Proven techniques for fin (and nosecone) survival at a bit over M5 and under 20Kft?

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

G_T

Well-Known Member
Joined
Dec 29, 2011
Messages
2,973
Reaction score
914
I've gone through simulations of too many motors for too long. Motors up through O, the largest I've made, have worked as designed within quite narrow specs. Burn time within 0.1 second of predicted. Flights coming in usually within 1 to 2 percent of predicted altitude. The only exception is when I pushed the case pressure too close to the limit and had a nozzle ejection at MEOP in the burn, which was designed essentially very close to the case limits in a 75mm. I didn't account for the extra force on the nozzle retention due to acceleration and was out of my best nozzle washers. I think the inferior one I used was not quite full diameter, warped the snap rings, and that was the end of that. Had I not been tired when assembling that motor, I'd have not used that washer. Lesson learned; move on. (PS - that motor was 1 success 1 fail)

I've modeled some motors in the larger O through baby Q range - triple taper monolithic core case bonded and not quite erosive - that I think I can probably make. There are some engineering challanges but I think I have ways of dealing with them. These are narrow motors for their total impulse, and do not have particularly long burns. They however do have pretty high mass fractions. This leads to survivability problems. Min diameter rockets based on these motors, in sizes from 88mm on up, all end up quite fast, in the M5 range +/- a hair at burnout per RasAero.

Making one of these for launch at BALLS in the next couple years is tempting. Though I have some of my own ideas that I think could work, I'd rather hear what people have done that has proven to work. This is a tougher problem than getting a second stage to survive these conditions as atmospheric pressure will be much greater.

This is still in the thought experiment stage. I'm not interested in discussing the motors further at this time, and certainly not in the open forums. The overall rocket concept is four smallish fins and a nosecone, with the fins attached to the motor tube.

I think I can handle the attachment, and getting the motor to survive the burn - at least in static test. But there will be considerable external heating and aero forces on the fins and nosecone, and even on the outside of the motor case. If aluminum gets too warm it loses most of its strength. Composites can peel apart.

I have experience and equipment for machining. I have experience making high quality composite parts. I have no experience with M5 rockets.

What has worked? What might work? What doesn't work? $$unobtainium$$ materials are not applicable. These motors were designed to be inexpensive for the total impulse and I'd like to keep it that way as much as possible. If possible solutions are too pricy or too unlikely to work, I'll drop the project.

Thanks,
Gerald
 
Last edited:
Gerald, I have never flown anything close to that velocity. I do have a couple thoughts though.

If you have machine availability and the skills could you use steel? Maybe a bolt on fin can similar to the ones Mike Fisher has been making. Steel will take a lot more heat than aluminum.

Next thought might be a composite carbon fin with a high temp ceramic coating. Was it cotronics that makes a ceramic based adhesives.

Then there is always carbon carbon. If it was good enough for the space shuttle it should be good enough for this. I have zero idea what it would cost though.
 
If you have machine availability and the skills could you use steel? Maybe a bolt on fin can similar to the ones Mike Fisher has been making. Steel will take a lot more heat than aluminum.

Aluminum will be fine for a short stint to M5.
 
I am not an engineer but the only way to know for sure would be to run the math. It sounds like you understand enough regarding atmospherically generated heat/friction at speed and thermal conductivity of metal to raise the question but not enough for a solution. Unfortunately from this forum I suspect the best answer you will get from the majority of those who chime in will be at best a guess. There are however, a few knowledgeable individuals at the level you are thinking whom peruse from time to time and hopefully will see your post and lead you in the right direction.

You mentioned your budget and that is understandable but it would be just plain sad to see a failure of any type at Mach 5 no matter how much dough you threw at the project. I know too little without doing my own research to offer a suggestion with any merit so speaking for myself, I would use titanium for the novelty alone!

You have an intriguing vision. Keep us updated!
 
I'm not a machinist, but I play one in my garage. Titanium is a PITA to machine. The pros complain about it all the time, so working with it in your garage even on a Bridgeport is going to be a bad experience. I vote for an aluminum fin can of some sort. The melting point for 6061 is 1080F so it should be good for short periods of time.
 
Then there is always carbon carbon. If it was good enough for the space shuttle it should be good enough for this. I have zero idea what it would cost though.

What altitude was the Space shuttle at when it went through Mach 3+ ?
 
You're out of the realm of hobby and into the realm of "professional aspirations," in my opinion. This sort of project requires a professional solution.

Hobby type band-aid solutions to ablation prevention of composites are likely out of the question. No "painting the leading edge with Proline or JB" here. I'd really recommend looking at what was used for the Loki dart (pretty sure you can buy a booster from Gary to take a look at) or other sounding rocket.

is-this-heavy-metal_o_703190.jpg
 
I am also interested.

I have never had a rocket go anywhere near that fast. That said, our team is talking with a professional aerospace company about doing some work with a hypersonic vehicle, which has us researching hypersonic nosecone design. It seems like the most foolproof solutions that fit within the safety code are either a blunt nosecone (high drag) or a drag-reducing aerospike that holds the shock off the nose.

Quicklaunch was planning to use a steel aerospike to withstand mach 20+ launches to orbital altitudes for gun launched projectiles. Something similar in aluminum should be quite effective.
 
Last edited:
Years ago, Kosdon used solid graphite nose cones on some of his high speed flights. (We actually found one of the a few years ago.) Whether he used it for easy of turning or because he had it laying around who knows. It will hold up, probably single use like the rest of the rocket but the weight will kill some of your speed.

Tony
 
I don't think ablatives are a band aid, all of the big launch vehicles I've worked on professionally have relied on them to protect carbon-epoxy structure. Aside from the Inconel nose cap on Orion LAS, silicone nose cap on Antares, cork is the choice. Heck, i have a shop full of Falcon 9 parts getting cork to protect them on the boost back burn to landing.

Granted, these are liquid engines and accelerate very different from a sounding rocket.
 
A few useful random links for those curious about some of the issues:

https://www.grc.nasa.gov/www/BGH/stagtmp.html (leading edge of fins, tip of nosecone, that sort of thing, where flow is stopped or radically diverted)

https://docs.engineeringtoolbox.com/documents/773/metal-modulus-elasticity.png

https://www.nakka-rocketry.net/pix/ftycht2.gif

https://www.nakka-rocketry.net/pix/hrate.gif - even with high rate heating, 2024-T3 which is better than the usual (and much cheaper) 6061-T6 cannot take much of a temperature rise. I was already looking at 2024-T3 for the motor case. 4x the cost of 6061-T6 though, and it might not be enough. I don't want a thick wall as that kills the mass fraction through hardware weight and loss of propellant cross section.

https://www.nakka-rocketry.net/therm.html for some more interesting reading. Unlike the usual motor case, for this application it will be hotter on the outside than the inside. And aluminum's high thermal conductivity might not be a plus in this instance. I can afford to have the motor tube lose a lot of its strength IF it does so only after the motor has depressurized.

For fins I've been considering a structure composed of stainless sheet folded skin around a spar which itself is solidly bolted through the motor tube into the nozzle retaining ring - itself essentially embedded in the nozzle structure. For these motors, the nozzle exit diameter is essentially just under full case diameter and even so they are a little underexpanded. The fins in my sims are simple wedge cross section, of relatively thick proportions compared to what many fly in high performance rockets. Being small helps. Having 4 should help negate any coning tendencies.

For a nosecone tip I was considering copper or more likely stainess as it is a poor thermal conductor, and an aerospike is definitely an option. I considered a hollow tip filled with water, with little holes around the base of the tip blocked with wax. Heating of the tip boils off the water and generates a steam and water envilope around the body of the rocket. I don't know if it would be an applicable or even useful technique or not. It probably wouldn't carry away enough heat quickly enough. Things are happening too fast. These designs are up to speed in a few seconds. It would be at max-Q likely before any real cooling would occur. Plus, sloshing liquid in the nose of a fast rocket sounds like a recipe for skywriting.

I wish I had info on thermal conductivity of some of the spray and cure ceramic coatings used in the gun industry. At least for the fins, and possibly everything, I'd consider such a surface coating. It may be useful under an ablative coating of some sort.

https://www.fabricatingandmetalworking.com/wp-content/uploads/2014/02/Alloy-Designation-Tree.jpg

pcalviln, I've considered thin cork as an insulation component in large long burn motors before. Metal, alodizing, bonded cork layer, EPDM+Kevlar thermal ablative, then propellant. It wouldn't be needed inside for these motors. I'm not sure how cork would be a survivable material on the outside at these speeds, unless phenolic loaded or some such? Anyway I don't want to add notably to the OD or mass of the rocket as either kills the performance.

A titanium tube would be out of my price range.

Were I allowed to shred a deliberately lightweight composite booster as a recovery method, I'd make a boosted dart motor + fin assembly which becomes unstable when the noseweight of the dart is released. A M5 tumble should take care of the rest. The dart could be solidly made to take the heat. No real payload capacity though if a payload was ever desired. For the larger end of the motors I was considering, it would take a lot of payload to keep it down and slow it down. I kept scaling down until the rocket wasn't quite so insane. That reduced it down to an 88mm O motor. I started much larger. I don't think I could readily make the motor any smaller than 88mm and 88 still sims to a bit over M5 and a bit over 100Kft, with conservative mass estimates.

Does anyone think BALLS would allow composite booster tumble/shred as a recovery method for a light single use booster, containing no metal?

Gerald

PS - Yes, in some ways the LOKI Dart is an analogue if it were not a dart. It is a good example case.
 
Last edited:
Titanium tube is really not very expensive. Try titaniumjoe.com
 
Gerald , I will suggest you look into a nice company called Techline Coatings . They make all sorts of high end coatings systems designed for different auto applications . I have used them a few times while doing a race motor build . One coating I will suggest for your fins and nosecone is made for high heat , turbo applications , and will reduce surface temps by 18% they claim . Another system they sell is made to go on your crankshaft and it makes the metal "slippery" so the oil in your pan won't coat the crank creating parasitic drag. I would recommend this for all surfaces .


Eric
 
Titanium tube is really not very expensive. Try titaniumjoe.com

Nothing seamless in larger sizes. Not much choice in wall thickness. They don't have useful material for this application, so the price doesn't matter. Thanks for the link though. I may have a use for something they carry sometime.

Gerald
 
First if you use titanium or steel it cannot be launched at a Tripoli event without petitioning the board for approval. Don't hold your breath on getting it.

I launched to M4 with aluminum nose cone and fins, both showed absolutely no damage. I machined the cone from bar stock and hollowed to leave 1/4" thickness. If I was going to do it again I would use spun aluminum. Fins were welded can by Kosdon.
Frank's graphite NC was because he had it.
Loki fins are are aluminum with square edges. Dart NC is steel, for weight not heat.

M
 
You shouldn't need to go too crazy with the materials.

About every sounding rocket I have seen that has gone close to that realm of flight used aluminum flanges to hold the fins on.

So if I was making the rocket as inexpensively as possible, I would go with Mike's fincan. Through I would probibly use a different set of fins, the high root to cord ratio of the stock fins isn't the best for flutter.

Then I would bond the fincan directly to the motor case with a high temperature adhesive. JB weld should work fine, mostly since aluminum has lost most of it's strength above 500 F anyway. Gluing the fins will actually save more weight the welding. When you weld aluminum you lose about half it's strength. So you have to make everything thicker to compensate, or you have to anneal the aluminum and re-precipitation harden it.

Though I would be more meticulous than normal on my bond prep. Getting the aluminum to pass the water break test: https://www.ctgclean.com/tech-blog/2011/08/is-it-clean-oil-and-hydrophobic-films-water-break-test/ would be a good idea.

It is actually pretty difficult to clean something well enough to pass the water break test, but if you can get close your adhesion properties are much better, as there is absolutely no oils present to prevent a good bond. Keep in mind if you get something to pass the water break test it will fail it again if you leave it out for a few hours, just from the trace oils present in the air.

For the rest of the airframe fiberglass would be fine. I'd make a fiberglass conic nosecone with a metal tip. I'd go with 2024 for the nose tip and the fins. Round stock large enough is ~$30 for a foot, and sufficient fin material is about $70. It would be worth the little bit of extra head room for flutter.

The last thing I would add is some ablative paint. I think I have talked about it previously, it's a 3M silicone RTV ablative. It works being 1 foot away from the nozzle of a minotaur IV taking off. So it should handle everything. I would paint it along the body tube of anything that stuck out, the transition on the nose cone, and the angled part of the nosecone up to the tip.

the primary purpose of the ablative would be to protect it from erosion so I can fly it for more than a few times. It would also keep the fiberglass from delaminating.
 
Last edited:
I think when you are talking about this it would be good to sit down and actually engineer the rocket. Find the loading on the fins, then start your design. The quote that comes to mind is "In God We Trust, all others bring data."

Edward
 
The last thing I would add is some ablative paint. I think I have talked about it previously, it's a 3M silicone RTV ablative. It works being 1 foot away from the nozzle of a minotaur IV taking off. So it should handle everything. I would paint it along the body tube of anything that stuck out, the transition on the nose cone, and the angled part of the nosecone up to the tip.
Xrain,
I'm not finding a 3M silicone RTV silicone ablative paint, but I've found a couple similar products. The regular high temp 3M silicone sealant, but no ablative and no paint. I've tried this, but wasnt happy with how well it bonded to composites (or anything for that matter). Are we thinking of the same thing? Another product I've been looking at is Dow Corning 3-6077. Supposedly some silicone based ablative. I have yet to try it.

I'm working on my own M3.5-M4 ishh project for BALLS, so its about time I stepped up my composites/thermal protection game.

BTW, Gerald - if you want some Titanium Round stock for cheap (basically just shipping), PM me. I have a big piece of 2in 6-4 ti round stock kicking around I'd love to see put to good use. It might not be necessary, but it's just another option.

Cool thread.
 
The DOW 3-6077 is interesting stuff. The stuff I have might be old stock and is very thick. I thin it and use it on my moon burners.

Tony

DOW.jpg
 
I will find out shortly with my "Q" project, it uses Mikes fin can, I have no fear and I know it'll work just fine.
 
I will find out shortly with my "Q" project, it uses Mikes fin can, I have no fear and I know it'll work just fine.

Want to see that on video please! What is the airframe - size, material etc.?
 
Xrain,
I'm not finding a 3M silicone RTV silicone ablative paint, but I've found a couple similar products. The regular high temp 3M silicone sealant, but no ablative and no paint. I've tried this, but wasnt happy with how well it bonded to composites (or anything for that matter). Are we thinking of the same thing? Another product I've been looking at is Dow Corning 3-6077. Supposedly some silicone based ablative. I have yet to try it.

I'm working on my own M3.5-M4 ishh project for BALLS, so its about time I stepped up my composites/thermal protection game.

BTW, Gerald - if you want some Titanium Round stock for cheap (basically just shipping), PM me. I have a big piece of 2in 6-4 ti round stock kicking around I'd love to see put to good use. It might not be necessary, but it's just another option.

Cool thread.

It was the dow corning 6077 that I meant to speak about. Was trying to remember off the top of my head and had 3m stuck in there for whatever reason.

I have a few pounds of old stock, though I also have a whole bottle of the primer. We would use it on launch stools to protect the steel from the flame of the rocket launching.

I think I'll PM you about the titanium, I have a pretty big project I am working on that could use it. I'll be detailing it in research section to start with soon.
 
FWIW aluminum Max Q Aerospace fin can has been to M4.2.

I have one for a smaller project. Thanks!

Due to the expected speed difference, I'd expect the aero loads to be on the order of 50% higher, and the heating loads would be substantially higher. Aero forces tend to be roughly a function of the square of the speed.

Gerald
 
...
Then I would bond the fincan directly to the motor case with a high temperature adhesive. JB weld should work fine, mostly since aluminum has lost most of it's strength above 500 F anyway. Gluing the fins will actually save more weight the welding. When you weld aluminum you lose about half it's strength. So you have to make everything thicker to compensate, or you have to anneal the aluminum and re-precipitation harden it.

Though I would be more meticulous than normal on my bond prep. Getting the aluminum to pass the water break test: https://www.ctgclean.com/tech-blog/2011/08/is-it-clean-oil-and-hydrophobic-films-water-break-test/ would be a good idea.

It is actually pretty difficult to clean something well enough to pass the water break test, but if you can get close your adhesion properties are much better, as there is absolutely no oils present to prevent a good bond. Keep in mind if you get something to pass the water break test it will fail it again if you leave it out for a few hours, just from the trace oils present in the air. ...

It turns out that the water break test is insufficient for aluminum. Aluminum is a special case. Aluminum is a super reactive metal. Exposed aluminum reacts in milliseconds with oxygen to form Al2O3 - AKA sapphire. This skin coating can be cleaned, certainly, but it is very difficult to bond solidly to it. That is because aluminum oxide is very stable. Chemical bonding will not be possible. That leaves mechanical bonding. Frankly I wouldn't trust that for anything structural. Here's a quick little blurb I found on the subject https://www.epotek.com/site/files/Techtips/pdfs/Tech_Tip_24_-_Bonding_to_AL.pdf

To improve the bonding to aluminum, one needs to do some surface prep beyond cleaning. There are at least two options:

1) If bonding with an adhesive such as epoxy which can act as an oxygen barrier (doubt JBWeld given its thickness and filling will do this, but it might if it is degassed first) then one can wet sand with the epoxy. Then wipe off the gunk except leave a layer of epoxy behind. Add fresh epoxy, and swim/squish the part into place. Wipe off excess. Due to the mechanical removal of the surface during sanding, epoxy has had a chance to directly contact aluminum. That is, IF the epoxy acts as a sufficient oxygen barrier.

2) Alodine or another surface treatment which is generally IIRC a chromium or phosphate surface replacement. https://www.aircraft-spruce.com/menus/me/metalprep.html is one source for some of the options. Note the chromium surface treatments are rather toxic and an environmental hazard.

When it comes to bonding to aluminum, there is another potential consideration. If a dissimilar material is bonded which is also conductive, it is possible to form a battery. Bonding carbon fiber to aluminum for instance will do this. The result is galvanic corrosion at the bond. This corrosion can be slow in a dry environment, or pretty quick in a humid environment. There was a military helecopter prototype which made this mistake, in spite of it being a well known issue in the industry... Usually a non-conductive coating of some sort is used to separate the aluminum from carbon fiber.

Gerald
 
That is extremely interesting, also extraordinarily pertinent to some things I am working on, thanks Gerald! I've done the water break test before out of curiosity on some stainless and never questioned if it worked on aluminum.

Some members on my team are in the process of designing some composite overwrapped aluminum tanks. Learning stuff like this early on before we committed to a process is so much nicer. I'm pretty sure they would have discovered some of these problems

That chromate and sulfuric acid preparation solution for aluminum sounds like pleasant stuff to deal with.

I am pretty interested in seeing where your project develops!
 
Back
Top