosion pressure for pure acetylene.

[ka9q]

Speaking of explosive nitrous oxide mixtures, I vividly remember an incident in Berkeley Heights, NJ when I lived there in the 1980s. A small lab in a commercial part of town specialized in analyzing gas mixtures for the semiconductor industry. Silane, SiH₄ is commonly used in fabrication, and it is often mixed with non-reactive carrier gases (e.g., helium) to dilute it. Several gas supply companies will mix up whatever you want to order.

Gould Semiconductor in Idaho had trouble with a cylinder of this stuff so they shipped it cross-country to New Jersey for analysis. The technician soon realized that the cylinder actually contained a mixture of silane and nitrous oxide. He tried to move the cylinder outside, but it blew up before he could do so. He and several others were killed and the building was set on fire. Because additional tanks with unknown contents were nearby, the entire surrounding area was evacuated for a day or so while they let the fire burn out.

A week or so later, I drove by the burned-out building. Tacked on the remains of the door was a nice, new crisp piece of paper: a violation notice from the EPA. Nice to know they're on the job.

Here are some old news stories about the incident. There were evacuations and bomb squad calls at several other places where these cylinders might have been shipped.

http://www.apnewsarchive.com/1988/Gas-Distributor-In-Fatal-Explosion-Issues-Nationwide-Warning/id-59352939c678357731c90cd23b23f5da

http://www.nytimes.com/1988/03/18/nyregion/3-die-in-blast-at-a-jersey-testing-plant.html

http://articles.latimes.com/1988-03-24/news/mn-388_1_gas-canisters

[ka9q]

I tested liquid nitrous oxide (N₂O) with liquid propane (C₃H₈).  It seems to work moderately well if we can run the engine at a fairly high pressure.  For example, I selected 34 atm (500 psi) and got a specific impulse of 235.8 s (sea level) with the nozzle expanded to 1 atm pressure.  Of course that's a very high pressure to run at for a pressure-fed system.  If we run at a pressure of 7 atm (which seems to be typical for many pressure-fed systems), we get an I_sp of only about 185 s.  That's pretty poor; we'd be better off sticking with solid propellants.

Why not run at the higher pressure? N₂O has a vapor pressure of 51.5 bar @ 20C, so you might as well use it. Propane is only about 8 bar, but you could conceivably use N₂O's much higher vapor pressure to push it out. As long as the propellants leave their tanks as liquids, the propane tank would absorb no heat and relatively little heat would be absorbed by the N₂O tank to boil enough liquid to make the gas to fill the ullage space.

Edited to add: Or you could use a small amount of CO₂ to pressurize the propane tank. Its vapor pressure is roughly the same as N₂O, a little higher actually.

Despite what Firestar says, I'd want a bladder or something to keep them from mixing in the tank...