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Gorn
2014-Mar-10, 07:55 PM
Hello. I have heard that 'all' rockets require a certain speed to reach orbit. And that it also takes roughly 8-10 minutes. Is it possible to reduce this time to like 4 minutes or even
less?

Might lead to a (greater)degree of engine re-useability.

Bye
SC

ngc3314
2014-Mar-10, 08:07 PM
It depends on what you're launching - the velocity needed is a hard limit, and the faster you reach that velocity the greater the acceleration. Already, some boosters are favored for crew launches (IIRC) because of the gentler acceleration profile. And it's not clear that a shorter period at higher thrust would favor engine life over longer ones at lower power. (clicking numbers) Zero to low circular-orbit velocity takes very roughly 12 minutes at 2g, 6 minutes at 4g (although in reality it matters a lot how fast you can get out of the drag-inducing lower atmosphere, which is why launch profiles go up then over). A Shuttle launch had mean about 2g, peak bit above 3. An Atlas V reaches about 5.5g (impressing me once again with what one can Google thse days).

JustAFriend
2014-Mar-10, 09:08 PM
Is it possible to reduce this time to like 4 minutes or even less?


Do you mind having the passengers reduced to sticky jelly puddles?

And even electronics can only take so many g-forces. That's why we don't use artillery to lob stuff into orbit.

publiusr
2014-Mar-10, 09:59 PM
LEO really isn't halfway to anywhere. At geosynch, it takes very little to escape the Earth--it is just that you don't need to waste energy doing that--simpler just to go "straight on--as in the figure 8 to the Moon.

Solfe
2014-Mar-11, 03:24 AM
I believe the space shuttle actually had to throttle back at various points in the flight to orbit: to protect the ship and then to protect the crew. One was an aerodynamic thing, the other was a crew comfort measure. 8 minutes is really short unless you happen to be riding in the space shuttle. Then it is a very long time.

I would think missiles could get to orbit faster, but if pure speed is your goal, you don't put a man in it. Some missile systems are so fast, they look like Star Wars lasers. Of course, they don't reach orbit, but they are very fast.

Amber Robot
2014-Mar-11, 11:12 PM
I believe the space shuttle actually had to throttle back at various points in the flight to orbit: to protect the ship and then to protect the crew. One was an aerodynamic thing, the other was a crew comfort measure. 8 minutes is really short unless you happen to be riding in the space shuttle. Then it is a very long time.

I believe that the aerodynamic thing was maximum dynamic pressure. Dynamic pressure goes as density times velocity squared. Velocity is increasing as you go up but density is decreasing. There will be a point of maximum dynamic pressure and then it drops off after that. For stability and integrity reasons they wanted to keep pressure from getting too high.

PetersCreek
2014-Mar-11, 11:42 PM
I believe that the aerodynamic thing was maximum dynamic pressure.

Also called Max Q, as one might have heard on mission audio. Speaking of the shuttle, IIRC, even the SRB propellant slugs were tailored so as to moderate thrust until Max Q had passed.

Jeff Root
2014-Mar-13, 12:57 AM
A smallish ICBM interceptor like the aptly-named Sprint had very
high acceleration and reached high altitude very quickly. A photo
of a test flight shows it glowing like a meteor from air friction.
But it was obviously designed for single-use only!

In general, getting to orbit with shorter burn times means larger,
more powerful engines, which can be more costly and have much
shorter useful lifetimes. And it requires a stronger, thus heavier
structure in order not to collapse under the larger accelerations
and aerodynamic loads. It does, however, have a smaller overall
fuel requirement since less time is spent just fighting gravity.

Astronauts could easily take a couple more g's than the Space
Shuttle gave them, since it would only last a few minutes.

-- Jeff, in Minneapolis

Jeff Root
2014-Mar-13, 01:34 AM
The thread title is "Escape Velocity", but the question is about
"speed to reach orbit". Escape speed at a given location is just
the square root of two times the circular orbital speed at that
location.

I like to say orbital "speed" (as you did) rather than orbital
"velocity" when I'm just talking about the speed and ignoring
the direction. I always prefer to say escape "speed" rather
than escape "velocity", since -- surprisingly -- the direction
makes no difference, as long as you don't run into anything.

-- Jeff, in Minneapolis