Russell McMahon <apptech <at> paradise.net.nz> writes:
 
> > 4) Rocket fuel is not anhydrous alcohol.
> 
> It would esentially be if you used alcohol. Few rockets do nowadays. 
> It has some advantages but it is a relatively low performance 
> propellant compared to viable alternatives. eg even LOX and Kerosene* 
> has substantially superior performance. (* "Jetex" and "RPx" and 
> similar are special kerosenes chosen for their relative purity and 
> absence of fractions which polymerise when heated and goo up your 
> injectors and chamber).

Yes, that's the point. An aluminium block modern engine probably would not like
being driven to the limit with anhydrous alcohol (thermally speaking - the
thermal load would increase by more than 40% going from gas at 8:1 to alcohol
and 15:1) although Indy cars seem to do just exactly that (but then engine life
between overhauls is a few hours). I wonder what copper heads would do here (or
even silver heads for the posh - I hope that silver does not catalyze anything
bad with alcohol and nitrogen from the air - I am no chemist). Was there any
experiment with graphite or composite head engines or such ? At such high
compression (15:1) it is probably best to use direct injection only because the
heat of compression would cause serious problems for mixture compression with a
high efficiency engine (with ceramic or such insets).
 
> > V2 rockets used 75% ethyl alcohol + 25% water
> 
> This was mainly to lower the combustion chamber temperature to suit 
> the materials used. It also helped a relatively rare resource go 
> further. It also didn't overly hurt net performance in this 
> application but modern methods tend (not always wisely) to optimise 
> performance. [[Truax et al BDB** approach compared with systems like 
> eg STS/Shuttle]]

Yes, exactly. That's the point. 'Low' tech is not asked for, what is asked for
is a good match between fuel and engine to maximize efficiency and durability
with a reasonable cost and MTBF.
 
> > And unlike hydrogen it is not really really dangerous or hard to 
> > handle.
> 
> Unless you drink it (whether 85% or anhydrous).
> But, then, liquid Hydrogen is also dangerous to drink, and has no 
> taste that you will ever taste if you do 

Yes, I think that the powers that be are very distressed about the issue of
drinkable fuel and fuelable drink. Come to think of it, setting equally low
taxes on both would fix the problem but that would delete or merge one
government department and lower (not really, by quantity) tax revenue, so that
is more unlikely than a perpetuum mobile. New bumper sticker idea: 'Moonshine
Power!' or 'Drink *or* Drive!' (hey, this is already true ... ) On the other
hand, there are anecdotal stories about torpedo technicians drinking torpedo
fuel on the Internet ... 

Money-wise alcohol is $1.23 to $1.50 / gallon wholesale in the USA (source:
Wikipedia). This is competitive with normal gas both by price and by power
(outside the US). Although alcohol has only about 70% of the heat value of
gasoline the alcohol engines (which do not yet exist in mass) have double the
compression of a gas engine and thus in theory twice the thermodynamic
efficiency. In reality this does not happen but alcohol is certainly used for
racing ... alcohol engines are no wimps for sure.

Peter P.


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