Mach 3.5 Loki L Altitude Record Attempt Build

[OverTheTop]

There is a great text book by John Anderson, Fundamentals of Aerodynamics. College level but really well written and understandable. IIRC (I think it was that bookof his. I have read a few.) there was a nice graph in there of what happens to total temperature (stagnation temperature), total pressure etc, when the Mach number increases. It is a really insightful graph. Hint: Search for the book title and "free pdf" .

It really was a good read.

 

[Not Quite Nominal]

Have you calculated the stagnation temperature at that Mach number to see what the aerodynamic heating could be?

I think stagnation temperature (zero velocity, thermal equilibrium) is the worst case scenario, likely to be seen only by the nose cone tip and the fin leading edge tips. Everywhere else you have more mass/thermal inertia, and some flow. Looks like we're dealing with the old 1960s engineering problems - a sharp leading edge is aerodynamically optimal for supersonic flight, but a blunt one is thermally optimal.

Perhaps it's time I revisit this for real. I am not an engineer, I'm a pilot. We're practically forbidden from doing any math that doesn't involve a 3:1 ratio.

For some reason, we all turn into math geniuses when it comes to calculating 117 limits and duty rig pay overrides.

 

[pbahorich]

Stagnation temperature (total air temperature) is actually very easy to calculate.
Ttot = Ta*(1+M^2*(gamma-1)/2)
Where:
Ttot is stagnation temperature
Ta is ambient temperature
M is mach number
gamma is ratio of specific heats = 1.4

The only tricky thing is that the temperatures are in absolute units, degrees Rankine (deg F +459.6) or Kelvin (deg C + 273.15).

So assuming a 50 deg F ground temperature, Ta = 50+459.6 = 509.6 deg R.
Assuming M=3.5,
Ttot = 509.6*(1+3.5^2*(1.4-1)/2 = 1758 deg R = 1298 deg F.

I am an Aerospace Engineer by degree, but have worked primarily in aircraft systems and not aerodynamics. I'm going off memory, but I still use that equation once in a blue moon.

The tip of that nose cone will probably see close to 100% recovery, so 1298 deg F for a few seconds. I expect the tip will indeed ablate. Due to the short exposure time it may not fail. I wouldn't risk it, but I'm not you. Why don't you try a 3D printed titanium from one of the various suppliers, like this one:
Online Titanium 3D Printing Service | i.materialise
You would probably want to make it hollow. If you need ballast maybe you could fill it with lead from openings on the bottom.

 

[pbahorich]

Wikipedia presents a nice summary here:
Total air temperature - Wikipedia

Really nice project BTW.

 

[OverTheTop]

Remember the fin leading edges only see a fraction of the total velocity due to their angle to the airstream. The total temperature there is reduced. The calculation of that velocity is a simple trig calculation concerning the flight speed and edge angles. Just calculate the air velocity normal to the leading edge angle.

The NC will be hit with the maximum Ttot at the tip.

 

[AeroAggie]

Save yourself some math and skip straight to the Normal Shock Tables to get T2/T1 for any Mach number. You'll still have to convert F to R and back again.

edit - pbahorich is correct below. The normal shock tables are useful for fins, but are not correct for the stagnation temperature on the nose. The Isentropic Flow Tables are the ones that tabulate the formula he gave above.

I wouldn't use ground temperature. Find the altitude that you hit your maximum Mach number and use the temperature at that altitude from the Standard Atmosphere for your preliminary calcs. You can get actual temperatures at that altitude on the day of the launch and recalculate as a sanity check if you're really concerned about it.

 

[pbahorich]

Save yourself some math and skip straight to the Normal Shock Tables to get T2/T1 for any Mach number. You'll still have to convert F to R and back again.

I wouldn't use ground temperature. Find the altitude that you hit your maximum Mach number and use the temperature at that altitude from the Standard Atmosphere for your preliminary calcs. You can get actual temperatures at that altitude on the day of the launch and recalculate as a sanity check if you're really concerned about it.

I agree that temperature at altitude from the standard atmosphere chart should be used.

But I disagree with using the Normal Shock Tables to calculate stagnation temperature on the nose. The tables show the increase of temperature across a shock wave, not due to normal stagnation. For example, they show zero temperature rise at subsonic speed, which is not correct.

They could be helpful in predicting fin temperature however. But in that case I think two shock waves would be relevant: one at the nose and another at the leading edge of the fin root (see picture) provided the fin leading edge is swept enough for the tip to stay behind the shock wave. I'd need to dig up my old textbooks to figure out how to do multiple shock waves.

 

[AeroAggie]

You're right - Should've been Isentropic Tables for the nose cone. Been a long time since I've opened Anderson.

 

[watheyak]

Thanks everyone for the info on aerodynamic heating. I'm about to start working through some of that, as I have some down time in the hotel.

Things will pick up again after I get home from this work trip on Wednesday. Up next is fin fillets and tip to tip with the prepreg. My goal is to get the have the whole fincan fabricated and polished in the six days that I'm home.

 

[watheyak]

Subscribed!
Are you still using the same roll of TenCate M40JB prepreg? Have you noticed any change in the stiffness/tackiness of the prepreg after all those years?

Yes, same stuff. I have noticed a slight decrease in tackiness, but not much. It's been well taken care of and is still going strong. I'm pretty surprised, but the end product is just as good as it's ever been. I really wish I had a source for more of that exact stuff, it's pretty amazing.

 

[watheyak]

There is a great text book by John Anderson, Fundamentals of Aerodynamics. College level but really well written and understandable. IIRC (I think it was that bookof his. I have read a few.) there was a nice graph in there of what happens to total temperature (stagnation temperature), total pressure etc, when the Mach number increases. It is a really insightful graph. Hint: Search for the book title and "free pdf" .

It really was a good read.

Found it! Thanks.

 

[grouch]

Oh so very cool Scott. Nothing to add other than my kudos! Will this be flying this spring at Eagle Eye?

 

[Rocket501]

I have been following this project with great interest. Your craftmanship is superb. Could you please share the mass of the nosecone?

I am also working on a flying case design, except with a stretched version of the J99 as the motor of choice. The design I am hoping to use as my nosecone ended up being rather similar to yours, so these threads have been very informative.

 

[watheyak]

Oh so very cool Scott. Nothing to add other than my kudos! Will this be flying this spring at Eagle Eye?

Thanks! Yep, Eagle Eye. Hopefully the February launch.

 

[watheyak]

Ok, slowly working back into this after the holidays. Actually, I worked over the holidays, I'm still catching up.

I got a cool part in the mail the other day from one Jordan Slavish. Jordan is a high school teacher who has access to a CNC mill. I had put out a request on the TRF FaceBook page looking for someone to make this part. Jordan agreed to do it for just a donation to his high school's rocket club. I sent him the Fusion file and he nailed it! Threads fit perfectly, great workmanship all around. Great work, and a great cause benefitted. Thanks Jordan!

This rocket has no airframe. It really is just a nosecone on a motor. Adding flight tresses to a snap-ring forward closure in addition to the stresses it's already experiencing could lead to any number of exciting, rapid, destructive sequences. Previous versions of this setup had been using a version of this part that didn't have threads. Everything worked fine, but as I mind sim how this all fits together, as well as conversations with Loki Scott, this whole assembly continues to evolve. My goal is to lock the forward closure in place the best I can, so any bending moment this joint sees wont be transferred to the snap-ring, o-rings and liner. It worked with less on a slightly smaller motor, I'm hoping this setup buys me a little insurance.









For reference, here's a cut away of hoy the whole shebang will for together. The eye-bolt will be long enough to thread fully down into the closure.

 

[AeroAggie]

I love it. That's a great piece of engineering you did there.

I'd love to buy one if Jordan is willing to make another one. I've got some other CNC work I need to farm out if he's up to the task.

 

[watheyak]

When it came to attaching the fins, I went down an adhesive rabbit hole for this project.

There's lots of good glue out there. Any number of laminating epoxies with various additives, Hysol, ScotchWeld, Rocketpoxy. There's a good chance that with a nice fillet, Hysol or my favorite, ScotchWeld DP-460NS would have been fine all by themselves with no other reinforcement. Also, tip to tip makes a whole bunch of it's own problems that need to be dealt with.

But I had some other external goals with this project. I wanted it to scale up to something more extreme, (N-5800s, Robert DeHate 6" motors, etc...), I wanted to use my prepreg, and I wanted to conquer tip to tip. I also wanted some cool factor, some art, if you will.

Well, the prepreg situation complicates the glue situation. Having to cure at 350°F kills all the adhesives mentioned above. No problem, there's Cotronics 4525, right? Well, I didn't have the best luck using it for fin fillets. It's really evident that it's a high temperature potting epoxy. Very runny, very black, and just plain weird. It also really doesn't like to have anything added to it. Thickening it with the usual stuff makes it very brittle, almost crumbly. Maybe I was just doing it wrong, I dunno.

So I started looking into other high temp epoxies, but with better structural qualities than the Cotronics. I found a few, and although not available to the public, I resurrected my aviation LLC in order to get quotes and maybe buy some of these amazing structural adhesives. And they are amazing. For instance, here's the price quote for my front-runner-



I was nearly speechless. (I'm never actually speechless, gets me into trouble.)

What the...? Who the heck would...? Ok, fine... Moving on.

So that brings us to what I actually used. What adhesive has great structural qualities, while at the same time has very high heat resistance, is affordable and readily available?

My guess is you'll know it when you see it.

 

[watheyak]

I have been following this project with great interest. Your craftmanship is superb. Could you please share the mass of the nosecone?

I am also working on a flying case design, except with a stretched version of the J99 as the motor of choice. The design I am hoping to use as my nosecone ended up being rather similar to yours, so these threads have been very informative.

Thanks!

The Kevlar part that came out of the mold is 60g. Once it's trimmed and complete, I'll add final weights.

 

[AeroAggie]

Try Hysol EA 9394 or 9396. They're room temp cure but can withstand multiple 350 oven cures. These are daily use adhesives in our aerospace applications.

 

[OverTheTop]

Have you thought about using CF tubing for fillets? That's what I did on my 54mm Tomach. M1.8 peak velocity no problems. No tip-to-tip. The CF tube segments provide the fillets and there is only a thin amount of epoxy joining them to the fins and airframe.


Details here:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758&start=10

 

[rocketlabdelta]

What adhesive has great structural qualities, while at the same time has very high heat resistance, is affordable and readily available?

Oh no, not J.B. Weld! The horror!

Please spend more money than that! Some of use have to live vicariously.

 

[watheyak]

Try Hysol EA 9394 or 9396. They're room temp cure but can withstand multiple 350 oven cures. These are daily use adhesives in our aerospace applications.

I looked at the 9394, but dismissed it due to the word "potting" and thought it might have the same issues as the Cotronics 4525. I'll give it a shot on the next project based on your recommendation.

Have you thought about using CF tubing for fillets? That's what I did on my 54mm Tomach. M1.8 peak velocity no problems. No tip-to-tip. The CF tube segments provide the fillets and there is only a thin amount of epoxy joining them to the fins and airframe.
View attachment 445021

Details here:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758&start=10

Interesting. Especially since in this project I need a very specific fillet radius. The CF tubes would have to withstand 350° oven temps, though. Something to think about.

Your interpretation of "wet sand" is way better than mine. I think if someone had mentioned Belvedere I would have gotten it straight.

Oh no, not J.B. Weld! The horror!

Please spend more money than that! Some of use have to live vicariously.

What if I told you I splurged for the jumbo sized tubes?

Well, here are the fillets, in all their tractor-axle-mending glory. It's your standard mask and wipe process. These fillets only extend to the edge of the prepreg reinforcements. Once that's in place, leading and trailing edge fillets will be added. Also, since the size and shape of the prepreg reinforcement patches was laid out in Fusion 360 and will be cut on a machine, the radius of the fillet has to be exactly .25". I made some fillet shapers out of K&S stainless steel rod. One 1/2" in diameter for fillet wiping, the other slightly smaller so that when sandpaper is wrapped around it makes it 1/2".







Tomorrow I'll cut the prepreg to shape with a roller cutter on a Cricut Maker vinyl cutter, stick it on and bake it. Very exciting an a lot of work.

At least I hope I can get to it tomorrow.

 

[OverTheTop]

Your interpretation of "wet sand" is way better than mine. I think if someone had mention Belvedere I would have gotten it straight.

Medicinal purposes only. And for scale comparison .

 

[Zertyme]

When it came to attaching the fins, I went down an adhesive rabbit hole for this project.

There's lots of good glue out there. Any number of laminating epoxies with various additives, Hysol, ScotchWeld, Rocketpoxy. There's a good chance that with a nice fillet, Hysol or my favorite, ScotchWeld DP-460NS would have been fine all by themselves with no other reinforcement. Also, tip to tip makes a whole bunch of it's own problems that need to be dealt with.

But I had some other external goals with this project. I wanted it to scale up to something more extreme, (N-5800s, Robert DeHate 6" motors, etc...), I wanted to use my prepreg, and I wanted to conquer tip to tip. I also wanted some cool factor, some art, if you will.

Well, the prepreg situation complicates the glue situation. Having to cure at 350°F kills all the adhesives mentioned above. No problem, there's Cotronics 4525, right? Well, I didn't have the best luck using it for fin fillets. It's really evident that it's a high temperature potting epoxy. Very runny, very black, and just plain weird. It also really doesn't like to have anything added to it. Thickening it with the usual stuff makes it very brittle, almost crumbly. Maybe I was just doing it wrong, I dunno.

So I started looking into other high temp epoxies, but with better structural qualities than the Cotronics. I found a few, and although not available to the public, I resurrected my aviation LLC in order to get quotes and maybe buy some of these amazing structural adhesives. And they are amazing. For instance, here's the price quote for my front-runner-

View attachment 445020

I was nearly speechless. (I'm never actually speechless, gets me into trouble.)

What the...? Who the heck would...? Ok, fine... Moving on.

So that brings us to what I actually used. What adhesive has great structural qualities, while at the same time has very high heat resistance, is affordable and readily available?

My guess is you'll know it when you see it.

Urg... I also went down the rabbit hole for adhesive for the exact same reasons: prepreg cure, scaling-up (to a hypersonic composite fincan) etc...
I ended going for the DP760: non-sag, high temperature up to 230°C (Tg = 145-150 °C)...
I paid 33,49 € per 50mL cartridge on farnell.
BUT, I can't find in on 3M USA website but can on the UK 3M website. interesting.

 

[AeroAggie]

I looked at the 9394, but dismissed it due to the word "potting" and thought it might have the same issues as the Cotronics 4525. I'll give it a shot on the next project based on your recommendation.

It's definitely not the same as the Cotronics stuff. It's a thickened adhesive made for sticking parts together. When they say potting, what they mean is stuff like potting honeycomb cells to give them compressive strength around the edge of a hole or to put a fastener through it, not an electrical potting.

 

[watheyak]

A quick aside...

Version ~20 of what I've been calling the "av-tree" finished printing this morning. I printed it on an Elegoo Mars Pro, and finished it in an Anycubic wash and cure station. The resin is Sirya Tech Blu resin, a "tough" type by comparison to other SLA resins. It's plenty strong for the application, and not very brittle at all. There were some weird out of wound bits on the top and bottom of this part, but all the parts that need to be dimensionally accurate, are. I may trim a bit so it doesn't bug me.









Between washing and curing I added press-fit inserts that will be used for the electrical connections. I've been using PEM nuts for a long time for tubes, but someone on here on TRF steered me to these versions, and how to use them for SLA printing. I can't find that post, but whoever you are, thanks.





The translucent resin glows under the curing light. Makes me feel like a mad scientist.



Ok, now fins.

 

[watheyak]

Since these fins have a recess for the tip to tip reinforcement, I needed a way to cut the carbon fiber to a very accurate size and shape, and I wanted it to be very repeatable. Adding to the headache was the fact that each of the four layers would be successively smaller, if just by a tiny bit.

To solve this problem, I ended up using the sheet metal feature in Fusion 360. I was able to add the carbon fiber sheet in the recess, then convert it to sheet metal. With that, I could flatten it to make a pattern that I could export as a DXF or even a PDF.



I had the idea to use our new Cricut Maker to cut the parts.
https://cricut.com/en_us/cricut-makerI figured this might work out well for a couple reasons. First, prepreg isn't like your usual carbon fiber that frays and warps every time you touch it. It's reasonably stuck together, and has a backing sheet, sorta like a big sticker. The Maker version of the Cricut vinyl cutters also has a wheel cutter that is capable of putting quite a bit of force into the cut.

That's basically how I would have cut it anyway, just smaller and automated. So I laid it all out in the Cricut software and cut a test piece on cardstock to check fit. I was able to fine tune the size in Cricut Design Space to get it just right.





Once I was satisfied, I started cutting 16 reinforcement pieces from the prepreg.





Here's all 16. In the parlance of Jesse Pinkman, Fat stacks, yo. Numbered and organized in order of use.



Here's a video of the action-

Cricut Cutting

Next up, applying them to the fin can, building a vacuum bag, and baking.

 

[rfjustin]

Very nice work, looking forward to seeing that fin can!

 

[watheyak]

Very nice work, looking forward to seeing that fin can!

Thanks! Me too, it's in the oven right now. Catching up on posting pics.

 

[watheyak]

Applying the first layer of prepreg is reasonably easy. I used a Monokote heat gun to soften it up a but so I could get the first bend started. Those SS rods I had for the fillets came in handy as rollers to smoosh it down. The successive layers are VERY frustrating. This is the third time I've built this same fincan, and I'm getting better, but holy smokes. The biggest issue is that it wants to stick to itself or the previous layer very badly. And once it sticks, it's no longer a fun and easy material to work with. Clamps and parchment paper between layers as I get it positioned help immensely. Amazingly I was able to go 16 for 16 without having to remake any pieces. A first. Hopefully a last. Some of these pics are from a previous attempt. Please ignore any continuity issues...







I also used the Cricut to cut most of the vacuum bagging materials.

Non-perforated release film. Since prepreg already has the correct resin to fiber ratio, I want to keep it all in there. It's also static-y as all get out.


Building the vacuum bag-



I fashioned a crude fixture to keep it upright in the oven. On previous attempts it was laying on it's side, which I believe was causing the fins to warp a bit. At least I hope that's what was causing it.



My new high temp vac line and bag connector.




It's currently in the oven, slowly ramping up to temperature.

And now, we wait.