# Idea for a high speed craft would this work?

1. Aug 6, 2007

### Jota

Idea for a high speed "craft"...would this work?

Note: since I am not a physicist, I can't be certain how foolish I might sound if the following hypothetical device would not be physically possible (or economically useful, for that matter), but I'm a curious sort of fellow, so I'd appreciate any expert opinion on it, anyway.

Imagine one were travelling in some kind of rocket-powered craft (in a vacuum) which had a maximum speed of about 1 mile per second, because the fuel simply cannot burn/expand any faster. But, this craft had a mineature version of itself on board. The mini-craft takes off. Could it reach a speed of 2 miles per second? And, would it be theoretically possible to reach much higher speeds by having a whole 'stack' of craft with progressively smaller versions of themselves, each taking off when the previous reached maximum speed, until eventually reaching near light speed?

2. Aug 6, 2007

### Janus

Staff Emeritus
You seem to be under the impression that rockets are limited to the speed of their exhaust. This is simply not true, the only thing that limits the top speed of a rocket is how much fuel it carries.

Other than that, what you suggest sounds like a stage rocket.

3. Aug 6, 2007

### Jota

So would that mean that, theoretically, assuming enough resources to make the craft, what I described indeed WOULD be able to reach nearly the speed of light?

4. Aug 6, 2007

### kach22i

Gravity is still at work in outerspace, it's just a lot weaker, right? All other forces such as accelerationand forces of decleretion would also apply, right?

The "classic myth" or example of falling in an elevator (constant rate of acceleration) and jumping up at the last second to avoid sudden decleration comes to mind. You will still die and be crushed, the moving body continues to move.

The TV program recently had an episode on this, they put springs on a mannequin and some kind of charge I think - it would of been a mess had it been a real person.

Back to our moving body coasting along with its mass as it continues to move in space, detach the probe craft/booster/stage.........................what happens?

The former single mass becomes two masses traveling at the same rate. For the new smaller stage rocket mass to travel faster it needs to expell a force greater than it's mass and greater than the mass/rate already being traveled at.

It's like me traveling in the back of my parents station wagon as a kid. If I stuck my butt out the tailgate and blew a fart would it make the car go any faster? Not unless the farts force was greater than the forces acting upon the car already.

Just a fart in the wind.

EDIT:
Conclusion; The new smaller stage rocket needs to expell more force than it's current mass is traveling at to go any faster.

It also will need to expell an equal force (F=MA) in the opposite direction to stop dead in space.

Question, will the loss of mass slow down the parent body? Is the speed or rate of accleration dependent on mass or is it only gravity which is dependent on mass? In a vacuum two bodies of different masses will accelerate/fall at the same rate (Galileo and the leaning tower of Pizza experiments). I still think they (the parent and the stage) will continue to travel at the same rate, attached or not.

Last edited: Aug 6, 2007
5. Aug 6, 2007

### Janus

Staff Emeritus
So far, so good.
Oops, no. any forward acting force will increase the velocity of the smaller stage. Remember f=ma, which can be rewritten a= f/m. Since m is constant, any force will produce acceleration, and acceleration is change in velocity. It doesn't matter how fast the rocket is already moving.
Yes, at least temporarily. When a car is driving at a constant speed, the froces acting on it are balanced. The forces of friction just equal the forward force produced by the engine. When you pass gas, the forward force is increased slighty and the car speeds up slighty until the forces balance again.(as the car speeds up, air resistance increases. Once you stop, the friction forces will be greater than the forward forces and the car will slow down back to its origianal speed.

Note, for a rocket in space, there are no friction forces to be overcome. once a rocket reaches speed, it will no longer need to run its engines in order to continue at that speed. Under these conditions any additional speed it gains from an extra push will be retained.
This is just plain wrong.

6. Aug 6, 2007

### Jota

So, Janus, you are saying that the craft I described would work?

7. Aug 6, 2007

### NoTime

As Janus mentioned before, your idea sounds like a multi-stage rocket.
This is existing technology that you can easily find on the net.

No. You will not achieve any meaningful percentage of the speed of light with any chemical rocket.

8. Aug 6, 2007

### Janus

Staff Emeritus
Well, sure it will work, but then again it is not exactly a new idea.

Here's a picture of the Saturn V rocket used for the Apollo missions, notice that it uses a series of stages. As each stage uses up its fuel, it is discarded.

This is done to save on the weight lifted to orbit. One drawback to this design is that when you discard each stage, you also throw away engines.

The reason that this design was used for the Saturn V was that it had to be aerodynamic in order to pass through the atmosphere.

In space, where this is not an issue, you can just use "strapped on" fuel tanks which you discard as they are emptied, and you just use the same engine.

As far as your idea making it possible to reach near light speed, As I said before, there is nothing preventing a single stage rocket from reaching that speed as long as it carries enough fuel. The real problem is, that with the efficiency of today's rockets, it would take more fuel than the entire mass of the Sun to reach even a fraction of the speed of light. Your idea doesn't address that problem. The total launch mass of your "nested ship" would have be of the same order. IOW, impossibly large.

9. Aug 7, 2007

### ank_gl

gee , why we cant reach speed of light is not bcoz of the fact that we cant get enough acceleration for enough time, its because of a guy called einstien gave some theory(i know ;)), theory of relativity). read it and you ll know, why speed of light cant be reached

10. Aug 7, 2007

### kach22i

For the "extra force" to contribute to additional velocity the rocket (or butt out the tailgate) must exceed the present/current velocity. I gotta squeeze one out at least 70 mph in other words for it to count. The particles/ions or expanding gasses must be expanding/exiting near or at the speed of light if you expect to go that fast.

The advantage of having a staged rocket in deep space would be being able to lessen the mass at will. Gravitational forces encountered such as passing by a large planet or "black hole" act on the mass of the rocket surely as gravity acts on us while on Earth.

Example-2: Merry-Go-Round
You sit on the Merry-Go-Round; your friend pushes the grab bar faster and faster until contact with his hand acts to slow you down more than to speed you up. Your friend must be able to exceed the velocity of his last push for you to go any faster.

11. Aug 7, 2007

### FredGarvin

The merry go round is not an applicable example.

The thrust of a rocket is proportional to the characteristic velocity of the exhaust. The speed of the rocket is not. As long as the rocket is putting out any kind of thrust, it will accelerate.

12. Aug 7, 2007

### kach22i

I don't want to play with words, but I want to learn and speak the same language.

Is it only considered "THRUST" if it's traveling faster than the rocket?

Let's say my rocket is traveling through space at 1/2 the speed of light. My main fuel tank runs out of propellant. I switch to another tank which for some odd reason shoots out gasses at only 1/4 the speed of light.

What would the result of this situation be?

13. Aug 7, 2007

### mgb_phys

It will acelerate.
Remember the fuel is already doing 1/2 the speed of light when it is still in the tank, when you shoot out at 1/4 of the speed of light 'relative to the engine' it is still moving relative to the engine and will provide thrust.

14. Aug 7, 2007

### kach22i

That answer eludes my primary processors.

#### Attached Files:

• ###### Physics-3.jpg
File size:
64.5 KB
Views:
42
15. Aug 7, 2007

### FredGarvin

I hate to say it, but the exhaust velocity used in thrust calculations is the velocity relative to the vehicle.

16. Aug 7, 2007

### kach22i

Now I'm really lost in space.

17. Aug 7, 2007

### Janus

Staff Emeritus
NO, exhaust gasses leaving the rear of the rocket at any speed will produce forward thrust on the rocket and accelerate it
The rocket will accelerate. The equation that describes this has been known for over 100 yrs:

$$\Delta v = v_e \ln (MR)$$

where
$\Delta v$ is the change of velocity
$v_e$ is the exhaust velocity relative to the rocket.
$MR$ is the mass ratio, or the mass of the fully fueled rocket divided by the mass of the rocket after the fuel is used up.

Notice that the starting velocity of the rocket does not enter into the equation at all.

Another point that should be made is that if your conception of how rocket propulsion works were true, then we would have never been able to put even one satellite into orbit. Our best rocket engines produce a exhaust velocity of about 4500 m/s. Low Earth orbital velocity(the speed you need to be traveling to maintain orbit.) is about 7750 m/s. If a rocket can't travel faster than its exhaust velocity, then none of our rockets could reach orbital velocity, and put a satellite into orbit.

But our rockets do put satellites into orbit even with exhaust velocities less than orbital velocity. Real life example shows your conception to be wrong.

18. Aug 8, 2007

### kach22i

I did not know this, thank you for taking the time to share this information.

I know this is somewhat off topic, but if our rocket is still in the atmosphere fighting friction and the forces of gravity, could the rocket still go faster than it's thrust velocity? Assuming there is enough thrust to overcome aerodynamic and frictional limitations of course.

I'm taking about close to earth not up in the very thin stratosphere, but I don't see how this could make a difference to the general rule.

Propulsion and forward motion occur because of pressure difference between the front and back of the craft (be it a propeller or a rocket). The front gets pulled forward by the pressure difference of the thrust in the back as they attempt to equalize.

If space is a vacuum how can we generate pressure differences at all?

It's a silly question, but the answer might surprise me.

19. Aug 8, 2007

### mgb_phys

Not quite, it gets pushed forward by the pressure difference between the front and back of the burning FUEL.
It's not the fuel going out of the back which provides the thrust it is the same fuel expanding and pushing against the top of the nozzle, which pushes the top of the nozzle, and so the craft, forward.
The only reason to let some thrust go out of the back is because otherwise it would push against the bottom of the nozzle and cancel the thrust against the top.

20. Aug 8, 2007

### kach22i

Most of my reading has been on propellers and fans including turbofan jet engines. I guess rockets are very different.