# Double Rocket Propulsion Speed

Discussion in 'Community Discussion' started by Mr. Durden, Dec 28, 2005.

1. ### Mr. Durden macrumors 6502a

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#1
OK. Need some help from our local smart-folk.

Being the holiday season and having a slow time at work, a co-worker and I got into a discussion of space travel. Funny thing, graphic designers discussing rocket propulsion, but I digress.

What I need is the answer to this question: If you were able to get a multi-staged rocket (not sure if thats the correct term) into space without firing it, could you then fire it in a designated direction at oh, say 5,000 MPH, then launch a totally seperate rocket from that original rocket at 5,000 MPH making the second rockets speed 10,000 MPH?

By multistaged rocket, I dont mean the kind that fall apart after its fuel has been spent. I mean a really big rocket, with another totally seperate rocket on its tip (or side or whatever). Once the main rocket has reached its top speed, the second rocket would then launch from the first rocket in an attempt to go faster than it could if it started from a dead stop.

Stupid, I know. But what do you expect from people with a lot of time on their hands?

2. ### XNine macrumors 68040

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Why are you wearing that stupid man suit?
#2
First off, I think you're my alter-ego (we both live in Colorado).

Second of all, if I get the logistics of it, it's like if you're falling with a giant rock above you, you couldn't thrust off of that rock to hit the ground faster. Of course, this might not hold true with NO GRAVITY while in space.

This is an interesting question. But... where the hell would you be firing this rocket to? IS there a purpose or is this just one of those "Hmmm, I wonder if..."
type of things?

3. ### Jesus macrumors 6502

#3
In a word, no, assuming the engines are burning the similar fuel, then the speed at which gases are expelled from the back will be roughly the same, so there would be no difference, you might as well put more fuel in the first engine.

E.g. you have a rocket engine that expels gases from the back at 5000 metres per second, and the rocket will travel at a little less than that speed because of efficientcy, or a lack of it. and if that takes you to your max speed, then you light another rocket engine that expels gases at 5000 metres per second, it will just maintain your speed, not increase it further. If your idea was true, nasa would simply put hundreds of simple solid rocket engines on a huge frame and ignite them one by one.

4. ### After G macrumors 68000

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Aug 27, 2003
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California
#4
Yes, in space.

I've had plenty of physics problems where you're driving a car at 50 mph and you throw a baseball forward at 20 mph and the baseball is going 70 mph because it really was going 50 mph (same as the car) before you threw it.

The reasons NASA rockets don't work this way is because you can't just ignite a stage when the other stage is attached. You have to turn off the stage, have it eject, burn up in the atmosphere, and then ignite the new stage, get it to a thrust where it can push against the force the earth applies to it. And all this time that the rocket is not going, the earth is pulling on the rocket, causing it to slow down. Plus the rocket has to go through air resistance, etc.

*** should read the OP's post ... yes, it would still work with the rocket you describe.

5. ### After G macrumors 68000

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#5
Actually, the speed of the gases is only part of how a rocket engine works. Rocket engines also work by the force they apply (the amount of gas). If a rocket's speed only depended on how fast the stuff from the engine came out, ion drives would be the fastest engines. You can't beat the speed of light.

6. ### Jesus macrumors 6502

#6
I'm trying to simplify this for general consumption.

7. ### Mr. Durden thread starter macrumors 6502a

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#7
Oh, cool. A fellow Coloradoaniteian. Where abouts are you? I'm just south of Denver. Being alter egos of ourselves, does that mean that while I'm at work pondering rocket propulsion, your actually at work working?

The purpose of the question was for some reason we were talking about travel to other planets and stars. It would take too long to travel to any stars with today's technology, so we were trying to figure out how you could speed up space travel. I'm sure that since NASA hasnt been able to solve this problem as of yet, I can, on my spare time at work, come up with a simplistic solution.

8. ### After G macrumors 68000

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California
#8
Oh, okay. I figured a good explanation would be enough, though. I for one find details fascinating, even if I don't quite understand all of them.

9. ### ibook30 macrumors 6502a

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2,000 light years from home
#9
Admirable ! And - hey, when you get the NASA thing worked out, can you figure out who's stealing my newspaper every morning?

10. ### Mr. Anderson Moderator emeritus

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Nov 1, 2001
Location:
VA
#10
I had a nice long reply ready to go and the power goes out - figure that

So here's a shorter version.

You would end up over 10,000 MPH

Why?

Because as you speed along to 5,000MPH with both rockets, you're depleting fuel and the weight you're pushing decreases. Say the the rocket weighs 1 ton and you have 1 ton of fuel, so the first rocket starts off pushing 4 tons and ends up with 3 tons. Now the second rocket is jettisoned and you start out with 2 tons and end with one ton. There's no question you'll be going a lot faster after the second one is finished.

The math can be done if you apply a little calculus, know the force of the rocket motor and the amount of fuel it consumes over time. I'm not going to do that - if someone feels ambitious, go right ahead

Also, I'm assuming you're flying dead in space, not near a gravity well (Moon, Earth, etc) and need to get out of orbit. Then you'll have to take into account the gravitational effects as well which change as you get farther away. Lots of good math here.

Also, escape velocity for the Earth is about 25,000 MPH, which is what you need to get into an orbit - so if you're breaking out of orbit, you'd have to add your 5k to the 25,000 or so...

I hope this makes sense,

D

11. ### Jesus macrumors 6502

#11

Yeah, but why not start of with one 1 ton engine and 3 tons of fuel?? More efficient, less down time between ignitions etc.

Joined:
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#12
Concept: if you are in weightless, frictionless space, acceleration over time is where it is at, not velocity.

If you are on a rocket that has accelerated in space to a velocity of 5000 MPH, and you turn off the engine, what happens? You keep going 5000 MPH, forever, until you hit something.

The reason you want a multi-stage rocket is that you need big tanks to hold enough fuel for blast off - and those go with big engines. THe weight of the empty tanks and engines is WAY more than the payload, so once your first big tank of fuel is used, you don't want to be hauling around the dead weight of the empty tanks and engines of the first stage, so you ditch it and fire your second stage, which has (relatively) much smaller tanks and engines.

Blasting off, you have to generate a horrendous amount of power in a short period of time to reach escape velocity (climb out of the Earth's gravity well) Once you are in weightless space, however, you can apply steady acceleration over a long period of time, you don't need to generate huge acceleration because you keep whatever velocity you have gained. So, you can use a small engine that fires for a long time (days, months) instead of ginormous engines that fire for seconds or minutes.

13. ### Mr. Anderson Moderator emeritus

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VA
#13
it was for ease of explanation - no other reason. The same concepts apply regardless of how much fuel you have - the only difference would be more advanced systems like ion propulsion or some sort of fusion system where the weight of the fuel vs. thrust is much greater.

I was assuming solid or liquid fueled rockets.

D

14. ### Jesus macrumors 6502

#14
Yes, you are right, but that is not what I am saying, the OPs idea is not real, and only if the second rocket was of a completly different type of propultion systems, eg ion drive or redshift rocket, or antimetter etc would it be any good. If you hae one rocket that takes you to, lets say, 20000 mph, and then you light another rocket which is identical to the first, then you will maintain your max speed, you have reached the highest speed you can go with that type of rocket.

FYI there is friction in space, caused by hydrogen atoms, and theoretically there are enough to power a rocket

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

15. ### stevep macrumors 6502a

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UK
#15
In answer to the original question, no one has mentioned Newton (the scientist, not the pda). Newtons second law - every action has an equal and opposite reaction. The second rocket will exert a force on the first one equal to its own propulsive force, tending to push it backwards. It would be like pulling yourself up by your own bootlaces.

16. ### XNine macrumors 68040

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Why are you wearing that stupid man suit?
#16
I'm in Aurora. *yay* And I think what's really happening is that I'm asleep at work and you're at work doing nothing. lol

This whole thing would really take someone with a lot of science background. I'm not that guy. With science of course comes math, which I also detest. However, I wonder if Gravity from other forces would cause the rocket to go faster?

17. ### Mr. Anderson Moderator emeritus

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#17
Sorry, you have it wrong - the speed of the rocket is dependent on the amount of fuel, not the type of rocket. Read what I mentioned before - the force of the rocket motor pushes the rocket, and it causes acceleration. You accelerate until you have no fuel left then you max your velocity. Particle resistance in space in negligible over the short term, so there isn't a critical velocity.

If you were flying on the rocket after it had spent all its fuel and you threw out your trash with a push in opposite the direction you were going, you'd increase your speed even more (although probably not in any measurable amount).

You might want to read up on this a little to get your facts straight.

D

18. ### mpw Guest

Joined:
Jun 18, 2004
#18
OK most of this space stuff is way over my head, literally and figuratively, but I've got a question.

What if Im in my rocket and Ive expended all my fuel and have reached say 20,000mph, if Im understanding this correctly the rocket will continue through empty, frictionless space at 20,000mph until it hits something, right?

And if theres no friction opening a window isnt going to have any drag effect on speed and Ill be able to climb out and wander along to the tip of my rocket without getting torn off by wind friction etc., right?

So Im at the tip and I jump off ahead of the rocket. Obviously Ill now have reached my maximum speed very quickly to 20,001mph.

Will I keep going like the rocket until I hit something slowly stretching my lead over the rocket?

19. ### Mr. Anderson Moderator emeritus

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VA
#19
Yep, that will happen - although, eventually you will slow down, but you'll have been dead for eons

And you'd probably be able to get up to 5 MPH over the rocket and the rocket would slow down a bit.

Again,

Newton's Third Law: For every action, there is an equal and opposite reaction.

D

20. ### mpw Guest

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Jun 18, 2004
#20
I doubt it, I'm quite fat & lazy and wasn't planning to put alot of effort into the jump.

21. ### Mr. Anderson Moderator emeritus

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VA
#21

ha! But you might be surprised what you could do in zero G

D

22. ### mpw Guest

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Jun 18, 2004
#22
Do NOT underestimate my sloth! or DO I really can't be arsed to stop you, whatever.

23. ### runplaysleeprun macrumors 6502a

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Jul 27, 2004
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Chicago, IL
#23
Its all relative. your going 20000 miles an hour away from something? towards something? Right now, relative to the sun, we're all traveling very fast. what that speed is, i don't remember. If you're traveling 20000 mph through dead space, its the same as going 0. or 70000000. it doesnt really matter over the short term for the sake of this problem. So, if you do jump of the tip, you will be going 5 mph from the point of reference of the ship. Maybe 100000 from the point of reference from the planet you are passing. maybe 20005 from the point of reference of earth. when adding force in (near) zero resistance, it doesnt matter what everything else is doing, only what you are comparing yourself to.

anyone ever read ender's game? this problem sort of reminds me of some of the concepts in that book.

ps- i'm taking an international flight in few hours, and am in a terrible rush, so pardon any errors. but i had to check macrumors before i left.

24. ### Lazyhound macrumors regular

Joined:
Jul 19, 2005
#24
From the same article:

The effect of friction would be negligible over a human life-span, unless the craft was miles wide or possibly if travelling close to c.

Also, as someone has already pointed out, the speed of particles ejected from the engine has no limiting effect on the maximum velocity (velocity=acceleration*time, acceleration=force/mass).

EDIT:
I'd say it's implied in the question that both rockets would be measured from the same point of reference.

25. ### Jesus macrumors 6502

#25

But my point still holds that there is no point having a second rocket once your are travelling in deep space, when you could instead have one big one with extra fuel instead of the bulk of the secind engine. And in the long times and distances involved in hopping between stars, the drag does have an effect on velocity, mild, but apparent.