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Glom
2003-Jan-07, 07:31 PM
The Nuffield Advanced Science Book of Data gives the standard enthalpy change of combustion of Dodecane as -8086.5kJmol^-1. That translates to -47.48kJg^-1.

The standard enthalpy change of formation of water, which results from the combustion of hydrogen in oxygen, as-285.8kJmol^-1. That translates to -142.9kJg^-1.

It seems that hydrogen is a better fuel. It also is more environmental, since it produces only water as a byproduct with no carbon dioxide and it is an easily renewable source of energy.

So why do the largest boosters (the most powerful being the Saturn-IC) use things like RP-1 as their fuel?

John Kierein
2003-Jan-07, 08:17 PM
It took a long time to learn how to safely handle liquid cryogenic hydrogen; especially for manned missions. For a long time the Centaur upper stage was the only user of LH2 and they had many problems making it highly reliable. It was very expensive ground handling equipment. Nowadays the Centaur and the Shuttle both use LH2.

Glom
2003-Jan-07, 08:41 PM
I suppose that in a booster with the combined energy of a small nuclear weapon, you'd want to use safer fuels.

cable
2003-Jan-07, 09:11 PM
India got seduced by the Vulcain engine (cryogenic), used on Ariane-5, and ready to pay big bucks to buy it. sorry, not for sale.
what possible goals that could be achieved by cryogenic engine only ??

http://www.esa.int/export/esaLA/ASELVQI4HNC_launchers_0.html
a nice animation here:
http://www.snecma-moteurs.com/fr/produits/moteur_fusee/vulcain.swf

John Kierein
2003-Jan-08, 10:17 AM
The shuttle is not only propelled by LH2/LOX engines, it generates electrical power using LH2 fuel cells. The fuel cell storage tanks are built by Ball Aerospace now after they bought the Beech design. I guess these spherical tanks are Beech Balls.

I am a believer in the hydrogen economy. But there are difficulties with cryogenics for everyday practical use. Uninsulated lines cannot be touched and actually condense oxygen out ot the air which is a high fire hazard. Spilled hydrogen escapes from the earth to outer space, lost forever.

Nice picture of French engineering. They can do such great things. Their nuc plants are superb. The high speed trains are fantastic, (I've ridden them). The Concorde and Ariane are tremendous. Why can't they make cars?

Karl
2003-Jan-08, 10:27 AM
On 2003-01-08 05:17, John Kierein wrote:
The Concorde and Ariane are tremendous.

I've had more spacecraft blown up on Ariane rockets than any other /phpBB/images/smiles/icon_frown.gif

http://rosetta.esa.int/content/news/index.cfm?aid=13&cid=36&oid=31284

kucharek
2003-Jan-08, 10:53 AM
On 2003-01-07 14:31, Glom wrote:
So why do the largest boosters (the most powerful being the Saturn-IC) use things like RP-1 as their fuel?

Other points are that RP-1 is denser than LOX, so you can use a smaller tank. And, the first stage of a rocket need not to be such elaborate on weight saving etc than upper stages. Remember, the achievable velocity change is proportional the the ln of the mass ratio. And for the first stage, empty mass means also the mass of all the other stages. So, the mass ratio is already pretty bad and didn't suffer very much when the first stage is designed heavier.

Harald

cable
2003-Jan-08, 05:37 PM
Nice picture of French engineering. They can do such great things. Their nuc plants are superb. The high speed trains are fantastic, (I've ridden them). The Concorde and Ariane are tremendous. Why can't they make cars?

obviously a car is more complicated than a rocket /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_smile.gif
let me put my question in other words:

we suspect India wants Vulcain engine for it's nuke program.
BUT why they want to go cryogenic ??
couldn't they do it with other fuel ??

John Kierein
2003-Jan-08, 06:39 PM
Actually, the zinc-air fuel cell may be better than hydrogen for automobile propulsion. I hear India wants to go to the moon.

liglats
2003-Jan-08, 08:57 PM
"Why can't they make cars? "

France does make cars - Citroens with a suspension system that makes rocket science look easy, and Peugots, loved by boy racers everywhere.

But only India still makes Royal Enfield motorcycles - if they built a road to the moon, only an enfield could get there and back without a major service. Though a petrol station every hundred miles would be handy... /phpBB/images/smiles/icon_wink.gif

calliarcale
2003-Jan-09, 03:01 PM
Cable, cryogenics are more efficient in the sense that you get more oomph from them. The specific impulse of most cryogenic engines is significantly better than for most kerosene-burning or hypergolic-burning engines. Loosely put, that means you get more thrust out of each pound of propellant. The problem, of course, is that they are a lot harder to work with, and this can make them less efficient from a economic standpoint. That's why there are kerosene-burning rockets and hypergolic-burning rockets still in use today. And solids, of course, which are also less efficient from a specific impulse (Isp) standpoint.

daver
2003-Jan-09, 09:24 PM
On 2003-01-09 10:01, calliarcale wrote:
Cable, cryogenics are more efficient in the sense that you get more oomph from them. The specific impulse of most cryogenic engines is significantly better than for most kerosene-burning or hypergolic-burning engines. Loosely put, that means you get more thrust out of each pound of propellant. The problem, of course, is that they are a lot harder to work with, and this can make them less efficient from a economic standpoint. That's why there are kerosene-burning rockets and hypergolic-burning rockets still in use today. And solids, of course, which are also less efficient from a specific impulse (Isp) standpoint.


People have already pointed out liquid hydrogen's low density as a problem, but in the context of tank size. It's also a problem in producing thrust--you have to pump an enormous volume of it to generate a given thrust.

Anyway, you can divide up a rocket's trajectory into two phases--an initial phase, where you're trying to get the rocket off the ground and out of the atmosphere and moving reasonbly quickly, and a later phase where you're in more or less a vacuum and trying to get up to orbital (or faster) speed. During the first phase, you want thrust. Specific impulse is good, too, but you're primarily interested in thrust. You're going essentially straight up, so you want as much acceleration as possible (subject to other constraints, like aerodynamic loads). During the second phase, you want specific impulse. Your rocket is lighter (you've burned a hecka lot of fuel getting up to altitude), you're adding velocity more than altitude, you've got more time.

Anyway, you can obviously use hydrogen for the first phase, but you get all the problems of hydrogen. Low density, which implies big tanks and bleeding edge turbopumps. Cryogenic fuels, which require insulation and funky materials and handling processes. If you're going for a staged rocket it makes sense to go for a hydrocarbon fuel for the initial phase and save the cryogenic fuels for later (there are obvious counter arguments. Maybe you only want one type of engine; you use 8 of them in your first stage, 3 in your second, 1 in your third, in which case maybe it's better to go cryogenic all the way).