Testing for Hobby-Accessible Ablative

[bobkrech]

I realize the OP is looking for comments on structural ablatives but I figured I'd mention my experience with ablatives used in combustion chambers for those interested. The comments below are targeting a purely amateur project.

I've had real good luck with ablatives used in my liquid engine over the years. I've used everything from simple E-Glass/resin forms to more complex cured S-Glass/phenolic structures. In the end, the application determined the best option which also turned out to be the easiest to produce. For "my" application, I needed an ablative liner that would operate in the combustor of my LOX/Ethanol engine and last for 15 seconds minimum without anything melting.

View attachment 123446

My ablative needed to perform three basic functions. First it has to act as an insulator, second it needs to move heat by ablation, and finally it needs to maintain some sort of structural integrity. The first engine liners I produced used standard store bought fiberglass as the structural component bonded with an ablating epoxy resin. These liners would go through a cycle of ablating the resin while also melting the glass. Molten glass would flow down walls and coat the nozzle which provided some protection against oxidation of the graphite throat.

View attachment 123447

I later switched to a silicia glass which wouldn't melt but had other problems. First was the ablative would vaporize leaving behind an unsupported silica cloth that would tear and the motor would then "cough" out fur balls of silica as it ran. I then switched resins, changed the layup structure to no avail. In the end I went back to the simple E-glass/epoxy system. I only needed 15 seconds and the liner always provided me with that.

In this photo you can see the molten glass deposited on the throat of the engine. The burn mark on the outer ring is just burned electrical tape. The fellow in the background is Tom Mueller (spacex).

View attachment 123450

Eric

Nice piece of work, Eric. As you discovered the epoxy pyrolizes and generates a reducing gas that sucks up a lot of thermal energy, and the molten glass flows and covers the downstream surface preventing oxidation. I'm curious if you measured the recession of the glass composite.

In some professional nozzles, overlapping tape is used instead of cloth. This permits an angled, lengthened thermal path to increase the strength and thermal resistance of the nozzle. If carbon fiber is used instead of glass, the phenolic epoxy pyrolizes in depth and the gases flow through the carbon fibers and prevent oxygen from reaching the carbon surface. Carbon fiber does not melt, and provides a radiative cooling mechanism for the surface, and becomes a true ablative, vaporizing into C, C2 and C3 molecules and in the process, carrying off a lot more heat.

You might be interested in googling "epoxy intumescent fire protection coating" for commercial thermal protection coatings that behave similarly.

Bob

 

[[email protected]]

Nice piece of work, Eric. As you discovered the epoxy pyrolizes and generates a reducing gas that sucks up a lot of thermal energy, and the molten glass flows and covers the downstream surface preventing oxidation. I'm curious if you measured the recession of the glass composite.

In some professional nozzles, overlapping tape is used instead of cloth. This permits an angled, lengthened thermal path to increase the strength and thermal resistance of the nozzle. If carbon fiber is used instead of glass, the phenolic epoxy pyrolizes in depth and the gases flow through the carbon fibers and prevent oxygen from reaching the carbon surface. Carbon fiber does not melt, and provides a radiative cooling mechanism for the surface, and becomes a true ablative, vaporizing into C, C2 and C3 molecules and in the process, carrying off a lot more heat.

You might be interested in googling "epoxy intumescent fire protection coating" for commercial thermal protection coatings that behave similarly.

Bob

I did make one liner that used silica tape that was wrapped at slant so the width of the tape went through the entire width of the ablative. I used a plenco phenolic resin that was vacuum cured in a curing oven I made. In the end the liner was still only good for a single use and was about 10x the effort to fabricate. I never made another one.

The low cost epoxy glass liner ablated at about 0.100" per 10 seconds of run time. Not great, but good enough for my application. My injector has one split triplet that impinges slightly offset and I always get a slightly deeper ablative hole right across from that injector. That region normally see's about .150" per 10 seconds of run time. I suspect some oxygen is getting to the wall and the boundary layer cooling injectors are then burning that free oxygen at the wall.

If I ever have the time to go back and fire up the engine again I'd probably convert the chamber to a full regenerative cooling.

Eric

 

[bobkrech]

I did make one liner that used silica tape that was wrapped at slant so the width of the tape went through the entire width of the ablative. I used a plenco phenolic resin that was vacuum cured in a curing oven I made. In the end the liner was still only good for a single use and was about 10x the effort to fabricate. I never made another one.

The low cost epoxy glass liner ablated at about 0.100" per 10 seconds of run time. Not great, but good enough for my application. My injector has one split triplet that impinges slightly offset and I always get a slightly deeper ablative hole right across from that injector. That region normally see's about .150" per 10 seconds of run time. I suspect some oxygen is getting to the wall and the boundary layer cooling injectors are then burning that free oxygen at the wall.

If I ever have the time to go back and fire up the engine again I'd probably convert the chamber to a full regenerative cooling.

Eric

I conducted some high radiance heating test on a COTS 45 mil 5-ply CF panel. In 10 seconds the ablation depth was ~36 mil. Measured surface temperature was over 3000 C.

Bob

 

[bobkrech]

Just to close the loop on an earlier post I made in this thread concerning AVCOAT.

Guess what flew into Hanscomb AFS Tuesday. A Super Guppy with a present from NASA to Textron Wilmington fromerly known as AVCO Wilmington.

https://www.boston.com/metrodesk/2013/03/26/guppy/xqkL7JizJx3Ie9z0MfTDwJ/story.html

[YOUTUBE]Z4jVpBMTPX0[/YOUTUBE]

Here's an historical Apollo document from 1967. Just like a good vintage wine....

Bob

 

[GregGleason]

In June 1967, the X-15A-2 rocket-powered research aircraft received a full-scale ablative coating to protect the craft from the high temperatures associated with hypersonic flight (above Mach 5). This pink eraser-like substance, applied to the X-15A-2 aircraft (56-6671), was then covered with a white sealant coat before flight. This coating would help the #2 aircraft reach the record speed of 4,520 mph (Mach 6.7).

cite: https://www.dfrc.nasa.gov/gallery/photo/X-15/HTML/ECN-1736.html

See also https://www.mach25media.com/x15chap10.html

Greg

 

[bobkrech]

Probably an early version of this DOW CORNING® 3-6077 RTV SILICONE ABLATIVE KIT

https://www.ebay.com/itm/Dow-Cornin...090?pt=LH_DefaultDomain_0&hash=item20cc83d05a

https://www.neelyindustries.com/p-2...ilicone-ablative-coating-11lbs-5-kg-pail.aspx

https://www.eis-inc.com/suppliers/productdetail.asp?prod_nbr=DC3-6077X5KG

etc.

Bob

 

[GregGleason]

Probably an early version of this DOW CORNING® 3-6077 RTV SILICONE ABLATIVE KIT

https://www.ebay.com/itm/Dow-Cornin...090?pt=LH_DefaultDomain_0&hash=item20cc83d05a

https://www.neelyindustries.com/p-2...ilicone-ablative-coating-11lbs-5-kg-pail.aspx

https://www.eis-inc.com/suppliers/productdetail.asp?prod_nbr=DC3-6077X5KG

etc.

Bob

From another source on the X-15 ablative testing:

On that date, Captain William J. Knight piloted the X-15 to a new unofficial world record speed of Mach 6.33. This record was
accomplished, in part, due to the use of an experimental ablative coating developed by the Martin Marietta Corporation under the name
MA-25S. [14] Heat shielding was a major experiment that the X-15 was used for and the heat shields of Apollo, Voyager, and the Space
Shuttle can thank this "little" aircraft for all the effort it put into testing different forms of heat shields.
Some of the other ablative coatings evaluated were Emerson Electric Thermo-Lag T500, Dow Corning DC-325, Armstrong Cork #2755,
Martin ME-32H and MA-45R, McDonnell B-44, NASA E-2A-1 Purple Blend, Molded Refrasil Phenolics, and the General Electric Series
materials. Of all the ablative coatings used on the X-15, the Thermo-Lag T500 was proven the most effective, but it was also the most
difficult to work with and very seldom achieved a uniform quality (both very important negatives towards its use).[15]

Cite: https://www.helium.com/items/1451460-the-grandfather-of-the-space-shuttle-the-x-15

Lockheed Martin still makes the ablative material MA-25S:

https://www.lockheedmartin.com/cont...rmalprotection/MA-25S Product Information.pdf

Greg

 

[butalane]

I too was an undergraduate back in the dark ages. You are correct that most undergraduates don't have a lot of money, but they do have a lot of time, and for the most part are quite intelligent and capable of solving problems. The biggest impediments are that most undergraduates don't know what they don't know, don't know what they need to know, and think it's really difficult to get the knowledge. I got my BS 40 years ago, and I let you in on a little secret. If you know you want to be an engineer or scientist, start reading and researching topics you are interested in. See what has been done before, and how it was done, and by doing this you will learn what has to be done in the future in that area. This is what you will do as a graduate student or as a practicing engineer or scientist for the rest of your career.

As for your project, go to your engineering library and look up the titles of a dozen EE trade magazines then get on-line to their website and research the topic. You'll find the latest and greatest on the topic.

.....

By the time you are a junior or senior, you have the basic engineering tools to research and solve problems, whether you know it or not. If you spend some up front time researching you problem, you'll come up with several ways to solve it. If you don't understand the problem and don't research to work out a measurement plan in detail before hand, at best you'll make an amusing video for YouTube....

Bob

I'm sorry but you missed my message which is that you don't have to reinvent the wheel. What Chris and Carlo wanted to do was developed, proven and documented in the '50s and '60s, and the information is readily available today. There was no reason for them to develop an ablator (which is not required for a Mach 4 flight) because the technology was developed 50 years ago, and it is there for the taking. If they had spend the same time researching the topic instead of playing around, they would have found this out, without wasting time and money, and probably would already have built and flown their rocket successfully. If they had needed an ablator, they would have found several ablator concepts that they could have formulated to make a proven ablator material.

While the may be a few successful extreme flights that were conducted by true amateurs, every successful one I am aware of has involved one or more unpaid professionals, and since they were not paid, they are amateur efforts. Certainly, the GoFAST and SugarShot efforts ha(d)(s) many unpaid professional participants.

Having access to and using simulation tools does not make someone an engineer. IMO I think that the current software tools can be an impediment to students learning engineering because they can be lulled into a false sense of security by a very precise equation solver that provides a solution even if the engineering model was not correct. You will almost always get a precise solution to your problem using a tool, however unless you understand the scientific and engineering principles behind your problem, you may find that you have generated an extremely precise but inaccurate solution because a boundary condition was omitted or misrepesented. If you had to solve the problem by hand, you might have found it earlier.

Before these tools existed, engineers and scientists would bound the solution space by quickly calculating limits by making certain simplifying assumption before performing a more detailed and time consuming calculation that would have more precision. If your solution was outside your limit calculations, it was obvious you missed something. I've helped out many young engineers and scientists having difficulties with complex system by performing limit calculation and helping them find what was left out of their model.

Again, my point is that you can do excellent engineering without using sophiscated tools if you understand the underlying principles. Once you learn what's behind the engineering you can work a lot faster with the right tools. With experience you will also recognize that it's perectly ok to use some else's proven solution if it fits your needs because it saves both time and money, and that's the other side of engineering you may not learn in school.

Bob

Damn good points.

 

[bobkrech]

From another source on the X-15 ablative testing:



Cite: https://www.helium.com/items/1451460-the-grandfather-of-the-space-shuttle-the-x-15

Lockheed Martin still makes the ablative material MA-25S:

https://www.lockheedmartin.com/content/dam/lockheed/data/space/documents/thermalprotection/MA-25S%20Product%20Information.pdf

Greg

Thanks Greg. Nasa's database is down so I couldn't look it up.... The data sheet describes it as a filled silicone rubber. It should be somewhat similar to the DC material I listed.

MA-255 is a respectable low temperature ablatative. With a heat flux of 33 watts/cm2. the Q* (mass loss or ablation coefficient) is 55.85 KJ/gm at 510 C at or about 24.7 times more efficient dissipating heat than generating steam at 100 C which absorbs 2.26 KJ/gm. At 33 watts/cm2 heat flux, the ablation rate is 0.01 mm/second.

Most of the heat flux problems I study are 3 to 300 times higher than this.

Bob

 

[CCotner]

I am probably risking an infraction by posting in this thread again, but I feel that it is constructive, and it is my thread.

Bob, you say that our 'amusing youtube videos' were unnecessary because we did not need to develop a new ablative (which we couldn't do anyway because we're not wealthy and don't have time) because you and the rest of the professional world developed them already, and we don't even need them for only mach 4 for a few seconds.

Perhaps a better way to start this thread would have been to list our design goals and the limits we had already placed on the solution space. Then, we would have more easily communicated the background for the test: the comercial products that have already been made do not meet the specific requirements we are seeking.

Another problem with the post is that we used the term 'ablative', and Bob and others have responded with vitriolic criticism in part based on a very specific and technical definition of the term. The word we should have used is probably closer to 'thermal protection system'.

Here are our design goals and solution space restrictions:

• Hobby-Accessible: Cheap, non-toxic (if possible), not reliant on sketchy ebay deals. (this rules out many/most professional products off the bat, such as Firex intumescing coats and the DOW stuff you linked to above.
• Low-Temperature: our epoxy/kevlar structure is only good to 300F at the maximum, which, while significantly higher than many conventional laminating epoxies, is not very high. Many commercial products (such as AVCOAT) would (potentially, maybe not on a single N5800, but on higher-thermal-stress flights) allow the underlying structure to overheat before the ablative mechanism becomes effective.
• Smooth Surface: this is where charring/intumescing materials (especially AVCOAT, the DOW silicone rubber products, and the firex) fail. I have seen first-hand the results of some Mach 3 plus altitude attempt flights (Von Delius's Falcon rockets come to mind), and the post-ablation surface is an aerodynamic nightmare for a small rocket. Does the X15 notice the surface roughness? No, but a 7' by 4" rocket does.
• RF-Transparent for GPS/Tracking/Telemetry: this is where RCC and graphite fail as simple insulating TPS.
• Other concerns: workability/machineability, porosity, density, chemical stability, ease to re-apply between flights, etc.

Hopefully this explains a little more. We have not ignored 60 years of research work by qualified professionals out of anti-intellectualist sentiment or blind ignorance, despite claims to the contrary. Neither have we attempted to reengineer the entire field of rocket thermal protection.

As Bob said, we are only going mach 4, and only for a few seconds. We *still* need to protect our nosecone structure from the stagnation heat somehow. Rather than slathering high-temp epoxy or paint on the surface and praying that it is good, as seems to be the standard response, we wanted to make some effort to do better. We chose to do this through research, theory, and experimentation on our version of a commercial product, the only one we found that met our requirements. If I called this 'engineering' and subsequently offended Bob or any of the other engineers on this thread, I apologize.

 

[troj]

I am probably risking an infraction by posting in this thread again

Nowhere even close.

-Kevin

 

[ECayemberg]

Probably an early version of this DOW CORNING® 3-6077 RTV SILICONE ABLATIVE KIT

https://www.ebay.com/itm/Dow-Cornin...090?pt=LH_DefaultDomain_0&hash=item20cc83d05a

https://www.neelyindustries.com/p-2...ilicone-ablative-coating-11lbs-5-kg-pail.aspx

https://www.eis-inc.com/suppliers/productdetail.asp?prod_nbr=DC3-6077X5KG

etc.

Bob

I've been playing around with the Dow 3-6077 for a while now. I have been unimpressed by its adhesion to a variety of base materials. I would fear it would slump or worse: peel off when subjected to high speed flow.

I have been impressed however with the moldability of the product. Applied to a mold with release, it forms a very smooth surface.

Like other Silicones, it is fairly soft and resilient even after full cure.

-Eric-

 

[bobkrech]

Silicone rubbers are funny materials. You almost always need to use a surface primer to get a good bond to the surface.

There are also usually 2 versions of a silicone product: one that can be sprayed and one that is troweled. From what you said, I think you got the trowelable stuff.

The sprayable MA-255 silicone used on the X-15 is probably a better option. As with any silicone, following the instruction exactly is a requisite for getting a coating that sticks to the article and has it rated properties.

Bob

 

[CCotner]

The MA-255, in addition to not being for sale directly to civilian buyers in small quantities (not hobby accessible), has an ablation temperature of 1200F, which is in excess of the 300F maximum structural temperature. Even if it ablates at the temperatures listed in the thermal conductivity tests it will not protect the epoxy/kevlar underneath.

 

[bobkrech]

Mach 4 rocket flights have been routinely conducted since the '50s, so it's frustrating to someone who has been tutoring and mentoring young scientists and engineers for more than four decades to encounter a young engineering student who can't be convinced of the necessity to calculate aerothermal loads when engineering a thermal protection system.

I'm done with this thread.

Bob

 

[CCotner]

Mach 4 rocket flights have been routinely conducted since the '50s, so it's frustrating to someone who has been tutoring and mentoring young scientists and engineers for more than four decades to encounter a young engineering student who can't be convinced of the necessity to calculate aerothermal loads when engineering a thermal protection system.

I'm done with this thread.

Bob

Again, Bob, I'm sorry if we gave the wrong impression or offended you or any one else on the forum. We have calculated the rough expected aerothermal loads for the rocket. We understand why it is important.

At max-Q, the vehicle is dissipating a terrifying 1.2MW into a variety of physical pathways. This number comes from the simple difference in power into the rocket (exhaust velocity and thrust) and the rocket's acceleration. The predominant physical sink for that dE/dT is the air surrounding the rocket, through forming and maintaining shock waves emanating from the nose tip and fins, and compressing and re-directing air around the body. Some portion of that power is spent in the boundary layer stagnation, and is then transferred into the nosecone (and the rest of the body) as heat. This is not a trivial calculation, as the dynamics of the boundary layer have to be estimated or analytically solved. Based on methods detailed in MIL-HDBK-762(MI), a bit of the NASA technical archive I copied before it went down because Congress can't legislate anymore, I estimated the nosecone thermal loading at around 25-30kW at max-Q, a number not entirely dissimilar to what you present at the beginning of this document in your little graph.

Temperature for that load is easier to calculate, as it is a simple analytical relation across the first shock. It is 1300K at the maximum, depending on how close to isentropic the boundary layer is.


In response to your earlier points about designing a new system; HMC teaches us that if no existing solution is a perfect fit, that one should develop a new solution if one can afford to do so, for the benefits of fully understanding the system's development and application, rather than adopting the design so a stock solution can be used.

 

[CarVac]

In response to your earlier points about designing a new system; HMC teaches us that if no existing solution is a perfect fit, that one should develop a new solution if one can afford to do so, for the benefits of fully understanding the system's development and application, rather than adopting the design so a stock solution can be used.

To clarify, this is more specifically for the purposes of learning. We can read test documents all day, but without seeing it ourselves (and recording it for posterity in a video), we won't learn as much as having developed a new solution and tested it.

This is not to say that they teach us to do this for every design problem we encounter for the rest of our careers; in fact much of our project work ends up teaching us the futility of developing a new solution when existing solutions are available. On the other hand, though, the project work has led us to not particularly trust others' test results, especially with regards to our ability to duplicate them with our limited experience, funding, and time. By performing our own tests, we can ensure that what is tested is something that we would be able to implement in our final design.

We made the choice to gain a better understanding of the material, and of our own capabilities, rather than engineer as efficiently as possible.

 

[JLRockets]

To clarify, this is more specifically for the purposes of learning.

Some quotes from educater Jean Piaget:

"Each time one prematurely teaches a child something he could have discovered himself, that child is kept from inventing it and consequently from understanding it completely.”

“The principle goal of education in the schools should be creating men and women who are capable of doing new things, not simply repeating what other generations have done; men and women who are creative, inventive and discoverers, who can be critical and verify, and not accept, everything they are offered.”

Kudos, guys. Keep discovering. I've enjoyed this thread.

Judy

 

[BLKKROW]

I would like to chime in and say I love this thread, and hope more information like this can be shared.

 

[IronSwan]

Looking for help finding a supplier who can bond cork to aluminum and EPDM rubber. Thank you

 

[A5tr0 An0n]

I am just curious if you guys have done any other testing? Also have you done anything in regards to your boattail testing? The last time I checked I did not see any updates. Thank you very much.

 

[butalane]

Looking for help finding a supplier who can bond cork to aluminum and EPDM rubber. Thank you

How many parts and are they shippable? Direct bonding? .