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Romanus
2007-Oct-06, 08:41 PM
From the limited reading I've done on the subject, I've seen that both alcohol (esp. methanol) and kerosene have been used as rocket fuels. However, it seems like kerosene is favored (e.g., Energiya), even though it seems to me that alcohol would be cheaper--ethanol especially--and have a higher impulse due to its lighter molecular weight. Is kerosene preferable because it packs more energy per unit than alcohol, despite its weight?

Thanks in advance!

astromark
2007-Oct-06, 09:32 PM
Hydrogen and oxygen are often used as rocket fuel. What is the solid fuel used in launching the solid boosters of the shuttle.. Its not Kero., or Alcohol.
Kero is used as jet engine fuel. Without atmospheric oxygen there is no ignition. Please expand your question; for I may have missed your point.

Romanus
2007-Oct-06, 09:53 PM
^
I mean, when kerosene or alcohol are used with LOX.

01101001
2007-Oct-06, 09:59 PM
Hydrogen and oxygen are often used as rocket fuel. What is the solid fuel used in launching the solid boosters of the shuttle.. Its not Kero., or Alcohol.
Kero is used as jet engine fuel. Without atmospheric oxygen there is no ignition. Please expand your question; for I may have missed your point.

I thought it was clear that Romanus was asking about liquid fueled rockets, not solid.

Alcohol and kerosene are both liquid fuels. See Wikipedia: Bipropellant rocket (http://en.wikipedia.org/wiki/Bipropellant_rocket). They don't need atmospheric oxygen. They carry their own oxidizer, often liquid oxygen, but also such as hydrogen peroxide or nitric acid.

I don't know enough about the chemistry to know why one would be preferred. As the article says, thousands of combinations have been tried, and many continue to be favored. I can only presume the ones selected have specific benefits for a particular application. It looks like alcohol/LOX is not used so much now.

Kaptain K
2007-Oct-06, 10:36 PM
I think that kerosene is preferred over alcohol because of energy per volume. Alcohol powered motors vehicles make great power but get lousy mileage compared to hydrocarbon powered vehicles.

If I'm wrong, please correct me.

Romanus
2007-Oct-07, 12:49 AM
^
Hmm...I'd heard about the decreased power available from alcohol compared to gasoline, but for some (probably illogical) reason I thought the situation might be different for rocket fuel.

Perhaps the answer is as simple as it seems.

Thanks for the replies, people.

:)

KaiYeves
2007-Oct-07, 02:54 AM
Isn't it obvious? You use kerosene so that your flight crew don't go stupid and drink your fuel tank dry. ;-)

joema
2007-Oct-07, 03:32 AM
...it seems like kerosene is favored (e.g., Energiya), even though it seems to me that alcohol would be cheaper--ethanol especially--and have a higher impulse due to its lighter molecular weight. Is kerosene preferable because it packs more energy per unit than alcohol, despite its weight?...
Forms of kerosene have been used in the Delta, Atlas, Titan I, Saturn IB, and Saturn V.

The V-1 and Redstone (used in early Mercury flights) used alcohol as fuel. Generally the oxidizer for these was LOX.

Kerosene contains about 134,000 BTUs (39,271 watt hours) of energy per gallon, and weighs roughly 6.5 pounds per gallon.

Ethyl alcohol contains about 84,000 BTUs (24,617 watt hours) of energy per gallon, and also weighs roughly 6.5 pounds per gallon.

So the problem is obvious -- alcohol weighs about the same per unit volume, yet only contains about 63% the energy per unit mass. That alone is probably why kerosene is often used, especially in lower stages.

Not sure whether alcohol would be any cheaper, but even if so, it's almost irrelevant. Propellant cost is a small % of the overall operational cost of a liquid-fueled launch vehicle. Therefore small differences in propellant cost (or even not-so-small differences) don't make much bottom line difference in the overall operational cost.

E.g, the total cost of the LOX and LH2 used to launch a space shuttle is about $12,328 for LOX + $206,000 for LH2. This compares to the typical marginal shuttle launch cost of about $100-$200 million.

Kerosene is more expensive the LOX/LH2, but it's still a small fraction of the overall operational cost.

G O R T
2007-Oct-07, 08:38 AM
Kerosene contains about 134,000 BTUs (39,271 watt hours) of energy per gallon, and weighs roughly 6.5 pounds per gallon.

Ethyl alcohol contains about 84,000 BTUs (24,617 watt hours) of energy per gallon, and also weighs roughly 6.5 pounds per gallon.

So the problem is obvious --

Data speaks volumes. Since weight is the overwhelming factor, kerosene wins.




astromark: The shuttle SRB's contain ammonium perchlorate and powdered aluminum. Iron oxides and a binder are used to slow the burn rate, otherwise you would have the worlds largest firecrackers. Nitrocellulose mixtures are used when high impulse is needed, such as in missiles.

astromark
2007-Oct-07, 08:40 AM
[QUOTE=01101001;1083728]I thought it was clear that Romanus was asking about liquid fueled rockets, not solid.

Where did he say that?...
If you want bigger bangs for your $ then solid fuel is the most energy efficient. The launch equation includes such parameters as weight, volume, and thrust obtained. I will bow to the knowledge of NASA. For they have opted for this mix of thrust engines not me. So which does deliver the highest thrust per kilo.? and why don't I know this?....and what is LOX? is it liquid oxygen... Its wrong to assume we all know.. I don't.:)

astromark
2007-Oct-07, 08:42 AM
Thanks GORT... you answered me before I asked...:)

neilzero
2007-Oct-07, 12:30 PM
Yes LOX is liquid oxygen. Does the perchlotate release significant dioxin (deadly at one part per billion) as it burns? Actually LOX and H2 have the most energy per pound, if you ignore the weight of the tanks and the ice that freezes on the tanks.
The solid rockets are less labor intensive, and stay ready instantly when the launch is delayed a day or two, while the LOX and H2 tanks need to have the ice removed from the outside and be topped off. The H2 = liquid hydrogen tank is huge and thus produces more air resistance than the solid rockets. Neil

Romanus
2007-Oct-07, 02:29 PM
Thanks again, all--Joema in particular.

:)

Jerry
2007-Oct-08, 08:17 PM
UDMH (Unsymetric dimethyl hydrazine) and fuming nitric acid are also used as liquid rocket fuels and oxidizers.The are both very corrosive, toxic and barely stable..definitely not for beginners.

Edited to add: UDMH and fuming nitric are both quite dense; so you can get more bang per unit volume than about any other liquid system.

mugaliens
2007-Oct-12, 02:59 PM
I think that kerosene is preferred over alcohol because of energy per volume. Alcohol powered motors vehicles make great power but get lousy mileage compared to hydrocarbon powered vehicles.

If I'm wrong, please correct me.

Also, it's less volatile than alcohol, and the fact that it's denser means more reaction mass.

Specific impulse means a lot, but not everything, when it comes to rocketry. How much fuel you're lifting compared to how much energy it'll provide (weight fractions) means a lot, too. For example, ion rockets have the highest specific impulse, but lousy thrust, and terrible weight fractions. But they're great once you're no longer fighting Earth's gravity and atmosphere.

cjl
2007-Oct-12, 04:17 PM
Data speaks volumes. Since weight is the overwhelming factor, kerosene wins.




astromark: The shuttle SRB's contain ammonium perchlorate and powdered aluminum. Iron oxides and a binder are used to slow the burn rate, otherwise you would have the worlds largest firecrackers. Nitrocellulose mixtures are used when high impulse is needed, such as in missiles.

Actually, the iron oxides are a catalyst used to speed up the burnrate, and the binder's purpose is simply to make it solid instead of a mass of powder, allowing a much more controlled and specific burn profile. Also, the binder acts as additional fuel for the motor, and is actually one of the main gas generating components.

Also, missiles tend to use similar compositions to the solid boosters on spacecraft with the exception that they use very low amounts of metals, resulting in a motor with much less smoke and visible flame. Some do use double base nitrocellulose, but these do not offer significant advantages as far as I know over more standard solid AP based propellants.

kzb
2007-Oct-15, 04:47 PM
Another way to look at it is, ethyl alcohol (the kind you drink) is CH3CH2OH, and methanol (toxic) is CH2OH. Kerosene is a mixture of hydrocarbons, CH3(CH2)nCH3, the salient point being no oxygen atom in the kerosene formula.

Alcohols are partially-oxidised hydrocarbons. They have already lost some potential energy with respect to oxidation compared to hydrocarbons.

It's possible alcohols could still win out if they burnt more efficiently in the engine than kerosene. But they have a lot of ground to make up, and my guess they don't.

Alcohols do have the advantage that they are (potentially) biofuels, that gets green points, therefore perhaps enhancing the political acceptability of space flight. They can be sourced from fermenting plant material.

However it's got to be said that currently the process requires 90% as much fossil fuel energy as you get out of burning the alcohol so it's not that green really.

galacsi
2007-Oct-15, 07:59 PM
Alcohol has an other advantage , the combustion temperature is lower than kerosene. So you can use less costly and more available metals to build your rocket motor.

I think it is the reason why the Germans used a mixture of ethanol with a little water as their fuel for the V2 at the end of WWII.

The power is less than with kerosene but your rocket don't melt in flight.

kzb
2007-Oct-16, 05:50 PM
Good point galacsi.

I forgot to mention in my last post

quote <<and have a higher impulse due to its lighter molecular weight>>

This is not the case -it's the reaction PRODUCTS that come out of the exhaust, which means its the molecular weight of the products that is important, not that of the fuel.

In the case of kerosene and alcohol, both burn to give H2O (MW=18) and CO2 (MW=34). Possibly there is a different mixture of CO and CO2 resulting from the different combustion temperatures but I wouldn't have thought there would be a massive difference in the average MW of products from alcohol and kerosene.

Romanus
2007-Oct-16, 08:47 PM
"Alcohols are partially-oxidised hydrocarbons. They have already lost some potential energy with respect to oxidation compared to hydrocarbons."

Ahh...something like that was always in the back of my mind, at least with respect to auto fuel, and to see it in print finally makes perfect sense.

Thanks again for the answers, all. :)

m1omg
2007-Oct-17, 04:01 PM
I am curious, what is the liquid fuel with most punch, except things like flourine or metallic hydrogen that were never tested and the exhaust is very toxic and are that are unsafe

Larry Jacks
2007-Oct-17, 04:40 PM
Off hand, it's probably liquid hydrogen/LOX. The specific impulse is high but the hydrogen propellant density is low. That means the tanks have to be larger and heavier. Most liquid fueled rockets used a denser propellant such as kerosene for the first stage. However, the Delta IV family uses LH/LOX for the first stage.

cjl
2007-Oct-17, 05:00 PM
The most punch of anything we currently use is LH2/LOX, which has a specific impulse of around 460 seconds ideally. There have been tests of engines using a combination of liquid lithium, liquid fluorine, and liquid hydrogen that have achieved over 500 seconds, but these have extremely toxic byproducts (and fuels), as you pointed out.

joema
2007-Oct-18, 02:28 AM
I am curious, what is the liquid fuel with most punch, except things like fluorine or metallic hydrogen that were never tested and the exhaust is very toxic and are that are unsafe
I think the highest specific impulse bipropellant that's actually been tested in a rocket is liquid fluorine and liquid lithium.

The highest specific impulse tripropellant rocket ever tested used liquid lithium, fluorine and hydrogen, specific impulse about 546 seconds.

These propellants require care in handling but the same chemicals are widely used in industrial facilities.

Other widely used liquid propellants such as nitrogen tetroxide and UDMH are also very toxic.

m1omg
2007-Oct-18, 12:18 PM
I think the highest specific impulse bipropellant that's actually been tested in a rocket is liquid fluorine and liquid lithium.

The highest specific impulse tripropellant rocket ever tested used liquid lithium, fluorine and hydrogen, specific impulse about 546 seconds.

These propellants require care in handling but the same chemicals are widely used in industrial facilities.

Other widely used liquid propellants such as nitrogen tetroxide and UDMH are also very toxic.

But nitrogen teroxide, niric acid, UDHM...release harmless exhaust.
Lithium or fluorine exhaust is another thing...
And UDHM is much less agressive than florine or lithium.
These things will ignite you if you smell it.
In the factories they do not burn it and exhaust the byproducts straight into air.
Not the UDHM.