# Altitude and fins question.

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#### shrox

##### Well-Known Member
At what altitude do fins begin to become ineffective?

At what altitude do fins begin to become ineffective?

For me, apparently 8 to 10 centimeters past the end of the launch rod! :clap::bangpan::confused2::roll:

Sorry, couldn't resist the straight line.

I'd guess it was somewhere in the 40-70K foot range - aircraft like the U2 and SR71 routinely got up that high and I don't remember hearing about them using steering jets to change directions. This site claims the SR-71 could fly up to 100,000, with others saying 85K is the max. That help?

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For me, apparently 8 to 10 centimeters past the end of the launch rod! :clap::bangpan::confused2::roll:

Sorry, couldn't resist the straight line.

I'd guess it was somewhere in the 40-70K foot range - aircraft like the U2 and SR71 routinely got up that high and I don't remember hearing about them using steering jets to change directions. This site claims the SR-71 could fly up to 100,000, with others saying 85K is the max. That help?

Yes, I was guessing about 6-8 miles. I am sure that speed factors in too.

Air density is reduced by 50% about every 18,000 ft (rough approximation for lower atmosphere). The forces exerted by aerodynamic control surfaces are proportional to air density and the speed squared.

So, if you double the speed for every 36,000 ft you gain, the effectiveness of the fins should stay about the same.

This example neglects a lot of physical details, but it should be good enough to get an idea.

Reinhard

You can use this calculator https://www.aerospaceweb.org/design/scripts/atmosphere/ to determine the EAS, equivalent air speed, of a rocket vs altitude. Provided that that rocket EAS is above the stall speed of the fins, they are still performing a guidance function.

Bob

The problem with the idea of stall speed is that it depends on the strength of the perturbation. Ideally, on a rocket, the fin force will be quite low, and therefore the Cl needed to maintain stability should also be quite low, which allows them to be effective at even quite high altitudes and low speeds. The typical fin design used (swept low-aspect delta) is excellent as well, as it has a fairly linear lift slope even at high angles of attack.

Realistically, they should be effective up to 100,000+ feet so long as there are no major perturbations, although for truly high altitude shots (up into that sort of region), it would probably be worthwhile to spin stabilize the rocket to at least some degree as well.

I'm not sure if you would ever get a fin into full stall during stable flight, having said that, if the 'upper surface' of the fin was stalled (separated turbulent flow) it would still provide a 'restoring force'. I think Bob is right, you need to look at the EAS and from that you get an idea how much restoring force the fins give.

An airfoil must be above its stall speed to develop lift which is the corrective force that you expect from fins. Airfoils develop increased lift as the angle of attack is increased until they reach the stall angle where the lift force is lost. I've seen stable model rockets do cartwheels in 15-20 mph winds 30 feet over the launch rod because the crosswind velocity created such a high angle of attack that the fins stall.

This type of incident is why https://www.nar.org/pdf/launchsafe.pdf recommends a rod leaving velocity that is 4 time the wind velocity. The basic 5:1 power to weight ratio rule of thumb fulfills this in winds to 5 mph, and but higher T/W ratios are required in stronger winds. It boils down to a T/W = the cross wind wind speed in mph is required for a typical launch rod length to insure a stable trajectory after leaving the launch rod.

The effective air speed is a normalization universally used to predict the performance of an airfoil as a function of density altitude and actual airspeed. It takes a much higher physical airspeed at higher altitudes to prevent stall because the air density, and therefore, the lifting force developed by the airfoil decreased exponentially with altitude. The lift is equal to
where L is lift force, &#961; is air density, v is true airspeed, A is planform area, and CL is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number
from https://en.wikipedia.org/wiki/Lift_(force)

If the minimum controlled air speed is 20 mph at sea level, if the air density is decreased by 2 (18,000'), your airspeed must increase by the square root of 2 (28 mph) to generate the same lift or an EAS of 20 mph if the temperature is constant. It isn't, so that why you need the atmospheric calculator. https://www.aerospaceweb.org/design/scripts/atmosphere/

At 100 kft density altitude, you would have to be going at least 168 mph (M=0.25) actual air speed for the fins to be effective. At 200 kft, you need to be moving at greater than 1340 mph (M>1.9) for the fins to be effective.

Bob

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At 200 kft, you need to be moving at greater than 1340 mph (M>1.9) for the fins to be effective.

Bob
Bob, does that Mach number reflect the speed of sound at 200 kft altitude?

At 100 kft density altitude, you would have to be going at least 168 mph (M=0.25) actual air speed for the fins to be effective.
Bob

Just out of curiousity, what happens if the rocket is at 100 kft and nearing apogee (i.e., slowing down below 168 mph). Does the rocket just tumble its way to apogee? Is the risk of damage minimal due to the thin air?

Jim

Just out of curiousity, what happens if the rocket is at 100 kft and nearing apogee (i.e., slowing down below 168 mph). Does the rocket just tumble its way to apogee? Is the risk of damage minimal due to the thin air?

Jim

Would a gyro keep it stable?

Bob, does that Mach number reflect the speed of sound at 200 kft altitude?

Yes. The sound speed depends on temperature, not density.

Additionally, the aerodynamic heating is proportional to the product of the density x the true airspeed cubed. You can get an idea by going to the atmospheric calculator I referenced.

Bob

Just out of curiousity, what happens if the rocket is at 100 kft and nearing apogee (i.e., slowing down below 168 mph). Does the rocket just tumble its way to apogee? Is the risk of damage minimal due to the thin air?

Jim
It's most likely to continue what it was doing before is slowed down.

Remember gravity is going to slow it down at approximately 21.8 mph per second, so it will reach apogee in about 8 seconds. If the rocket had a residual turning moment it could rotate, but I think this is unlikey. It is lilely to backslide and gain downward velocity after reaching apogee. 8 second later it will be dropping at 168 mph so it might flip over to go nose first or it could stabalize out to some intermediate angle if an asymmetry were introduced at apogee (such as deploying a wing).

The CSXT GoFast rocket backslid for almost 100 kft after passing apogee before it flipped nose down IIRC!

Bob

It's most likely to continue what it was doing before is slowed down.

Remember gravity is going to slow it down at approximately 21.8 mph per second, so it will reach apogee in about 8 seconds. If the rocket had a residual turning moment it could rotate, but I think this is unlikey. It is lilely to backslide and gain downward velocity after reaching apogee. 8 second later it will be dropping at 168 mph so it might flip over to go nose first or it could stabalize out to some intermediate angle if an asymmetry were introduced at apogee (such as deploying a wing).

The CSXT GoFast rocket backslid for almost 100 kft after passing apogee before it flipped nose down IIRC!

Bob

Do we know what the roll rate was for the backsliding CSXT? There are some ideas out there for rockoon flights to orbit that depend on using fins to passively perform most of a gravity turn between 100-200 kft, followed by a stage that uses the thin atmosphere to spin it up and then goes high enough for the aero forces to be negligible, at which point the spin keeps it inertially pointed for a nose-mounted circularization motor to kick in after half an orbit. I've always thought that would be a fine line, but I think I read that such a feat was accomplished in the 60s. I'm curious how much spin is required to overcome residual atmosphere acting on fins.

Do we know what the roll rate was for the backsliding CSXT? There are some ideas out there for rockoon flights to orbit that depend on using fins to passively perform most of a gravity turn between 100-200 kft, followed by a stage that uses the thin atmosphere to spin it up and then goes high enough for the aero forces to be negligible, at which point the spin keeps it inertially pointed for a nose-mounted circularization motor to kick in after half an orbit. I've always thought that would be a fine line, but I think I read that such a feat was accomplished in the 60s. I'm curious how much spin is required to overcome residual atmosphere acting on fins.

The range safety required roll rate at burnout was 8 rps.

I think I mentioned the backsliding distance in the Sport Rocketry article. If not, I seem to recall that they determined the backsliding from the accelerometer data and I believe the backsliding distance was about 100,000 feet.

Another account is here. https://highpowerrocketry.blogspot.com/2009/03/narcon-2009.html

I know of no successful balloon launched orbital spacecraft. AFAIK all air launched orbital vehicles have been launched from aircraft. https://www.astronautix.com/lvfam/airnched.htm

And the one that is a successful LV is the Pegasus https://www.astronautix.com/lvs/pegasus.htm

The rockoon program was a high altitude balloon launched suborbital rocket program. It was quickly abandoned as inexpensive and more reliable sounding rockets became available. https://www.astronautix.com/lvs/rockoon.htm

I think if you do a complete analysis of the problem you will find that a balloon launched rocket can not get into orbit because a balloon can not loft a rocket with sufficient propellant mass fraction to accelerate a spacecraft to the required 7.8 km/s orbital velocity.

Bob

The range safety required roll rate at burnout was 8 rps.

I think I mentioned the backsliding distance in the Sport Rocketry article. If not, I seem to recall that they determined the backsliding from the accelerometer data and I believe the backsliding distance was about 100,000 feet.

Another account is here. https://highpowerrocketry.blogspot.com/2009/03/narcon-2009.html

I know of no successful balloon launched orbital spacecraft. AFAIK all air launched orbital vehicles have been launched from aircraft. https://www.astronautix.com/lvfam/airnched.htm

And the one that is a successful LV is the Pegasus https://www.astronautix.com/lvs/pegasus.htm

The rockoon program was a high altitude balloon launched suborbital rocket program. It was quickly abandoned as inexpensive and more reliable sounding rockets became available. https://www.astronautix.com/lvs/rockoon.htm

I think if you do a complete analysis of the problem you will find that a balloon launched rocket can not get into orbit because a balloon can not loft a rocket with sufficient propellant mass fraction to accelerate a spacecraft to the required 7.8 km/s orbital velocity.

Bob

The program in the 60s I was referring to was not balloon launched, but it was passively/aerodynamically stabilized, from what I understand. I'll check into those links.

An airfoil must be above its stall speed to develop lift which is the corrective force that you expect from fins. Airfoils develop increased lift as the angle of attack is increased until they reach the stall angle where the lift force is lost. I've seen stable model rockets do cartwheels in 15-20 mph winds 30 feet over the launch rod because the crosswind velocity created such a high angle of attack that the fins stall.

This type of incident is why https://www.nar.org/pdf/launchsafe.pdf recommends a rod leaving velocity that is 4 time the wind velocity. The basic 5:1 power to weight ratio rule of thumb fulfills this in winds to 5 mph, and but higher T/W ratios are required in stronger winds. It boils down to a T/W = the cross wind wind speed in mph is required for a typical launch rod length to insure a stable trajectory after leaving the launch rod.
Absolutely, though as I said, a highly swept, low-aspect delta tends to be fairly stall resistant due to the lift caused by the leading edge vortex.

The effective air speed is a normalization universally used to predict the performance of an airfoil as a function of density altitude and actual airspeed. It takes a much higher physical airspeed at higher altitudes to prevent stall because the air density, and therefore, the lifting force developed by the airfoil decreased exponentially with altitude. The lift is equal to
where L is lift force, &#961; is air density, v is true airspeed, A is planform area, and CL is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number
from https://en.wikipedia.org/wiki/Lift_(force)

If the minimum controlled air speed is 20 mph at sea level, if the air density is decreased by 2 (18,000'), your airspeed must increase by the square root of 2 (28 mph) to generate the same lift or an EAS of 20 mph if the temperature is constant. It isn't, so that why you need the atmospheric calculator. https://www.aerospaceweb.org/design/scripts/atmosphere/

At 100 kft density altitude, you would have to be going at least 168 mph (M=0.25) actual air speed for the fins to be effective. At 200 kft, you need to be moving at greater than 1340 mph (M>1.9) for the fins to be effective.

Bob
Not exactly.The minimum controlled airspeed is usually (as you said earlier) defined by crosswind requirements. At 200kft, you would need to be moving at greater than 1340mph only if you had a danger of crosswind components equal in relative magnitude to those that caused the 20mph initial requirement. I have seen rockets that were stable at amazingly low speeds (mostly clusters with incomplete ignition) in still air, demonstrating that without the crosswind requirement, rockets are capable of flight at much lower speeds than might be expected.

Overall, as I said above, stability is determined by the strength of the perturbation. At liftoff, the limiting factor is typically crosswind mitigation, as well as the mitigation of any minor construction asymmetries. At high altitudes and speeds, the perturbation from a crosswind is reduced by the same factor as the correcting force (both depend on density), so the airspeed requirement is the same in either case. The construction asymmetries are a different case. Some of them (aerodynamic asymmetries) will scale with altitude, and again will not affect the minimum speed required for stability. Others will not (specifically, those having to do with the motor), and will require a higher speed at high altitude. The motor-related asymmetries though (the only ones I can think of at the moment that are not aerodynamic) are only present until burnout though, and as a result, they would not be much of a concern at ultra high altitude.

Would a gyro keep it stable?

When the CATS Prize was still un-won I had it planned to go for the \$50,000 prize then use that to try for the \$250,000 prize useing basically modified off the shelf things like the Dr. Rocket Modular rocket in 2 stages and using Weather service baloons and lots of Helium.

Part of the stabilazition problem we hope'd to solve was would a gyro help? I had found surplus gyros that would have fit the airframe but we never got to the point of testing. When the radio station DJ's that I had lined up were going to get behind the effort as a long term promotion for the station and find me the local money to make the effort was suddenly sold and the DJ's were fired as the station went Soft Pop. That was the end of that

I still have the 1/10 scale model that I've never flown, maybe this year.

When the CATS Prize was still un-won I had it planned to go for the \$50,000 prize then use that to try for the \$250,000 prize useing basically modified off the shelf things like the Dr. Rocket Modular rocket in 2 stages and using Weather service baloons and lots of Helium.

Part of the stabilazition problem we hope'd to solve was would a gyro help? I had found surplus gyros that would have fit the airframe but we never got to the point of testing. When the radio station DJ's that I had lined up were going to get behind the effort as a long term promotion for the station and find me the local money to make the effort was suddenly sold and the DJ's were fired as the station went Soft Pop. That was the end of that

I still have the 1/10 scale model that I've never flown, maybe this year.

I've seen an old photo of some rocket hobbyists in France I think from over 100 years ago. They had a gyro in the top of the rocket that was started by pulling a cord.

I've seen an old photo of some rocket hobbyists in France I think from over 100 years ago. They had a gyro in the top of the rocket that was started by pulling a cord.
Technicians used the same method to get hard drives going several decades later...

MarkII

Technicians used the same method to get hard drives going several decades later...

MarkII

I thought pulling the cord was how you got the hard drive to stop...

I thought pulling the cord was how you got the hard drive to stop...
I was referring to something like this.

MarkII

I thought pulling the cord was how you got the hard drive to stop...

I thought it was referring to one of these:

:roll:

That's to dump the hard drive...

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