98mm N5800 MD rocket for BALLS

cjl said:

Composites are absolutely better than steel, both in terms of heat resistance and strength to weight ratio. Of course, that's assuming you can get your hands on the correct composites. Sadly, I can't get my hands on any truly high-temperature composites (much as I would like a rocket made of RCC, it isn't going to happen for obvious reasons), so I'm using a combination of very high Tg epoxies and ceramic coatings to hopefully get a similar effect. Interestingly though, with high Tg epoxy, you can get composites that retain strength up to around 500F, which is pretty much as good as aluminum in terms of heat resistance.

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I hate to dwell on this but I see tradeoffs in any composite materials. You already mentioned one, the cost of RCC makes it basically impossible to use on our projects. CF is lighter but certainly not as heat resistant as steel in almost any conceivable layup that we can do, including your high temp layup. CF may conduct heat away from exposed surfaces faster, however.

CF is really crippled by the resins available, and I am unaware of any conditions where it can beat steel thermo-mechanically in high speed rockets. Correct me if wrong but I have not seen it.

The survivability, cost, strength to weight of steel, and other factors combined make steel the best possible choice for high performance rockets.

Also consider the fact that if a rocket project fails once and then works on the second try, it is twice as expensive. If a more expensive metal rocket works the first time, that may actually save money in the long run.

New Ocean said:

Better than steel?

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Yes.

What are you flying at BALLS New Ocean?

New Ocean said:

I hate to dwell on this but I see tradeoffs in any composite materials. You already mentioned one, the cost of RCC makes it basically impossible to use on our projects. CF is lighter but certainly not as heat resistant as steel in almost any conceivable layup that we can do, including your high temp layup. CF may conduct heat away from exposed surfaces faster, however.

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By that same argument, there are tradeoffs in any material though, not just composites. Take steel for example, since you keep using it as an example. Steel is heavy. Extremely heavy. As such, a rocket made of steel would end up with a lot more mass, meaning it would not perform as well and it would strain the recovery system more, potentially making it substantially less safe. This can somewhat be mitigated using high strength steel alloys, which would allow for thinner, smaller components with the same strength. However, these high strength alloys are expensive, hard to machine, and sometimes hard to obtain. If I could build this entire rocket out of, say, maraging steel, it would be great, yes, but I neither have the equipment to work with that kind of alloy, nor do I have the budget. If you chose ordinary steel instead, you might as well use high strength aluminum instead - 7075 or 2024 aluminum are as strong as mild steel, at a fraction the weight. And finally, of course, there's the fact that steel construction of any kind is somewhat frowned upon at amateur rocketry events.

Basically, all materials are a tradeoff, and based on my performance goals, available equipment, and budget, I feel that CF is the best all around material for this rocket. Also, you can put ceramic coatings on a composite that can take higher temperatures than any steel. Yes, they're just coatings, but this doesn't need to withstand the heat for more than 10-15 seconds.

I'm just going to say one word: titanium.

Titanium has the strength of steel, density of aluminum, and various other properties that make it the best metal to use for aerospace projects. It saves weight compared to steel, and melts 500 degrees above steel, while also having a higher heat capacity. Why anybody is mentioning steel above titanium I don't know. I would've also used titanium on my rocket's fins if the stock wasn't so expensive, and if the metal weren't so difficult to machine.

Oh, and as a progress update:

I just finished the two and a half hours of sanding it took to profile the remaining two fins, so all four fins are now nicely profiled. The body tube also arrived today, and I've got the e-bay layout started, though it's only preliminary at the moment.

edwinshap1 said:

I'm just going to say one word: titanium.

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Titanium?! Why not get really exotic--how about a metal-ceramic alloy? or how about a spun Aerographite structure?

edwinshap1 said:

I'm just going to say one word: titanium.

Titanium has the strength of steel, density of aluminum, and various other properties that make it the best metal to use for aerospace projects. It saves weight compared to steel, and melts 500 degrees above steel, while also having a higher heat capacity. Why anybody is mentioning steel above titanium I don't know. I would've also used titanium on my rocket's fins if the stock wasn't so expensive, and if the metal weren't so difficult to machine.

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Cost is an issue of course, as is machinability. Ti 6-4 has the dubious distinction of being the biggest pain in the backside out of every alloy I've worked with throughout my undergraduate and graduate projects. It can be done, of course, but it takes a huge amount of time and experience to properly machine. It's also near twice the density of aluminum, with triple the strength, so it is an excellent choice if heat resistance is needed (but it is substantially heavier than Al, so if the heat resistance and absolute strength isn't needed, aluminum is actually a better choice). Interestingly, I would actually chose a high strength maraging steel over titanium if I was going all out for performance. It's nearly twice the density, but the strength potential is absolutely enormous - some of the best have tensile yield strengths of around 2500 MPa, compared to around 1000 MPa for a high strength titanium (Ti 6-4), or around 350 MPa for a high strength aluminum. This incredible strength would allow for extremely thin fins and body tube, which could give extremely low drag.

I'm looking at materials that can be easily produced and ordered, my basis is McMaster

That does limit things a bit

No body tube is thinner than ours, so there is no room for improvement in airframe materials. The fins could be thinner if we used a stronger material than 7075 aluminum, though.

CarVac said:

No body tube is thinner than ours, so there is no room for improvement in airframe materials. The fins could be thinner if we used a stronger material than 7075 aluminum, though.

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Very true. I will admit to being a bit curious how you're attaching the body tube to the motor though, and how you plan to minimize the lip created at the airframe join. Also, are you planning a tailcone, or just the back of the motor?

cjl said:

By that same argument, there are tradeoffs in any material though, not just composites. Take steel for example, since you keep using it as an example. Steel is heavy. Extremely heavy. As such, a rocket made of steel would end up with a lot more mass, meaning it would not perform as well and it would strain the recovery system more, potentially making it substantially less safe. This can somewhat be mitigated using high strength steel alloys, which would allow for thinner, smaller components with the same strength. However, these high strength alloys are expensive, hard to machine, and sometimes hard to obtain. If I could build this entire rocket out of, say, maraging steel, it would be great, yes, but I neither have the equipment to work with that kind of alloy, nor do I have the budget. If you chose ordinary steel instead, you might as well use high strength aluminum instead - 7075 or 2024 aluminum are as strong as mild steel, at a fraction the weight. And finally, of course, there's the fact that steel construction of any kind is somewhat frowned upon at amateur rocketry events.

Basically, all materials are a tradeoff, and based on my performance goals, available equipment, and budget, I feel that CF is the best all around material for this rocket. Also, you can put ceramic coatings on a composite that can take higher temperatures than any steel. Yes, they're just coatings, but this doesn't need to withstand the heat for more than 10-15 seconds.

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7075 is near impossible to obtain in large tube sizes, but its mechanical properties are appealing. Also keep in mind that steel is frowned upon at TRIPOLI launch events, not amateur rocketry events. When it comes to hobby rocketry steel is not really a useful consideration - it is heavy and the large benefit one would gain it its use; a high Pc is lost due to the fact that you already have an aluminum case. If you could make your motor out of steel it would be beneficial both in reduced tube thickness and higher delivered ISP, unfortunantly in this case the motor is not a knob to turn.

New Ocean, I dont know what you're talking about ... carbon is awesome, as an example of its use...a large number of commerical rockets made today? Hopefully I will have some information on composite tube as an airframe and motor tube in the near future, I'll let you know how it goes.

endwinshap1, you answered your own question: $$$, machinability.

We have a machined aluminum piece which transitions the 3 mil (or something on that order) diameter difference between the Hawk Mountain coupler and the motor tube. The coupler slips over this part with a 3-inch overlap, and a whole lot of bolts staggered around it keep it from coming off. The aluminum piece is bolted to the forward closure, and its alignment is ensured by a machined aluminum ring between the delay well and the outside of this aluminum piece.

We do have a tailcone planned.

Chris,

What are you using for recovery? Descent rate and recovery mass?

Edward

Descent mass should be around 18 pounds, give or take, and the main chute will be a 36 inch toroid, giving a descent rate around 30fps. I'll post the recovery system design as I build it - it's extremely compact, but not 100% finalized yet.

Single deploy?

Dual - drogueless (or maybe a small streamer) at apogee, and main at 4 or 5 thousand feet.

cjl said:

Descent mass should be around 18 pounds, give or take, and the main chute will be a 36 inch toroid, giving a descent rate around 30fps. I'll post the recovery system design as I build it - it's extremely compact, but not 100% finalized yet.

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Why something so small? Why not go with a 48" or 60" that will fit into <3" of 98mm airframe and give you a much better descent rate?

Edward

Because the chute is going in a 2.5 inch tube, not a 4 inch.

Thanks for showing your build Chris, I'm glad CTI is running this promo. As for the cotronics 4461, do you plan on using a post-cure? Reason I ask is that the catalog seems to imply a 500F Tg from a room temp cure, yet the bottles suggest a post-cure for 4 hours at 250F. I'm using the same stuff on my (non-N5800) 98mm 6GXL research flight. Might be worth asking a rep, but I post-cured mine just in case.

cjl said:

Because the chute is going in a 2.5 inch tube, not a 4 inch.

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How many inches of 2.5" tube? I know I can get a 60" into about 4". Also, it may pay off just to use a circular chute even though it is less efficient. The seams on any gored chute add more packing volume than you think a flat chute will pack much tighter.

Edward

For our N5800, we're using a Fruity Chutes 60" Iris Ultra, which packs tighter and weighs less than and supports more weight than the 60" elliptical. We originally budgeted 7 inches of 2.5" tube, which the Fruity Chutes 60" elliptical fits in perfectly, but now we're using 1/2 of the room available inside a 3.9" coupler around a 29mm tube, which has exactly the same cross-sectional area as a 2.5" tube.

Why not use wood for the nose cone. Seal with alumina slurry and paint with engine paint, some of which are good to 1000 deg F. For fins, use a non-melting resin such as phenolic. G3 is the same as G10 but phenolic instead of epoxy binder.

The 60" that I have is a toroidal, but takes up less room than the Iris Ultra chutes. I really like them for projects where you have some room to pack them - the seams can add up. But, you can get them quite small. I put mine in a vacuum bag inside of the coupler I used it for. Vacuumed it very small and left it for a couple days. Then immediately transferred it to the rocket day of flight.

Edward

CarVac said:

For our N5800, we're using a Fruity Chutes 60" Iris Ultra, which packs tighter and weighs less than and supports more weight than the 60" elliptical.

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Ditto

butalane said:

Yes.
What are you flying at BALLS New Ocean?

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You could be Robert von Braun Goddard and still be wrong about the use of steel in rockets. In this case you are not, and are, respectively.

Steel has better thermo-mechanical properties than CF under all but heroic layups or RCC. It is denser, but can be made far thinner than CF structures (these fins an ideal example.) If 1/8th inch steel can recreate what these 1/2 inch fins can do, suddenly the density is less of a problem. Furthermore, thinner fins compensate for extra mass by reducing drag. And on a nearly hypersonic rocket, getting thin fins is well worth hundreds or thousands of extra grams.

The argument ultimately comes down to this: composites are struggling to survive these kinds of flights. If going to steel helps a rocket survive the flight, steel is the superior choice. This OP is working on something else so I completely understand his use of composites as a challenge. But the statement that composites are better than metals, even if you set aside inconel, titanium, beryllium, etc., is very hard to support. Aluminum has been the go-to material for high performance rockets in this hobby for a long time... more than 20 years. There is a reason for that. Composites may replace metal in that lower end, but in the mach 4+ realm? I seriously doubt it.

Let's break it down into two parts: Metals are better for a significant range of hobby and amateur rockets, particularly those that achieve very high mach numbers in the lower atmosphere. And since composites are a huge part of the core of the HPR hobby, it is great that people are pushing the limits of composites to see how far they can go. I bet they can handle an N5800 when done right, and I get a good feeling about this project. Best of luck and post more pics!

cjl said:

Descent mass should be around 18 pounds, give or take, and the main chute will be a 36 inch toroid, giving a descent rate around 30fps. I'll post the recovery system design as I build it - it's extremely compact, but not 100% finalized yet.

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cjl- try and keep it under 30 if you can. We are trying to stop the rockets tha come in really fast with tiny chutes. We have let it be done for a long time at BALLS, TRA rules even make some exception for it, but we want to slow down the desents.

We launched MANY O10,000's minimum diameter and never had a stuctural issue with the welded Al fin cans. Yea they are heavier but the rocket goes higher in one piece. we hit nearly M4 with a 2 stage with no fin isssue.

Mark

Also, doesn't the motor itself weigh 12 pounds after burnout? You should be planning for 20-22 pounds given your expected empty weight.

I was considering at once point making a chute myself for compactness, but I don't trust myself making a chute that'll slow 25 pounds from 150fps. My school's USLI team's rocket got utterly destroyed when their elliptical Fruity Chute deployed during a ballistic descent, and the chute is still perfectly fine, so we're going with them for Bare Necessities.

The motor might weigh 12 - I was going off of memory, and I may have gotten the descent mass wrong. I do know that the decent rate was between 26 and 31fps though, depending on the final empty mass and the temperature (and thus the air density) at landing. Although the chute is only a 36, it is a true 36 (open diameter), and it's the toroidal design, so it should slow it down to a safe speed. It's also pink and black, so it should be extremely visible.

Amusingly, the USC RPL is using a streamer for their Carmack prize attempt, which they just registered on the arocket mailing list.

A streamer.

12"x180", but still, the rocket itself is 8"x160".