# I Rocket vs wheel driven

1. Dec 7, 2016

### Mohd Abdullah

Hey guys,

We all know that weight plays an important role in determining the speed of a vehicle, such as dragster like Top Fuel. But how about a rocket-powered dragster that uses hydrogen peroxide? Rocket-powered vehicles indeed accelerate faster than any wheel driven vehicle, but do weight still played an important role in determining its speed? The record for the fastest elapsed time for the quarter mile straight race/drag race is held by a rocket-powered hydrogen peroxide dragster at mere 3.22 seconds.

2. Dec 7, 2016

### Staff: Mentor

What do you think and why?
And why does weight matter for the top speed, as opposed to the acceleration?

3. Dec 7, 2016

### Mohd Abdullah

I don't know. I don't really have much knowledge about it. Don't misunderstand me, I'm not asking answers for a homework.

But in my own opinion, I think weight don't play a role in determining the acceleration speed of a rocket-powered vehicle. Am I correct?

4. Dec 7, 2016

### CWatters

Have a look at Newtons Laws.

5. Dec 8, 2016

### Mohd Abdullah

Is there anything with very light weight moves very fast besides light? For example at 50 miles in one second with energy equals 80000000 joules per second?

6. Dec 8, 2016

### jbriggs444

80000000 joules per second (80 megawatts) is not a measure of energy. It is a measure of power. What is it that you are trying to accomplish with this fast light weight? A kinetic energy weapon?

7. Dec 8, 2016

### CWatters

Protons in the LHC (very close to the speed of light).
Cosmic rays (Protons very close to speed of light)
Alpha particles (about 5% of speed of light).
Beta particles (about 60% to 90% of speed of light)
Plasma ejected by black holes (99.9% of speed of light from http://www.space.com/694-blazing-speed-fastest-stuff-universe.html)

Last edited: Dec 8, 2016
8. Dec 8, 2016

### Mohd Abdullah

No, I'm not talking about weapons afterall. Actually what I wanna say is, say, if a bullet weigh at mere 4 grams moves at 80 megawatts, or equal to 107282 horsepower, how much distance that it can cover in less than a second?

Are there anything, such as something that is lightweight other than particles, that can accelerate over 5 miles in a second in the real world with power equivalent to millions of horsepower?

9. Dec 8, 2016

### jbriggs444

In addition to not being an energy, 80 megawatts is not a speed. What are you really trying to ask?

10. Dec 8, 2016

### Mohd Abdullah

You're familiar with the sequentially timed all-optical mapping photography” (STAMP) that was created by a team of Japanese scientists with 4.4 trillion frames per second, right? Do you think this type of camera can record or capture a still image of those things that you had mentioned?

11. Dec 8, 2016

### Mohd Abdullah

Okay, if you are familiar with Top Fuel dragsters, those dragsters can cover a quarter mile in just around 4 seconds with 10,000 horsepower. Top Fuel dragsters weigh at 2300 pounds. Are there anything with much lighter weight that can also accelerate fast with power equivalent to 10,000 horsepower or more in the real world?

12. Dec 8, 2016

### CWatters

No.

13. Dec 8, 2016

### jbriggs444

A rifle achieves a muzzle energy of between 2000 and 6000 joules over an interval of a few milliseconds. That's one megawatt.
A kinetic energy penetrator (e.g. 125 mm APFSDS) might have a muzzle energy around 6 megajoules. That's into the terrawatt range.

[APFSDS = Armor Piercing, Fin Stabilized, Discarding Sabot]

14. Dec 8, 2016

### Mohd Abdullah

15. Dec 8, 2016

### Mohd Abdullah

Is it possible for a much heavier object to move as fast as those things with the same power just like those things that you had mentioned? The record for the fastest elapsed time for the quarter mile straight race/drag race is held by a rocket-powered hydrogen peroxide dragster at mere 3.22 seconds. Can something heavier break this record with assumed same power at 5000 pound thrust?

16. Dec 8, 2016

### jbriggs444

If you hold thrust constant and increase mass, what happens to acceleration? See post #4

If you increase mass and hold velocity constant, what happens to energy?
If you hold power constant and increase energy, what happens to time?

17. Dec 8, 2016

### CWatters

So you are interested in things that can accelerate fast?..

Let's do some back of the envelope calculations and work out how fast your dragster is accelerating. We will assume the acceleration is constant to make the figures easier...

s = ut + 0.5at2

If starts from rest then u=0 and it simplifies to
s = 0.5at2
rearrange to give an equation for a..
a = 2s/t2

A quarter mile is 402 meters, t = 4 seconds so the acceleration is roughly..
a = 2*402/16 = 50 m/s2

If you prefer it in "g" then that's about 50/9.8 = 5g.

Now lets do the same for a bullet..

Google says 300m/s is a reasonable muzzle velocity for a rifle and the barrel is say 1m long so. We can use another SUVAT equation to work out the acceleration..

v2 = u2 + 2as

again u=0
so
a = v2/2s
= 3002/2
= 45,000m/s2

which is about 45,000/9.8 = 4600g

So compare..

Dragster = 5g
Bullet = 4,600g

No contest.

18. Dec 8, 2016

### CWatters

19. Dec 8, 2016

### rcgldr

In the USA, NHRA reduced drag distance from 1320 feet to 1000 feet for top fuel dragsters and top fuel funny cars, mostly due to not having enough run off distance at many tracks.

As for weight, it still makes a difference, which is why some high speed rocket sleds are multi-stage. This one achieved 157 g's of acceleration on it's final stage.

20. Dec 8, 2016

### Mohd Abdullah

I thought what you meant by your previous answer "No", was even fastest camera with 4.4 trillion fps can't capture a still image or record those things that you had mentioned earlier. I was wrong.

What things in the universe that no matter how fast or advanced our technology is, we will never be able to capture a single frame of it or record a video (be it slow-mo or otherwise) of it?