Rocket engine

[ZTDesigninc]

Hi there Everyone,

 

I am new to this forum but certainly looking forward to some good conversation. Anyway really the question ive had on my mind for years now is "What speed do you need to travel to space at"

 

Why I ask is that, this whole strap a big rocket to your bum and try and go as fast as you can model is good for a lot of things but BAD for most things. (Just imagine a house)

 

Now if i was able to build a structure (the size and shape is not important) and strap a couple of small but adequate rocket engines to it, that would give me a terminal velocity of say 500mph, whats to stop me from just reaching this terminal velocity and then carrying on for like an hour until im in space??

 

Am i missing the point, because my friend just laughed, but to me, it means a lot less stress on structures

[Spyman]

The largest parts of a big rocket is the fuel tanks. Lower speed will certainly reduce stress on the structure and eventually you will also reach space but slower speed means burning fuel for a much longer duration, more fuel equals a bigger tank, which will need even more fuel to lift.

 

Saturn V diagram from http://en.wikipedia.org/wiki/Saturn_V

Edited November 29, 2012 by Spyman

[CaptainPanic]

You are not entirely wrong, but for the most part you are. (I'm not trying to make fun of you here).

 

Orbit

There is one good reason why rockets go as fast as they do. They need to get into orbit (wikipedia).

In order to get into orbit, you need such a high velocity that the gravity of the planet Earth cannot pull you back to the surface. This is why satellites and spaceships can stay in orbit without burning their rocket engines once they are up there. The velocity needed for a Low Earth Orbit (often abbreviated "LEO") is about 7.8 km/s. Note that the rockets go straight up at first, but then bend their trajectory to go nearly parallel to the earth's surface. They need to get up to 7.8 km/s in a circle around the earth, so parallel to the surface.

 

But... the further away from the earth you get, the lower the velocity required to stay in an orbit. If you are at 35800km altitude, you only need to go 3.07 km/s. This is called geostationary orbit.

 

And there are orbits where you only need to go as little as 500 mph to stay in orbit. But those are at a very large distance away from earth... and your spacecraft would have to keep going at a steady 500 mph for multiple days to get there... burning its engines all the time!

 

Getting into space is maybe not the hardest part. The most difficult is staying there.

 

Straight up (no orbit)

There is a different way to look at this.

The escape velocity you need to get away from planet Earth is about 11 km/s. At such a huge velocity, the gravity of earth is not strong enough.

 

But if you get away from earth far enough, the gravity just doesn't act on your spacecraft anymore. At a certain distance, 500 mph is sufficient to keep going. However, that is at quite a huge distance. And again, until you get there, you need to keep burning those engines.

 

Conclusion

Basically, in both cases, it is easier to get up to an orbital velocity, so that the Earth's gravity stops slowing you down. And if you want to get away from the planet altogether, you can then slowly increase your velocity (while you are still in orbit). And this is exactly what is done sometimes: a big rocket to get to orbit, and a puny little ion thruster to increase the velocity after that.

[ZTDesigninc]

Well that is an impressive answer and certainly give more food for thought, I really hadn't considered what to do once an object had simply made it into space.

 

Do you have any more information on an ion thruster??

 

You are not entirely wrong, but for the most part you are. (I'm not trying to make fun of you here).

 

Orbit

There is one good reason why rockets go as fast as they do. They need to get into orbit (wikipedia).

In order to get into orbit, you need such a high velocity that the gravity of the planet Earth cannot pull you back to the surface. This is why satellites and spaceships can stay in orbit without burning their rocket engines once they are up there. The velocity needed for a Low Earth Orbit (often abbreviated "LEO") is about 7.8 km/s. Note that the rockets go straight up at first, but then bend their trajectory to go nearly parallel to the earth's surface. They need to get up to 7.8 km/s in a circle around the earth, so parallel to the surface.

 

But... the further away from the earth you get, the lower the velocity required to stay in an orbit. If you are at 35800km altitude, you only need to go 3.07 km/s. This is called geosynchronous orbit velocitygeostationary orbit.

 

And there are orbits where you only need to go as little as 500 mph to stay in orbit. But those are at a very large distance away from earth... and your spacecraft would have to keep going at a steady 500 mph for multiple days to get there... burning its engines all the time!

 

Getting into space is maybe not the hardest part. The most difficult is staying there.

 

Straight up (no orbit)

There is a different way to look at this.

The escape velocity you need to get away from planet Earth is about 11 km/s. At such a huge velocity, the gravity of earth is not strong enough.

 

But if you get away from earth far enough, the gravity just doesn't act on your spacecraft anymore. At a certain distance, 500 mph is sufficient to keep going. However, that is at quite a huge distance. And again, until you get there, you need to keep burning those engines.

 

Conclusion

Basically, in both cases, it is easier to get up to an orbital velocity, so that the Earth's gravity stops slowing you down. And if you want to get away from the planet altogether, you can then slowly increase your velocity (while you are still in orbit). And this is exactly what is done sometimes: a big rocket to get to orbit, and a puny little ion thruster to increase the velocity after that.

 

 

 

 

I guess this all comes down to what type of fuel you choose to use and the efficiency of the engines I reckon.

 

Hi there Everyone,

 

I am new to this forum but certainly looking forward to some good conversation. Anyway really the question ive had on my mind for years now is "What speed do you need to travel to space at"

 

Why I ask is that, this whole strap a big rocket to your bum and try and go as fast as you can model is good for a lot of things but BAD for most things. (Just imagine a house)

 

Now if i was able to build a structure (the size and shape is not important) and strap a couple of small but adequate rocket engines to it, that would give me a terminal velocity of say 500mph, whats to stop me from just reaching this terminal velocity and then carrying on for like an hour until im in space??

 

Am i missing the point, because my friend just laughed, but to me, it means a lot less stress on structures

 

 

 

[CaptainPanic]

Do you have any more information on an ion thruster??

I would google for it, so I don't see any point in doing that for you. I am no ion-thruster-expert, and I have no secret sources of information

 

So, sorry... but I don't have anything more myself.

[ZTDesigninc]

Are you sure you don't have some secret stash of info??? FBI maybe

I would google for it, so I don't see any point in doing that for you. I am no ion-thruster-expert, and I have no secret sources of information

 

So, sorry... but I don't have anything more myself.

[InigoMontoya]

Ion thrusters are very efficient, but they have terrible thrust/weight ratios. Cutting to the important part.... They can't even generate enough thrust to lift *themselves* off the ground, let alone a payload. They're great once you're *IN* space, but they're utterly worthless when it comes to getting you *TO* space.

 

The bigger point being that there's more to a "good rocket" than efficiency. Thrust to weight ratio matters. Availability of materials matters. Environmental impact matters. Lots o' things matter.

Edited November 29, 2012 by InigoMontoya