# The acceleration of a rocket

## Main Question or Discussion Point

i heard about that when we lauch a rocket, the astronaut must endure acceleration which can be many times of g.
why cant we accelerat it unceasingly for longer time and more slow down the burning of fuel.

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russ_watters
Mentor
There is no way to avoid positive Gs, since any acceleration at all causes them. The shuttle accelerates for about 9 minutes, and taking longer would mean doing more work against gravity (as if it were hovering). The space shuttle only subjects the astronauts to 3 g's, which really isn't that much. You can get that at a carnival ride. Re-entry for Mercury astronauts had them at 12.

but the work it depend on F and distance,why does it count on time?

Hootenanny
Staff Emeritus
Gold Member
but the work it depend on F and distance,why does it count on time?
The work done also depends on time. A rocket is propelled forward by ejecting fuel downwards, to eject the fuel the fuel must be accelerated and hence work must be done on the fuel. The longer the flight, the longer the rocket must keep ejecting the fuel.

tiny-tim
Homework Helper
but the work it depend on F and distance,why does it count on time?
I think it's a bit like Alice through the looking glass, who was told that you have to run very hard just to stay still.

If the rocket hovered for an hour just above the launch-pad, and then carried on, that would take a huge amount of extra energy, even though no distance was involved, because in gravity you have to work very hard to stay still!

The sooner you get to where the gravity is less, the less of this Alice-type work you have to do!

thank you for responds!

i think i can quite understand the question now.

i just wonder if i can ride on the rocket, since someone told me that it's impossible for ordinary people to suffer it.

tiny-tim
Homework Helper
thank you for responds!

i think i can quite understand the question now.

i just wonder if i can ride on the rocket, since someone told me that it's impossible for ordinary people to suffer it.
I think you have to go through lots of tests first - some people get blackouts, or are sick (which is a big problem in "zero" gravity!), or have weak hearts.

But I expect its ok for most people.

(erm … if someone's selling you a cheap ticket, I would check the watermark! And … it just so happens I have a spare one which I could let you have at a discount … )

it just so happens I have a spare one which I could let you have at a discount … )

appreciate it! please notify me if it happens!

tiny-tim
Homework Helper

appreciate it! please notify me if it happens!
It's a really good ticket, it's for a seat by the exit, so there's plenty of leg-room.

The only problem is … if the rocket breaks down, you're the one who has to get out and push!

It's a really good ticket, it's for a seat by the exit, so there's plenty of leg-room.

The only problem is … if the rocket breaks down, you're the one who has to get out and push!

do you mean the rocket's exit?

HallsofIvy
Homework Helper
There was a rather silly program a few years ago starring Andy Griffith as a Junk dealer who goes to the moon to salvage debris left there by NASA (go figure the "profit" margin on that one!). They went through a rather elaborate explanation in the first program about how they are going to use a low but constant acceleration to finally reach the high speed necessary without stress on the crew and reducing the cost of fuel. Ignoring the fact that fuel use would be the same, a "low but constant acceleration" won't work for two reasons: 1) the earth's gravity falls off as 1/r2: the faster you can move away from the earth, the better; 2) the atmosphere causes a lot of friction: the faster you can get out of the atmosphere, the better.

I have a friend who was recently on a "safari" in Kenya. While looking at a pride of lions, the little bus got stuck in the mud. There was a lot of discussion about who was going to get out and push! I'm sure the lions were rooting for a fat person. (Of course, they radioed for another bus that came and pushed them out.)

I think I remember reading that the main purpose of the SRBs on the Space Shuttle is to use their high (but relatively short duration) thrust to get the space shuttle clear of the thickest part of the atmosphere as quickly as possible - then the rest of the shuttle's main engine fuel is used to start hauling butt tangentially to achieve orbital velocity.

Funny aside: I was listening to one of the MIT lecturers on the Space Shuttle available on iTunes, and one engineer guest lecturer that had worked on the program once had an opportunity to talk with a Russian counterpart whom had worked on Buran. Because of the orientation of the Shuttle's launch pad (which was originally used for Apollo) it has to do that roll maneuver soon after launch to orient itself properly for an eastwards trajectory. The engineer had noticed the Russians had done the same thing on their single unmanned Buran launch, even though their launchpad design made it unnecessary. In response to the Shuttle engineer asking why the Buran did this maneuver, the Buran engineer's response was "Because you did."

mgb_phys
Homework Helper
They went through a rather elaborate explanation in the first program about how they are going to use a low but constant acceleration to finally reach the high speed necessary without stress on the crew and reducing the cost of fuel.
I remember that show - wish I could remember what it was called.

The total energy needed to put something into orbit is the same but using a few 1000 tons of explosive to do it isn't the most efficent way.
If you had a motor that provided a small thrust for a long time you would do better, and atmospheric friction isn't much of a problem if you are only doing 50mph vertically.

rcgldr
Homework Helper
but the work it depend on F and distance,why does it count on time?
Rocket engines produce thrust by accelerating burnt fuel, the fuel undergoes very high acceleration one direction, while the rocket is accelerated at a moderate speed in the opposing direction. Work is done on both the fuel and rocket, and this is independent of speed and distance the rocket moves, once in outer space, and close to this while in the atmoshpere. In the case of rockets there is a "specific impulse" (thrust x time) per unit of fuel.

http://en.wikipedia.org/wiki/Rocket

Janus
Staff Emeritus
Gold Member
I remember that show - wish I could remember what it was called.
It was called Salvage 1

tiny-tim
Homework Helper
Work is done on the fuel

Rocket engines produce thrust by accelerating burnt fuel, the fuel undergoes very high acceleration one direction, while the rocket is accelerated at a moderate speed in the opposing direction. Work is done on both the fuel and rocket {snip}
I agree the total work done on the rocket to get it to a particular height is independent of the time taken or profile of velocities used.

But, although the rocket rises to a particular height, the fuel does not go to a particular height, and the same argument does not apply.

The momentum transferred to the fuel is always substantially more than the momentum transferred to the rocket, because the fuel must produce an acceleration of 1g for the rocket before the rocket even starts to accelerate upward.

This "wasted" momentum depends on the strength of gravity, and the time taken.

It therefore saves a lot of fuel if the time spent "fighting gravity" (which ends once the rocket has achieved sufficient height and sideways motion to be in orbit) is reduced as much as possible.

In other words, if the rocket accelerates as fast as possible.

rcgldr
Homework Helper
In other words, if the rocket accelerates as fast as possible.
Agreed, the faster the rocket accelerates, the less fuel it takes. I was only trying to explain that force times distance of the rocket without considering the acceleration of the fuel is not a good method to describe how a rocket works, in terms of work done.

THX

i ask this question because a classmate asked me in highschool. in that time, our teacher was talking about rocket. and i cant answer him. after so many years, i finally revived it.
hoping he can read this, since i not sure whether i can answer him in person or not.

Danger
Gold Member
Salvage 1 was silly as hell, but I loved it. (Okay, I loved seeing Trish Steward in a jumpsuit.)
Believe it or not, Isaac Asimov was the technical advisor for the show. The show itself was inspired by the life of Bob Truax.
http://en.wikipedia.org/wiki/Robert_Truax" [Broken]

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