The LM maneuvres pre docking

[VQ]

To be consistent, I again used a combustion chamber pressure of 7 atm and a 50:1 expansion ratio.  Here's what I got:

O₂ (liquid) + SiH₄ (gas)  --->  334.4 s (vacuum)

N₂O₄ (liquid) + SiH₄ (gas)  --->  322.6 s (vacuum)

N₂O (liquid) + SiH₄ (gas)  --->  298.3 s (vacuum)

Surprisingly I got almost the exact same I_sp using N₂O regardless of whether it was liquid or gas.  Not quite sure why it worked out that way.

What I found a bit unusual with this fuel is that the mixture ratio needed to attain maximum specific impulse is extremely fuel-rich, more so than usual.  It's better to leave most of the hydrogen unoxidized to drive down the molecular weight, even though this results in a significant drop in temperature.  At its optimum mixture ratio the combustion temperature is only about 2700 K.  With O₂ the optimum formula is (ignoring dissociation),

SiH₄ + 0.65 O₂ --> SiO + 0.3 H₂O + 1.7 H₂

Awesome, thanks! Looks like similar performance to ethane, but with a cooler 10 bar boiling point (210 K vs 235 K for ethane). Not sure if SiO is stable at combustion chamber temperatures. SiO and SiO₂ (silica) are both ceramics with melting points above 1600C so there could be issues with fouling the exhaust bell - another reason to run the mixture fuel-rich. I guess the advantages to a silane engine would be carbon-free fuel chemistry for in situ fuel generation and less-toxic hypergolic (maybe) operation.

[Bob B.]

Not sure if SiO is stable at combustion chamber temperatures. SiO and SiO₂ (silica) are both ceramics with melting points above 1600C so there could be issues with fouling the exhaust bell - another reason to run the mixture fuel-rich.

The calculations definitely show that some SiO₂ will be present in condensed phase.

[Bob B.]

C₂N₂ is far better than I expected but that's because it burns incredibly hot, probably too hot.  While everything else burns at about 3000-3400 K, N₂O and C₂N₂ burns at about 4200 K.  Of course we could run oxidizer-rich and bring the temperature down to a more manageable level, but that will lower the specific impulse and defeat the purpose.  We'd be better off using acetylene.

Why not mix in some excess hydrogen? That will cool things down and lower the average molecular weight.

I tried this by simply adding hydrogen without making any changes in the initial amount of N₂O and C₂N₂.  Interestingly, the relative change in T/M is nearly constant as the proportion of hydrogen is increased.  That is, the specific impulse stays about the same.  So while it brings the temperature down, the added complexity of a tri-propellant system doesn't seem worth it in terms of performance.

[Peter B]

They didn't. They use gaseous nitrous oxide, and don't get much performance from it. SS1 only had to hop up above 100 km altitude, and it appears SS2 can't even do that. There's rumors that they're looking at all-liquid systems now.

Really? How could you get anywhere near enough gaseous N₂O on board to burn the fuel? The fuel grain usually requires a liquid N₂O tank of roughly the same size to burn it.

Yes, the hype around SpaceShip One really bugged me, especially all the breastbeating about how great the private sector was. People simply don't understand that just getting to altitudes we call 'space' takes only about 4% of the energy required to stay there, i.e., achieve orbit. Nearly all of the energy in low earth orbit is kinetic, not potential.

Is it fair to be bugged about this, though? I was certainly one of the large number of ordinary members of the public who had no idea that the percentage was as low as 4%. If I'd been pressed for a figure I would have said something closer to 20%.

The thing is, there are all sorts of things about any profession that non-experts simply won't know. When people make uninformed statements about that profession, isn't the best response is to educate people rather than get bugged about their uninformed statements?

[cjameshuff]

Is it fair to be bugged about this, though? I was certainly one of the large number of ordinary members of the public who had no idea that the percentage was as low as 4%. If I'd been pressed for a figure I would have said something closer to 20%.

It's not just random members of the public who are spouting misinformation like this. Branson really should know better than to say nonsense like:

We have reduced the cost of somebody going into space from something like two weeks of New York’s electricity supply to less than the cost of an economy round-trip from Singapore to London.

They haven't reduced anything, they aren't getting anywhere close to orbit with their approach to "spaceflight", yet Branson is bragging about the supposed superiority of SpaceShipTwo.

For what it's worth, even the energy comparison exaggerates their achievement. The rocket equation means they don't just need 25 times the energy input. SpaceShipTwo has a mass ratio of somewhere around 2, half the takeoff mass being propellant. Reaching orbit with a single stage and similar propulsion system would require a mass ratio closer to 40-50. There's a reason SSTO concepts all use exotic materials and liquid hydrogen fuel.

[ka9q]

Apropos to this discussion of hybrid rockets and especially of N₂O as an oxidizer, the recent SpaceShipTwo disaster got me reading up on the hazards of this stuff.

It's not quite as benign as I had thought. While remarkably stable at STP, it can be decomposed with sufficient pressure and temperature -- and its positive enthalpy of formation means it will happily continue the process once started.

The necessary pressure can be supplied by the N₂O itself (because of its high vapor pressure at room temperature) and the ignition temperature can be supplied by any number of unintentional mechanisms that are often all too obvious only after an accident has occurred: adiabatic compression of gas in a pipeline, or the implosion of bubbles in a mixture of gas and liquid.

I know it's much too early to know for sure, but I already strongly suspect that autocatalytic decomposition of N₂O had a lot to do with it. Scaled Composites had one prior fatal accident with the stuff in 2007 during a ground test that simply filled a tank and then expelled it through an injector, with no intended ignition. Three people were killed and more were injured.

[Bob B.]

The latest news is that SpaceShipTwo broke apart when its feathering device accidentally deployed during acceleration.  The reason why is not yet known.  There apparently was no explosion.

[ka9q]

I've seen that too. If the rocket engine and oxidizer tank were recovered intact, then I guess they couldn't have exploded.

Was the rocket still burning when the breakup occurred? At what point?

[Luke Pemberton]

The latest news is that SpaceShipTwo broke apart when its feathering device accidentally deployed during acceleration.  The reason why is not yet known.  There apparently was no explosion.

I arrived home from work (at 7 pm ) to the news. According to ITN news, the feathering device should deploy at Mach 1.4, but deployed earlier in the flight at a little over Mach 1.

[Bob B.]

Was the rocket still burning when the breakup occurred? At what point?

I believe that it was, but don't quote me on that.  The breakup occurred just 11 seconds after detaching from its carrier aircraft.

[cjameshuff]

I've seen that too. If the rocket engine and oxidizer tank were recovered intact, then I guess they couldn't have exploded.

Was the rocket still burning when the breakup occurred? At what point?

That doesn't mean much: the engine's a big nearly indestructible mass of plastic, and the tank's a heavy duty pressure vessel. They both withstood impact with the ground and stayed individually intact. The weak point was the connection between them, an explosion outside the oxidizer tank could easily have separated them without causing much damage to either.

As far as I've heard, they still don't have a reason for the aircraft entering the feathered configuration: the mechanism was unlocked, but the feathering action was not commanded. It might have been forced by the aerodynamic environment just above Mach 1, but it's also possible that an issue with the rocket motor caused it.

[ka9q]

An explosion outside the oxidizer tank wouldn't be very big. The real disaster happens when a detonation wave somehow ripples through the tank itself. N₂O is capable of detonating under the right (wrong?) conditions.

Scaled Composites put out a lengthy white paper on N₂O safety after their fatal 2007 accident. It does seem they got very serious about it.