Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nasa's Shuttle

  1. Nov 4, 2007 #1
    I don't know much about Nasa or their shuttles but i am wondering once they get into space can't they go as fast as they want, or is there something i don't know about.
  2. jcsd
  3. Nov 4, 2007 #2

    D H

    Staff: Mentor

    Vehicles in space still have the same mass as they had on the ground and are still subject to the laws of physics.

    Think of it this way: If your conjecture were true we would have been able to send people to other stars as soon the we learned how to get out of the atmosphere.
  4. Nov 4, 2007 #3


    User Avatar
    Gold Member

    That's not much of an answer. I would wager he knows we can't go to other planets and is looking for an explanation as to why.

    noagname, the only way a spacecraft can go faster is by using fuel to propel it. The big limitation on the shuttle's performance is the amount of fuel it can take with it - which is very little on top of the huge amount it takes just to get to orbit.

    Even if they filled the cargo hold with fuel, this would increase the liftoff weight of the shuttle, meaning it would use more to get to orbit, meaning they would use some of that extra fuel they brought, which would mean their outward journey would be correspondingly curtailed.

    And we are sitll in a gravity well that we'd have to climb out of (using yet more fuel) to head away from Earth.

    And if we wanted to explore once we got somewhere, we'd use more of that fuel to slow down at our destination.

    And then we'd use more fuel to break out of orbit there, and again to get into orbit here on Earth.

    That's the big limit: fuel.
  5. Nov 4, 2007 #4
    well see if you forget about the amount of fuel
    I am asking can the shuttle handle going at those speeds
  6. Nov 4, 2007 #5


    User Avatar
    Gold Member

    Yes. If the shuttle had a magical bottomless fuel tank, it could reach a speed arbitrarily close to the speed of light if it fired its engines long enough.

    The occupants, other than getting bored or hungry, would experience no untoward effects. Indeed, they cannot demonstrate they are going fast at all except in relation to nearby objects.
    Last edited: Nov 4, 2007
  7. Nov 4, 2007 #6
    just wondering what would happen after it hit the speed of light
  8. Nov 4, 2007 #7

    D H

    Staff: Mentor

    Well gee, thanks. Was that really necessary?

    I interpreted the OPs question as "once a vehicle gets into space it is weightless. That means it can accelerate as fast as it wants and go as fast as it wants, or am I missing something?" The main thing he was missing was that the spacecraft still has mass. It still has weight too, and quite a bit. The actual weight of a spacecraft in low Earth orbit is nearly the same as the weight of the vehicle on the surface of the Earth.

    noagname, when people say things are weightless in space, what they really mean is that things have no apparent weight in space. The actual weight of a spacecraft in low Earth orbit is nearly the same as the weight of the vehicle on the surface of the Earth. Even more importantly, the vehicle in orbit has exactly the same mass that it had on the surface of the Earth. This means that Newton's third law is just as restrictive in space as it is on the surface of the Earth.

    Getting to low Earth orbit is just part of the battle. A vehicle has to flinish climbing out of the Earth's gravitational well just to begin going somewhere else in our solar system. As DaveC mentioned, that takes fuel. Then it has to use more fuel to get where it wants to go.

    Here's where things get really nasty: There isn't any fuel in space. All of the fuel needed has to be carried up into space along with the vehicle. A lot of fuel is needed just to launch all the fuel the vehicle needs once it gets into orbit. The vast majority of the launch mass of a vehicle is fuel. Look at the Apollo missions. In order to send a fairly small vehicle to the moon we had to build the biggest vehicle ever built by mankind.
  9. Nov 4, 2007 #8
    ok thanks
  10. Nov 4, 2007 #9
    i've been told that if you get close to the speed of light and lets say you've loadsa feul left, so you burn it in hopes to accelerate further that instead of reaching [or exceeding] c, you'll just gain mass!!
    i do not understand what i've just said!! i'm just repeating something someone once said to me!
  11. Nov 4, 2007 #10


    User Avatar
    Gold Member

    As you get closer to c, yes your mass will increase, and it will take more and more force to accelerate further (you will not experience any change within your ship - though the universe around you will start to look strange). You will never reach c but you can get artibitrarily close. If you then came back to your starting point, you would find that hundreds or thousands or even millions or years had passed.
  12. Nov 5, 2007 #11
    ok then
    so i am guessing that is how you would time travel
  13. Nov 6, 2007 #12


    User Avatar
    Gold Member

    Well, it only works in the forward direction. You can never go home again.

    i.e. You would accoomplish the same thing with a good suspended animation technology.
  14. Nov 6, 2007 #13
  15. Nov 6, 2007 #14


    User Avatar
    Homework Helper

    Getting back to the original question, it's an issue of fuel to payload. An "ideal" single stage rocket starts off as 91% fuel, the rest being the rocket and it's payload. The advantage of multiple stages is a reduction in the mass of final rocket stage and payload.

    Space shuttles are designed to only acheive a low earth orbit around 17,300mph (a bit more to get to the space station). There are 2 main stages and a detachabe fuel tank. The pair of solid rocket boosters can be considered to be the first stage. The shuttle also has 3 engines and that big oxygen / hydrogen tank that detaches (and burns up on re-entry). This is just barely beyond the outer edge of the atmoshpere.

    In constrast, the Apollo space craft had 3 main stages, to get to the moon, then a lunar module and a lunar lander to return. The Apollo spacecraft went a bit over 24,000mph, not quite escape velocity, but enough to orbit far enough out to where the moons gravity had more force than the earth's gravity. Fastest speed a man has traveled was around 24,700mph just before re-entry from one of the Apollo missions.

    Sattellites, which are relatively small payloads, exceed escape velocity during launch.
  16. Nov 6, 2007 #15
    i am taking it that hydrogen is a very flammable fuel
    why do they need that much fuel for re-entry couldn't they just use the earth's gravity
  17. Nov 6, 2007 #16


    User Avatar

    Staff: Mentor

    Hydrogen, by mass, is the best fuel.

    And they don't use much fuel for re-entry - just enough to re-shape the orbit so it crosses into the atmosphere.
  18. Nov 6, 2007 #17


    User Avatar
    Homework Helper

    liquid hydrogen + liguid oxygen are the most overall "efficient" fuels for rockets.

    Note I stated return, not re-entry. On the return trip, they only needed enoug fuel to transtion from a moon dominated gravitational pull to an earth dominated one. My guess is the reason Apollo 10 had the fastest return speed was because the moon was further away from the earth when Apollo 10 transitioned into the Earths pull, and so there was more distance to "fall" before re-entry.
  19. Nov 6, 2007 #18
    re-entry is right before they enter earth's atmosphere
    and return is heading to earth right
  20. Nov 7, 2007 #19


    User Avatar
    Science Advisor

    In terms of what? Liquid hydrogen has a lot of pluses to it, but I could argue that H2 is the least efficient because its low density requires a large volume and thus requires large storage tanks. That increases vehicle weight and thus requires more fuel. It also requires, usually, jacketing of all of the piping that handles it in the cryogenic state. In terms of the overall system requirements, that's a killer.
    Last edited: Nov 7, 2007
  21. Nov 7, 2007 #20


    User Avatar
    Homework Helper

    Higher density fuels are used for the first stage(s) of a multi-stage rocket, but the later stages, which the first stage has to accelerate, use lighter fuels. The final stages are almost always liquid hydrogen and liquid oxygen.

    I'm not sure what fuel is used to slow down the shuttle for reentry. The small maneuvering thrusters use a fuel that doesn't have the storage issues of hyrodgen and oxygen, but it's toxic.


    Different fuels can be used for different purposes. Hydrogen Peroxide, 80% or more pure, only requires silver as a catalyst and is one of the few single component fuels. An example usage:

    H2O2 usage.avi
    Last edited: Nov 7, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Nasa's Shuttle
  1. NASA at home (Replies: 1)

  2. Lost Shuttles (Replies: 10)

  3. Space Shuttle Tiles (Replies: 37)