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  #1  
Old 04-22-2003, 11:08 PM
ghjm ghjm is offline
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Join Date: Apr 2003
How much fuel does a rocket use at launch?

I frequently run across statements like:

"The Space Shuttle uses half its fuel just to get off the ground"
"The Space Shuttle uses 96.2% of its fuel to get one foot into the air"
"The Space Shuttle uses nearly all its fuel in the few seconds immediately after launch"

This does not seem reasonable to me. The solid rocket boosters, which provide a majority of the thrust to get into orbit, burn at what appears to be a pretty continuous rate for a good couple of minutes. Also, the acceleration of the shuttle must be more-or-less continuous since the crew experience a maximum of something around 4 or 5 Gs. If all the fuel got used up in the first few seconds, it would mean (a) most of it is wasted or (b) most of the energy to get to orbit is imparted in the first few seconds, neither of which is easy for me to believe.

Any ideas? I have a colleague who strenuously insists on this assertion because he has been using it in motivational seminars for years, so if I'm going to prove him wrong, I'll need solid references. On the other hand, it's possible I may owe him an apology. :-)

-Graham
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  #2  
Old 04-22-2003, 11:45 PM
scr4 scr4 is offline
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All I can do is agree with your impeccable logic. This article from howstuffworks describes the launch sequence. You can see that the main engines don't even reach maximum thrust till T+60 seconds. I don't know the thrust profile for the SRBs but they burn for 2 full minutes. And here's another thought: if it takes that much fuel just to get off the ground, why not use an umbilical hose to supply the fuel for those few seconds?

I think the quotes are corrupted from Apollo-related information. It's correct to say that the Saturn V used almost all the fuel "just to
get off the ground" (i.e. reach low earth orbit). Tell your colleage to use that as an example instead of the Shuttle. Also, I believe some ICBMs and sounding rockets do have very high and short-duration acceleration (over 100 Gs for a few seconds).

Oh, and welcome to the board!
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  #3  
Old 04-23-2003, 02:29 AM
Duckster Duckster is offline
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You can learn quite a bit from NASA, rather than an unattributed source ...
Quote:
The two SRBs provide the main thrust to lift the space shuttle off the pad and up to an altitude of about 150,000 feet, or 24 nautical miles (28 statute miles). In addition, the two SRBs carry the entire weight of the external tank and orbiter and transmit the weight load through their structure to the mobile launcher platform. Each booster has a thrust (sea level) of approximately 3,300,000 pounds at launch. They are ignited after the three space shuttle main engines' thrust level is verified. The two SRBs provide 71.4 percent of the thrust at lift- off and during first-stage ascent. Seventy- five seconds after SRB separation, SRB apogee occurs at an altitude of approximately 220,000 feet, or 35 nautical miles (41 statute miles). SRB impact occurs in the ocean approximately 122 nautical miles (141 statute miles) downrange.

The SRBs are the largest solid-propellant motors ever flown and the first designed for reuse. Each is 149.16 feet long and 12.17 feet in diameter.

Each SRB weighs approximately 1,300,000 pounds at launch. The propellant for each solid rocket motor weighs approximately 1,100,000 pounds. The inert weight of each SRB is approximately 192,000 pounds.
Source: http://spaceflight.nasa.gov/shuttle/...f/srb/srb.html
Quote:
The three main engines of the space shuttle, in conjunction with the solid rocket boosters, provide the thrust to lift the orbiter off the ground for the initial ascent. The main engines continue to operate for 8.5 minutes after launch, the duration of the shuttle's powered flight.

After the solid rockets are jettisoned, the main engines provide thrust which accelerates the shuttle from 4,828 kilometers per hour (3,000 mph) to over 27,358 kilometers per hour (17,000 mph) in just six minutes to reach orbit. They create a combined maximum thrust of more than 1.2 million foot-pounds.
...

Each space shuttle main engine operates at a liquid oxygen/liquid hydrogen mixture ratio of 6 to 1 to produce a sea level thrust of 179,097 kilograms (375,000 pounds) and a vacuum thrust of 213,188 (470,000 pounds). The engines can be throttled over a thrust range of 65 percent to 109 percent, which provides for a high thrust level during liftoff and the initial ascent phase but allows thrust to be reduced to limit acceleration to 3 g's during the final ascent phase. The engines are gimbaled to provide pitch, yaw and roll control during the ascent.
Source: http://spaceflight.nasa.gov/shuttle/...sme/index.html

You may want to suggest to your colleague that they check their "facts" with NASA.

Your colleague owes you an apology.

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  #4  
Old 04-23-2003, 02:39 AM
Nanoda Nanoda is offline
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Location: Edmonton
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If it used so much fuel to get 1 foot up, I'd make the launchpad 2 feet higher. Duh.
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