Minimum Diameter 54mm - Let's see yours.

[CoyoteNumber2]

I'd like to know more about this.

Anecdotally, I've seen it written that having an airframe section aft of the fins smooths out turbulent airflow from the fins, thereby reducing drag. I have no data to cite though.

 

[OverTheTop]

Hoping not to hijack here, why are these MD bird's fins not mounted against the aft end of the rocket, but 2-3 inches up?

Doing that does reduce stability but makes the rocket more likely to survive the landing. That is the main reason I build rockets that way.

Anecdotally, I've seen it written that having an airframe section aft of the fins smooths out turbulent airflow from the fins, thereby reducing drag. I have no data to cite though.

I could imagine that being true for non-zero AoA, but rockets fly almost exclusively at almost zero angle of attack. Maybe there is a small improvement in keeping the air streamlined, but it would be diminishing returns I think. Anyone with a better idea on this?

 

[airforcetp1991]

I agree. MD rockets tend to accelerate at a very high

Tonimus said:
Hoping not to hijack here, why are these MD bird's fins not mounted against the aft end of the rocket, but 2-3 inches up?



I'd like to know more about this.

If the only goal in mounting the fins higher than the bottom end of the rocket was to create less stability, then making the fins smaller would seem to be a better choice.

I'm certainly no aerodynamicist (is that even a word?), but I suspect there might be something more to it like, moving the fins to an area of less turbulent airflow makes for less drag and more effective stabilizing.

My guess is that that's probably a gross oversimplification if it's even accurate. I'd welcome more input...
[/QUOT

Doing that does reduce stability but makes the rocket more likely to survive the landing. That is the main reason I build rockets that way.


I could imagine that being true for non-zero AoA, but rockets fly almost exclusively at almost zero angle of attack. Maybe there is a small improvement in keeping the air streamlined, but it would be diminishing returns I think. Anyone with a better idea on this?

My understanding of the Aero would lead me to believe that most of the turbulent flow from the fins is away from the airframe and so any part of the airframe would have little effect. A boat tail reduces pressure drag but that is a different concept.

 

[manixFan]

Anecdotally, I've seen it written that having an airframe section aft of the fins smooths out turbulent airflow from the fins, thereby reducing drag. I have no data to cite though.

My son is an aerospace engineer and I asked him about this. He thought that any reduction in drag would be offset by the increase in wetted surface (needed to maintain stability by moving the CP forward). (I don't recall if he did any math to back up his opinion so it could have been more of a guess.) But he is right of course that moving the fins forward moves the CP forward which has to be compensated for somehow, all of which likely increase drag or otherwise affects performance. I have basically settled on the Mongoose fin shape which so far has held up very well even when placed at the rear of the rocket. I have built a fair number of both 38mm and 54mm MD rockets and have flown the 54's well past Mach 2 on a number of occasions so it seems to be working. (Just using surface mounting and fillets, no tip-to-tip or other reinforcement.)

Another very simple test is a visual survey of sounding rocket designs. The vast majority of designs have the fins basically as far back as they can go. I realize they don't care about the fins surviving a landing, but I suspect if there was an aerodynamic advantage to having the fins placed forward, it would be seen on more designs, especially since they are all essentially MD rockets.

I suspect the higher MD fin placement started as way to protect fins and things went from there. And unless I misunderstand weathercocking, it's mostly a function of being over-stable, which can be corrected by just making a shorter rocket or using smaller fins, both of which would help altitude and speed. As far as I've been able to discover, there seems to be very little practical effect on aerodynamics by having the fins forward. I suspect bigger gains might come from a boat tail or a long burning smoke grain to reduce base drag. But that's just a guess based on some informal research.


Tony

(OpenRocket shows essentially no difference in altitude based on fin placement, at least with my testing.)

 

[G_T]

An advantage of moving the fins forward is to get them out of the heat soak zone of the nozzle. Most adhesives hobbyists use don't handle heat well.

Gerald

 

[airforcetp1991]

An advantage of moving the fins forward is to get them out of the heat soak zone of the nozzle. Most adhesives hobbyists use don't handle heat well.

Gerald

That is a good point. I wonder though how long that heating lasts given the high speed at burnout and associated airflow over the rocket airframe/fins?

 

[airforcetp1991]

Custom carbon 54mm MD using Wildman airframe and PML custom fins. Raven4/RRC3/featherweight tracker inside. Hit Mach 2 on the first flight with no cosmetic damage.

 

[G_T]

The high speed adds additional heating.

Gerald

 

[airforcetp1991]

The high speed adds additional heating.

Gerald

True but that will be most pronounced on the leading edge of the fins and the nose cone. Once there is a boundary layer below Mach 1 I would expect the airframe to rapidly cool due to the airflow and colder temperatures at altitude.

 

[G_T]

The deceleration of the air from freestream to below mach 1 relative to the rocket requires accelerating the air - adding energy to it. If you didn't accelerate it, or otherwise deflect it, it wouldn't be hotter. Get it all the way to zero relative speed (stagnation points) and you hit the stagnation temperature. Heat is a kinetic energy thing. Probably way oversimplified, but you might be able to get there from here: http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/kintem.html comparing the relative velocities of the freestream and the near boundary layer. There are probably papers and charts which can be referenced to avoid the back of the envilope math. I'm sure shock waves and compressible flow add additional levels of complexity - at these speeds air may no longer be treated as incompressible. That threshold is perhaps somewhere around 100m/s so pretty slow by rocketry standards.

IIRC, the heating is roughly proportional to the cube of the speed in the supersonic range of speeds. But don't trust my memory on that one!

Gerald

 

[airforcetp1991]

The deceleration of the air from freestream to below mach 1 relative to the rocket requires accelerating the air - adding energy to it. If you didn't accelerate it, or otherwise deflect it, it wouldn't be hotter. Get it all the way to zero relative speed (stagnation points) and you hit the stagnation temperature. Heat is a kinetic energy thing. Probably way oversimplified, but you might be able to get there from here: http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/kintem.html comparing the relative velocities of the freestream and the near boundary layer. There are probably papers and charts which can be referenced to avoid the back of the envilope math. I'm sure shock waves and compressible flow add additional levels of complexity - at these speeds air may no longer be treated as incompressible. That threshold is perhaps somewhere around 100m/s so pretty slow by rocketry standards.

IIRC, the heating is roughly proportional to the cube of the speed in the supersonic range of speeds. But don't trust my memory on that one!

Gerald

That checks with my undergrad. The stagnation points will definitely have the most heating and generally below Mach 0.3 we can assume incompressible flow.

My overall point was that I wouldn’t expect there to be a significant amount of airframe heating on a portion of the airframe near the aft end of the rocket near the nozzle. I won’t guess as to whether the surface temp would decrease linearly or in someo other way starting from behind the nosecone but it would likely drop quickly especially in the subsonic region.

 

[G_T]

I think I was trying to point out that it will be higher temp than ambient at that altitude. Because, you had to put energy into the air - accelerate it - to get it to be subsonic there. At the stagnation point you simply had to accelerate it more. So if you are talking cooler than the center tip of the nosecone, then yes. Cooler than ambient? No.

Gerald

 

[airforcetp1991]

I think I was trying to point out that it will be higher temp than ambient at that altitude. Because, you had to put energy into the air - accelerate it - to get it to be subsonic there. At the stagnation point you simply had to accelerate it more. So if you are talking cooler than the center tip of the nosecone, then yes. Cooler than ambient? No.

Gerald

Agreed. It won’t be colder than ambient for sure. I saw a paper on the X-15 heating data but it was just won’t leading edge of I remember right so not the surface temps.

 

[kbRocket]

I designed "Pure Impulse" to fly shorter long-burn 54 mm motors.. It is all self-made thin walled s-glass. In 2020 it flew to 14,291' with an I-65W as configured below:


The tailcone was an inner tube made from JB-weld bonding fiberglass, Styrofoam, and an outer covering of fiberglass. These were all glued directly onto the motor. They cooked a little in flight:


In 2019, prior to tailcone, it flew on the same motor to 11,565' and 12,482' and on the longer J135W to 14,016'.

 

[G_T]

Agreed. It won’t be colder than ambient for sure. I saw a paper on the X-15 heating data but it was just won’t leading edge of I remember right so not the surface temps.

Random quote on SR-71 "Titanium skin helped protect the aircraft's aluminum airframe from the intense heat of supersonic flight. Temperatures ranged from 450 degrees F (232 C) near the back part of the aircraft to 950 degrees F (510 C) near the engine exhaust."

Gerald

 

[airforcetp1991]

Random quote on SR-71 "Titanium skin helped protect the aircraft's aluminum airframe from the intense heat of supersonic flight. Temperatures ranged from 450 degrees F (232 C) near the back part of the aircraft to 950 degrees F (510 C) near the engine exhaust."

Gerald

Valid, the SR-71 leaked fuel like crazy on the ground to to allowances for skin expansion at those temperatures. Although I wonder how long it took for those temps to be reached in flight. Even the L3 concept I am working on with an O3400 will only be above Mach 3 for a total of 10 secs or so. Hopefully this summer I will have some personal data haha

 

[OverTheTop]

The expansion shock waves should be less intense than the compression ones you get on front-facing edges. Also the isentropic expansion of the gas, if it has compressed, would result in some cooling. Lots of effects happening. Need someone with CFD and some time to model it up!

 

[manixFan]

An advantage of moving the fins forward is to get them out of the heat soak zone of the nozzle. Most adhesives hobbyists use don't handle heat well.

Gerald

In all my years I've never seen fin adhesion adversely affected by heat soak from a motor. But I've concentrated on smaller MD rockets using commercial motors that far outperform anything I can make up. The research propellant I use is pretty conservative to preserve motors and rockets, so it's not a great performer if going fast and high is the goal. With commercial motors using non-graphite nozzles, the heat transfer seems to be more evenly distributed. But I've never seen any measurements of temps along the length of the case during a firing so that's just based on observation of cases removed shortly after landing. It would be very interesting to see in-flight data of temps along the length of the case.

(I vaguely recall some very long burn motors failing due to heat soak of the aluminum case at the nozzle, but that's a different issue. If the case fails, it doesn't matter what the fins do. And that heat soak can be mitigated by using a carrier as I recall.)

I'll be flying my first 4" MD rocket at BALLS this year with a research motor (CF Mongoose with a 5 grain John Bolene case) and now that you raise the issue, I'll have to look for damage to the epoxy in that area. But the fin root is so long compared to the nozzle length that I don't think it would have any real effect on the overall strength of the fin attachment.

But just one more thing to worry about.

However, the issue of the aerodynamic benefits of fin placement is still outstanding it seems. I think the closer to the rear the better since it moves the CP farther back, which is critical in rockets trying to achieve the minimum wetted area for drag reduction. But maybe I am missing something else.


Tony

 

[Rocketjunkie]

Cloudbuster 54's and the Vulcan L750-30's for power. They went 18-19K ft. This was in 1991 at LDRS X and used motor ejection. Didn't have electronic deployment then.
These were not optimized for altitude, just a scaled down version of the 4.5" ones I flew the previous year.