rockoon rocket

[MClark]

I check back on this thread in a few months.
Sounds like you have a plan.
Please post updates.

 

[lr64]

I check back on this thread in a few months.
Sounds like you have a plan.
Please post updates.

I'm not actually interested in doing it, or any big rocket project, except if someone pays me. In that case, I might, but there are other people who would do it more quickly and with fewer screw ups along the way. Meanwhile, the problems are fun to think about and discuss.

 

[MClark]

So you have been wasting our time.

 

[Spacedog49Krell]

So the air can be thin enough that fins won't work, but thick enough to create significant drag? That seems counter-intuitive. I am not familiar with TVC mechanisms. Could one make a motor with a long end burning section, like a slightly overpowered delay charge, to allow TVC most of the way to apogee?

Fins have a small cross-sectional area compared to a rocket tumbling, exposing the complete cross-sectional area of the vehicle. Yes, the drag becomes significant. Review the Space X Starship re-entry video. Ionized plasma formed at 105 km.

 

[lr64]

How much drag was that? At 105 km, the air density is something like 8 X 10^-6 kg/m^3. That's not very much air to ionize and I don't think it takes very much energy to do that. If we knew what the actual drag was, that would be more useful. There wasn't enough energy to get a good glow going around this huge thing for a few more seconds, at a slightly lower altitude. Plus, of course, the Starship (can I PLEASE call it the BFR?) was going 3 1/2 or 4 times faster than our rocket would be.

Let's say that max q for our rockoon is at 120,000 feet and 2,000 meters per second. Air density at that height is something like .0064 kg/m^3. If we assume that supersonic drag scales roughly as the square of the speed (I have no idea), then that's like Mach 1 at 47,000 feet, so I guess the drag CAN be significant. But do the fins really stop working when the drag of a rocket going crosswise is still significant? Obviously, if the thrust is significant, and slightly misaligned, the fins will be less and less able to cope, but isn't that what TVC is for, and doesn't that problem go away the moment the motor stops?

I don't pretend that I know it would work, but if not, it would be interesting to know why not.

https://www.digitaldutch.com/atmoscalc/
https://ntrs.nasa.gov/api/citations/19770003812/downloads/19770003812.pdf

P.S. The fins have a small cross sectional area compared to a rocket tumbling at any altitude, including at the bottom of the Marianas Trench, unless your rocket looks like an Astron Scout or something.

 

[jsdemar]

Troll alert.

 

[Spacedog49Krell]

How much drag was that? At 105 km, the air density is something like 8 X 10^-6 kg/m^3. That's not very much air to ionize and I don't think it takes very much energy to do that. If we knew what the actual drag was, that would be more useful. There wasn't enough energy to get a good glow going around this huge thing for a few more seconds, at a slightly lower altitude. Plus, of course, the Starship (can I PLEASE call it the BFR?) was going 3 1/2 or 4 times faster than our rocket would be.

Let's say that max q for our rockoon is at 120,000 feet and 2,000 meters per second. Air density at that height is something like .0064 kg/m^3. If we assume that supersonic drag scales roughly as the square of the speed (I have no idea), then that's like Mach 1 at 47,000 feet, so I guess the drag CAN be significant. But do the fins really stop working when the drag of a rocket going crosswise is still significant? Obviously, if the thrust is significant, and slightly misaligned, the fins will be less and less able to cope, but isn't that what TVC is for, and doesn't that problem go away the moment the motor stops?

I don't pretend that I know it would work, but if not, it would be interesting to know why not.

https://www.digitaldutch.com/atmoscalc/
https://ntrs.nasa.gov/api/citations/19770003812/downloads/19770003812.pdf

P.S. The fins have a small cross sectional area compared to a rocket tumbling at any altitude, including at the bottom of the Marianas Trench, unless your rocket looks like an Astron Scout or something.

If you were in one of my classes, I would advise you to look at a new major.

 

[Antares JS]

So the air can be thin enough that fins won't work, but thick enough to create significant drag?

Yes.

For example:

 

[Art Upton]

Yes.

For example:

View attachment 645413

Notice or Remember that the capsule has to enter at just the right angle from the Moon, or it will Skip Off into space and be lost forever.

Edit: lots of studies for Re-entry warheads were done even firing rockets from near space downward to speed up the re-entry devices [like Beryllium cones that were made 40 miles from here]

More tests were done for understanding Moon return re-entry. Real stuff had to be done to prove the math.

 

[lr64]

It would be neat to have an explanation of why the fins don't work in this case rather than just to hear that they don't. If it makes me a troll that I don't take things on people's personal authority, I guess maybe I am one. Maybe just point me at a paper that shows why they don't?

BTW, examples with velocities several times higher, and going down into increasing density instead of up into decreasing density, and with no fins, aren't really very convincing.

 

[Antares JS]

It would be neat to have an explanation of why the fins don't work in this case rather than just to hear that they don't. If it makes me a troll that I don't take things on people's personal authority, I guess maybe I am one. Maybe just point me at a paper that shows why they don't?

The air is too thin for fins to have a significant correcting effect on attitude.

The X-15 had to use RCS thrusters during its high altitude flights.

 

[lr64]

Why do the fins stop working before the drag does? Or did they just want to be able to control the X-15's attitude when it was too high for either?

 

[Antares JS]

Why do the fins stop working before the drag does? Or did they just want to be able to control the X-15's attitude when it was too high for either?

Because the amount of air required for a reasonably-sized set of fins to have an effect on attitude is different from the amount of air required to exert noticeable aerodynamic drag. You're talking about enough air to have an effect on your fins vs. enough air to have an effect on the entire surface area of your vehicle. Obviously, your entire vehicle has much more surface area than just your fins, hence drag can still be pronounced even if your fins are having negligible effect on aerodynamic stability.

Offhand, I think a set of fins could theoretically work at those high altitudes, but the fins would have to be extremely large and the corrective response to the rocket pitching would probably be too slow to be useful.

 

[lr64]

Certainly it would take longer for the fins to stabilize things, and if the motor was on, with a less than perfect direction, the fins might be hopeless. I wonder if the damping effect is non-linear or something.

 

[Spacedog49Krell]

The V-2 fins were very large for the rocket. They were an attempt to maintain aerodynamic flight control after the motor, with its TVC vanes, shut down. The V-2's ballistic trajectory reached 50-65 mile altitudes. Flights conducted at White Sands, carrying movie cameras, showed that the V-2 missile starts tumbling at altitudes above 40 miles.

 

[jsdemar]

Certainly it would take longer for the fins to stabilize things, and if the motor was on, with a less than perfect direction, the fins might be hopeless. I wonder if the damping effect is non-linear or something.

The response to the aerodynamic forces is a second-order differential equation with 6 degrees of freedom (ignoring elasticity of the rocket body and fins). Do you think it might be "non-linear or something"?