# Model Rocket Blast off @ 100,000'

1. Jun 3, 2012

### erock

Hello Everyone,

If I were to blast off my model rocket from a weather balloon what altitude would it reach?

I have a model rocket that routinely reaches 10,000' @ sea level. If this were attached to a weather balloon and launched at 100,000', what would happen? Are gravitational pull, reduction of atmosphere, etc. significant enough that this rocket would reach a much greater height then it current capabilities? or would it be relatively negligible? I'm finding a bunch of conflicting information on the magnitude of these forces at a 100,000' launch? Does any one have any experience or information on this? Thanks so much for reading any information is greatly appreciated.

2. Jun 3, 2012

### pingu111

You may find the LOHAN (Low Orbit Helium Assisted Navigator is the painful backronym) project interesting (search dot theregister dot co dot uk/?q=lohan) - basically a rocket on a balloon experimental project running on enthusiasm and beer, all a bit tongue in cheek but with some interesting elements.

It is a follow-up to PARIS (Paper Aeroplane Released Into Space)

3. Jun 4, 2012

### enigma

Staff Emeritus
It would go a little higher than a sea level launch, but for the most part, it would be negligible. Shooting from the hip, I'd guess 10% or less

4. Jun 4, 2012

### Staff: Mentor

At first thought I would say the significantly lower drag would make for a significantly longer flight (50%?) but that would only be true if it is supersonic. If it can't break the sound barrier it won't go any further.

5. Jun 5, 2012

### Mech_Engineer

I don't think this is true. Since it's a rocket, it's thrust will be the same regardless of altitiude, but its drag will be much lower in far lower-density atmosphere. Since the thrust will be the same but drag less (nearly neglegible by 100,000ft), its net acceleration will be more constant (assuming of course the rocket's thrust is constant) and therefore it's total altitude gained will increase. Not sure by 50%, but it will be significant.

6. Jun 5, 2012

### Staff: Mentor

If it isn't shaped for supersonic flight, breaking the sound barrier can mean an enormous increase in drag, not to mention unstable flight. If this were purely subsonic, i'd agree.

7. Jun 5, 2012

### Mech_Engineer

Are most amateur rockets limited by the speed of sound?

8. Jun 7, 2012

### MattRob

You could try simulating it using a spreadsheet and Euler's method, say calculate it's state every .1 seconds or so. Just ask if you don't know what that means.

The only problem then is getting accurate information in terms of it's drag coefficient, and atmospheric density at those altitudes. I know of one equation that gives atmospheric density per altitude, but it's not very accurate at higher altitudes.

Do you know what it's drag coefficient is, or can you find out?
What's the forward-facing surface area?
What kind of engine is it going to use?

9. Jun 7, 2012

### Mech_Engineer

I've been thinking about this, and although I'm not really an aero guy I'm thinking the increase in coefficient of drag won't actually increase the net drag on the rocket at 100,000ft. If you look at this (admittedly simplistic) drag equation, and ignore the instability issues:

$F_{d}=-\frac{1}{2}\rho*v*A*C_{d}$

... looking at the chart I've attached which is from a while ago in a calculation sheet I played around with for simulating a railgun's range, the Cd of a sphere transitioning to supersonic flow might go up by a factor of 2.5. However, the air density at 100,000 ft is a factor of 73 lower than that of sea level. Since the drag force is linearly related to both density and Cd, there is still a net decrease in drag of about a factor of ~30.

If we assume the drag equation above is valid in supersonic flight, and that the coefficient of drag for a rocket passing the sound barrier has a jump in Cd of around 3, the net drag on the rocket will still be reduced by a factor of 25, inceasing its top speed and therefore increasing max altitude delta.

Anyone disagree?

#### Attached Files:

• ###### Sphere Supersonic Cd.jpg
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Last edited: Jun 7, 2012
10. Jun 9, 2012

### Topher925

Mech Engineer, I think your reasoning is sound (no pun intended). However, if this is just a small hobby level model rocket I don't think it will even approach the sound barrier. If we knew the mass of the rocket and the engine being used we could easily figure this out.

11. Jun 11, 2012

### Mech_Engineer

I agree, it's impossible to know much more without specifics on the rocket being used. I suspect it would be a high-grade hobby level if it can reach 10,000 ft though.