Type of Shuttle Fuel?

[skippystars]

Good evening all,<br /><br />With the Space Shuttle Discovery shortly to launch (T-Minus 20 mins/reading now) I'd like to know why they use liquid hydrogen as opposed to normal "jet fuel." In space you can't use "good old fossil fuels" correct, that's always been my assumption?<br /><br />Thanks all & goodluck to Discovery!<br /><br />SK

 

[MeteorWayne]

AFAIK, the Liquid oxygen and Hydrogen gives the most thrust for the weight.<br />I'm not an expert, so get that confirmed. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[vogon13]

Most thrust for the weight is essentially correct.<br /><br />Liquid hydrogen/fluorine is a tad better, but the corrosive nature of the fluorine makes it not worth the extra bother.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[qso1]

They actually use a combination of liquid and solid propellant to get the shuttle to orbit. The solid rocket boosters are loaded with ammonium perchlorate and powdered aluminum along with some chemical binders.<br /><br />The shuttle main engines are fed with liquid hydrogen and liquid oxygen by the external fuel tank. Rocket engines are far more powerful than jet engines pound for pound. Jet engines would also require oxidizer to operate above 35 to 50 thousand feet.<br /><br />Jet engines were proposed in early studies for the shuttle to be able to land with a go around option. The weight of the jets cut too deep into useful payload capacity so the plan was scrapped once it was established (With X-24 and other lifting bodies) that the shuttle could glide to a landing dead stick. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[rfoshaug]

"Good old fossil fuels" can be used, and are used by most Russian rockets as well as the Atlas V rocket (which, in fact, uses Russian engines).<br /><br />Also, the Saturn V used fossil fuels for its first stage. It's a type of kerosene that is not quite the same mix as standard jet fuel, but it's not very far from it.<br /><br />All liquid rocket fuels need two components: the fuel and the oxidizer. If they didn't need the oxidizer to go boom, they would go boom already inside the tank... <img src="/images/icons/smile.gif" /><br /><br />So the main difference between a jet engine and a rocket engine, is that the jet engine gets its oxidizer (oxygen) from the air that goes through the air intake, while the rocket engine carries all its oxidizer inside a tank and can therefore work in a vacuum.<br /><br />Usually, both for hydrogen and kerosene, the oxidizer is liquid oxygen. Other oxidizers like liquid fluorine can be used, but are often not favourable because they tend to be very corrosive, toxic etc.<br /><br /><br />When deciding on which fuel to use for a rocket, one has to consider several factors. The most important one is Specific Impulse (abbreviated Isp), which is measured in seconds. This is a measurement of the efficiency of a rocket engine. For example, the main engines on the Space Shuttle has an Isp of 453 seconds when it is operating in the vacuum of space. This number means that if the engine produced 1 kg thrust and had 1 kg of fuel in its tank, it would be able to produce that amount of thrust for 453 seconds before running out of fuel. The same would be true for 1 lbf thrust and 1 lb of fuel, or 1,000 kg thrust and 1,000 kg of fuel etc.<br /><br />If you double the amount of fuel the engine would of course be able to run for twice that amount of time before running out of fuel, and if you doubled the thrust, it would run for half that amount of time. So the Isp doesn't directly say how long the engines can burn, as this depends on the thrust of the engines and t <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

 

[MeteorWayne]

Very nice and thorough answer. Thanks, I definately learned something today! <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[Swampcat]

I agree with MeteorWayne, except for one point...<br /><br /><font color="yellow">"All liquid rocket fuels need two components: the fuel and the oxidizer."</font><br /><br />Rocket scientists refer to both fuel and oxidizer as "propellants." Oxidizer is one propellant, fuel the other. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[MeteorWayne]

I guess you're right. One is worthless without the other <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[CalliArcale]

Yep, although of course chemically there is a difference between fuel and oxidizer.<br /><br />Propellant = anything that you toss out the back end of the vehicle to make it go forwards, and of course in a chemical rocket, both fuel and oxidizer get shot out the back as a result of the reaction. You can even have non-burning propellants. Ion drives electrically charge gasses, such as xenon, to make them fly out the back of the engine at fantastic speeds without burning anything. These are the most efficient engines of all, but you have to be very patient to get up any speed; the thrust is so slight they can barely flicker a piece of paper. And of course a popular example of a non-burning propellant is water. You can buy little toy rockets that use tap water and a bit of air as their propellant. You just pressurize them with some air from a hand-pump and then let 'em go. <img src="/images/icons/wink.gif" /><br /><br />Confusingly, there are some chemical propellant combinations where it's not entirely clear which is the fuel and which is the propellant....and sometimes the oxidizer is something that has no actual oxygen in it! Chemistry can be fun. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[Swampcat]

<font color="yellow">"One is worthless without the other"</font><br /><br />In general, that's true, but there are such things as monopropellants that don't require two components. Hydrazine is one such monopropellant, but is generally only used for such things as attitude control thrusters. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[Swampcat]

As usual, you explain things better than me <img src="/images/icons/smile.gif" />. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[MeteorWayne]

Good point, Calli.<br />"Fuel" can also include anything thrown out the back end <img src="/images/icons/smile.gif" /><br /><br />F=ma<br /><br />Ain't physics wonderful? <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[rfoshaug]

<blockquote><font class="small">In reply to:</font><hr /><p>Swampcat:<br /><br /><br />I agree with MeteorWayne, except for one point...<br /><br />"All liquid rocket fuels need two components: the fuel and the oxidizer."<br /><br />Rocket scientists refer to both fuel and oxidizer as "propellants." Oxidizer is one propellant, fuel the other.<br /><p><hr /></p></p></blockquote><br /><br />That is correct. I was a bit too quick there. It should have been "All liquid rocket <i>propellants</i> need two components: the fuel and the oxidizer".<br /><br /><br />Calli has explained the difference between propellant, fuel and oxidizer very well - there's nothing to add to that. <img src="/images/icons/smile.gif" /><br /><br />The ultimate goal of any space vehicle is to move forward by tossing something out in the back. <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

 

[CalliArcale]

Glad you liked. <img src="/images/icons/wink.gif" /><br /><br />One popular demonstration of propellant as "stuff tossed out the back" is a guy sitting on a skateboard (with really really really slick wheels -- imagine loads of WD-40 on there) and throwing baseballs in one direction. It'll actually push him forwards, although not very quickly; he weighs a lot more than the baseballs, after all.<br /><br />Scale the baseballs down to individual molecules or atoms, and scale the dude on a skateboard up to a spaceship, and that's the principle behind all rocket propulsion.<br /><br />A recent "Mythbuster's" episode revisited their "Confederate rocket" episode, in which they built a rocket out of salami. (Seriously!) It was a hybrid rocket motor: the fuel was salami, and the oxidizer was liquid nitrous oxide which flowed through the hollow core they'd drilled out of the salami. The first time they did the experiment, they thought they'd succeeded. But when they revisited it and did some actual thrust tests, they found that they'd actually failed. The rocket did go, but not because it was burning salami. The nitrous oxide was coming out of a pressurized bottle under some force; that was their sole propellant. They did eventually get it working right, though. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[MeteorWayne]

I'll have to watch for a rerun of that episode <img src="/images/icons/smile.gif" /><br /><br />A salami rocket!<br />That's a new one on me. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[qso1]

One other thing about jet engines. There are no jet engines I'm aware of in existence that provide the sheer thrust levels of rocket engines. The most powerful jet engines I know of provide about 50,000 to 100,000 lbs thrust a piece. The SSME provides 375,000 lbs thrust at sea level and the Saturn V F-1 provided 1.5 million lbs thrust per engine.<br /><br />Another thing is ISP. Rocket engines are rated by ISP or initial specific impulse which is a power per pound of propellant rating of sorts. The shuttle LH2 LOX combination yeilds around 450 ISP while a hypergolic rocket such as the Titan II provides somewhere around 300 ISP. This rating is not used in jet engines but if it could be known or if someone here knows...jet engines would probably have much less ISP than rockets. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[vogon13]

Since the aircraft derives the oxygen from the atmosphere and does not have to carry it, the Isp of jet engines is enormous. Like 3000 to 5000 if I recall correctly.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[qso1]

Much higher than I would have expected. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[rfoshaug]

Not only do jet engines not have to carry the oxygen with them, but they also get vast amounts of "propellants" (i.e air) for free, stuff that they can toss back to accelerate forward.<br /><br />This air does not move back very fast (exhaust velocity is not very great), but there are huge amounts of it, which the aircraft itself doesn't have to carry.<br /><br />All this gives an aircraft better Isp than just a rocket that doesn't have to carry its own oxidizer. <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

 

[qso1]

Unfortunately, it doesn't do much for jets being used in space where carrying oxidizer probably makes them less efficient or we would not have had to develop rockets at all. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[rfoshaug]

Of course, since a jet engine uses the oxygen in the air as oxidizer and the air itself is an important part of what it tosses back to accelerate forward, it will not work where there is very little or no air. <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

 

[qso1]

Thats for sure. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>