Why do the space shuttles typically use 104 % thrust?

Question

They throttle back when approaching the point of MADP, but in every shuttle transmission I've heard, standard throttle is 104%. Why? They can set these percentages to mean whatever they want. So why go over 100%? And what the heck did they use to determine 100%?

Answer 1

If your *** was on the line, would you settle for just 100 percent.  I don't think so.

Answer 2

I don't think they use 104%.

Answer 3

"Thrust" depends on two things - the amount of fuel the engine can burn per second, and the amount of fuel the engine can *get* in a second.  

In the initial design of the engine, the fuel pumps were delivering fuel at the original design limit of the engine's ability to burn it - 100% at maximum.  The engines, after further testing, were found they could burn fuel *faster* than originally calculated, and when they "throttle up", it's getting 4% more fuel per second than originally designed.

As I understand it, they left the designation of "104%" because the computer has to tell the pump the rate at which to deliver fuel - which is designated at '104% of maximum.'

Answer 4

Maybe its like Spinal Tap's Amps.

"Nigel Tufnel: The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven and...

Marty DiBergi: Oh, I see. And most amps go up to ten?

Nigel Tufnel: Exactly.

Marty DiBergi: Does that mean it's louder? Is it any louder?

Nigel Tufnel: Well, it's one louder, isn't it? It's not ten. You see, most blokes, you know, will be playing at ten. You're on ten here, all the way up, all the way up, all the way up, you're on ten on your guitar. Where can you go from there? Where?

Marty DiBergi: I don't know.

Nigel Tufnel: Nowhere. Exactly. What we do is, if we need that extra push over the cliff, you know what we do?

Marty DiBergi: Put it up to eleven.

Nigel Tufnel: Eleven. Exactly. One louder.

Marty DiBergi: Why don't you just make ten louder and make ten be the top number and make that a little louder?

Nigel Tufnel: [pause] These go to eleven. "

It's America. Our stuff has to be better. You see, the Russians only go to 100%, but ours goes to 104%.

Answer 5

Because when the space shuttle was initially designed, the engines were given a "set" rate of performance, when they were tested prior to having been used for launches.

When the shuttles started flying, NASA noticed that the shuttle's main engines were operating at 4% higher thrust than expected.  (Thus, when the Columbia shuttle first launched in 1981, mission controllers noticed that the shuttle "lofted" a bit, and that the SRB cutoff was about 10,000 feet  (3,048 m.) higher than they expected.

BTW, when the shuttle's engines light about six seconds prior to liftoff, the main requirement is that all three of the main engines must fire at least 90% of rated thrust, otherwise shutdown of the engines will occur.  (This has happened at least 4 times in shuttle history).

Also, after about 35 seconds, the shuttle's engines are throttled back to between 65%-67% before maximum aerodymanic pressure (Max-Q).

After about 65 seconds, the main engines are again throttled back to 104% rated performance, until about eight minutes after takeoff, when the engines are AGAIN throttled back to 65% thrust to ease the "G-loads" on the vehicle.

And, finally, during the "abort calls", you might hear them say "Single engine press, 109", (which means--late in the launch sequence--if two engines fail, one engine at 109% rated performance can make a trans-Atlantic landing).

And, about 15 seconds later, if all of the engines are performing well, another call is given "Single engine press, 104", which means said trans-Atlantic landing can be made if both engines fail, and that the single operating engine can perform a trans-Atlantic abort on only one engine at 104% rated thrust.

The only abort during a shuttle launch occurred during the 19th space shuttle flight (Challenger-Spacelab 2) in July of 1985, when an "abort-to-orbit" was called.