The LM maneuvres pre docking

[ka9q]

I've arrived home to find that this thread is now examining the use of HCN as a fuel. I don't know whether to laugh or feel alarmed at this deviation.

I admit to having my tongue in my cheek when I made that suggestion, but there's a method in my madness. Everybody knows HCN is toxic; it has a long and notorious history. What everybody doesn't know is that many other materials are just as toxic, or worse.

H₂S, for example -- it's just about as toxic as HCN, and a major occupational risk in the oil and gas industry. Yet I still encounter teens who think it would be fun to stink up the place with some. The only real difference is that most people can smell H₂S more easily at low concentrations than HCN.

Hydrazine and N₂O₄ are also of roughly equivalent toxicity, although they all kill or injure by different mechanisms.

[Echnaton]

Sure, what could possibly go wrong with that?

[ka9q]

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.

(ETA) These calculations assume gaseous reactants.

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

[VQ]

I've arrived home to find that this thread is now examining the use of HCN as a fuel. I don't know whether to laugh or feel alarmed at this deviation.

HCN is about as toxic as UDMH, isn't it?

[ka9q]

HCN is about as toxic as UDMH, isn't it?

Yeah, but in different ways. The hydrazines are both acutely toxic and carcinogenic. HCN is infamous for being acutely lethal in sufficient concentrations, but we've evolved mechanisms to safely metabolize trace amounts because it occurs in many foods, e.g., apricot pits, apple seeds, and especially cassava. We Westerners know it mainly in tapioca pudding but it is widely consumed in the tropics.

So chronic low level HCN exposure is probably less hazardous than chronic low level hydrazine exposure.

[Glom]

Who cares?  This is a great discussion.  Makes me wish I'd paid more attention in Chemistry.

You might say it's been a breath of fresh air.

[Bob B.]

Bob, what kind of performance could one get from SiH4 and the usual oxidizer suspects?

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

[Luke Pemberton]

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.

It's difficult to write this without sounding sarcastic, but I genuinely found this snippet very interesting. It's counter intuitive, but makes perfect sense. After reading these discussions I'm beginning to wish I took a different career path.

I'm beginning to feel that I lost sight of why I studied physics. I took a left turn instead of a right as this is much more interesting compared to the technical route I took.

[Bob B.]

It's difficult to write this without sounding sarcastic, but I genuinely found this snippet very interesting. It's counter intuitive, but makes perfect sense.

Yes, it can be a little counter intuitive.  Adding more oxygen drives up the temperature (until reaching a stoichiometric mixture), but that results in heavier products (e.g. CO₂ rather than CO).  Exhaust gas velocity is proportional to (T/M)^1/2, so somewhere there is an optimum point that's hot enough and light enough to produce the maximum velocity.

I generally initially balance the combustion equation with enough oxygen such that all the carbon and hydrogen produce CO and H₂O.  I'll then adjust the amount of oxygen up or down (usually down) until I find the optimum point.  In the case of SiH₄, I kept adjusting down and down, and the exhaust velocity keep going up and up.  I ended with less than half as much oxygen as I started with.  All rockets run fuel-rich but this was extreme.
 

After reading these discussions I'm beginning to wish I took a different career path.

I don't have any regrets with the career I chose, but I definitely wonder sometimes how things would have worked out had I decided to go into aerospace instead.

[Al Johnston]

H₂S, for example -- it's just about as toxic as HCN, and a major occupational risk in the oil and gas industry. Yet I still encounter teens who think it would be fun to stink up the place with some. The only real difference is that most people can smell H₂S more easily at low concentrations than HCN.

Having nearly killed myself with the stuff, I have to agree - it's worth noting that at fatal concentrations you can't smell H₂S at all...

[JayUtah]

I don't have any regrets with the career I chose, but I definitely wonder sometimes how things would have worked out had I decided to go into aerospace instead.

Anything you do for a living becomes your occupation, and it results inevitably into normalizing its erstwhile glamor down to daily drudgery, and associating with it all the subordinate and attendant problems that accompany anything you do for a living.  If you like doing something, and you're good at it, you don't necessarily have to make it your profession in order to enjoy it.  In fact, you may enjoy it more if you keep it "pure" and unsullied by occupational concerns.

[Bob B.]

Anything you do for a living becomes your occupation, and it results inevitably into normalizing its erstwhile glamor down to daily drudgery, and associating with it all the subordinate and attendant problems that accompany anything you do for a living.  If you like doing something, and you're good at it, you don't necessarily have to make it your profession in order to enjoy it.  In fact, you may enjoy it more if you keep it "pure" and unsullied by occupational concerns.

Yep, I agree with that.  I've heard it said that if you really love doing something, make it a hobby and not a profession.  When I first started working full time I really enjoyed it and was a workaholic.  After a while though it just became a job.  That's when I started to develop hobbies outside of my profession.

[Luke Pemberton]

I don't have any regrets with the career I chose, but I definitely wonder sometimes how things would have worked out had I decided to go into aerospace instead.

In fact, you may enjoy it more if you keep it "pure" and unsullied by occupational concerns.

Absolutely. That's why I enjoy reading these boards, I learn so much. More so when hoax theorists turn up. Well, with the exception of awe130. That episode did not really stimulate much more than the blueprint issue, but I understood that already from Clavius and your treatment of Collier's claims.

My original 'regret' was nothing more than 'the stuff here amazes me, I wish I had taken it further earlier in life.'

[Luke Pemberton]

Having nearly killed myself with the stuff, I have to agree - it's worth noting that at fatal concentrations you can't smell H₂S at all...

Phosgene is an interesting gas from a similar perspective. The troops in WW1 would run through clouds of the stuff unaffected, unlike chlorine which has an immediate action on the lungs. It was not until a few hours after exposure that troops had an onset of symptoms.

[ka9q]

And N₂O₄, for that matter. It is certainly acutely quite irritating, but the real damage doesn't appear for a day or so when the pulmonary edema sets in.

The ASTP astronauts brushed off their encounter with it after landing until they happened to mention it casually to a flight surgeon. He knew his stuff, so off they went to the ship's clinic and then a two-week stay in Honolulu for observation. Good call.

The stuff is just plain evil. It even looks evil.