Chuck,
There are a number of design elements of rockets of this magnitude that don't scale well from "normal" high power rockets. I'm no expert, but the KloudBuster Max we launched in 2015 has a similar scope, size and ambition. The design and build of this project was very fluid as we figured out what we could do and couldn't accomplish and as the weight started to become extreme. The discussion above brings some of them to light.
- Finding another big project you can emulate will be difficult. Every huge rocket is one-off based upon the budget, the commitment by the champion of the project, the time available for the team, the skills of the team members, launch window, motor(s) available, etc. Many are single builder and take years to build and perfect. Think Vern Hoag and Steve Eves.
- The notion of couplers and shoulders on nose cones pretty much goes away. How do you reliably push a 30" diameter nosecone with a 20" - 40" shoulder out of the tube? It isn't easy without blowing out the side of the airframe. When I took over the KBMax project design/build team, the nosecone had a 20" shoulder and there was a lot of discussion regarding getting it off the rocket. We learned from the Nebraska team's designs instead to use mating bulkplates and explosive bolts. We deployed two parachutes with a total of about 6g of black powder.
- Bringing the rocket down in intact (flyable) condition is very difficult. The sheer size and weight of the components damage themselves. Bring the thing down in as many pieces as you need to in order to be safe, but remember, every joint is a complication and a failure point. My recovery thoughts were consumed by the safety of each piece coming down and safety if something(s) didn't go nominally (broken airframe structure or motor CATO). I always considered the rocket to be single use and just wanted to get the motor cases to the ground safely. Interestingly, we didn't significantly break anything (some cracks in the nosecone on landing) and could have jammed reloaded motors in the thing and tried again.
- You're doing right thinking about the load/weight bearing parts of the structure first. Design the internals to deal with the thrust of the motor and weight of the components. In a huge way, a body tube is mostly aesthetic.
- Your concern of transferring the thrust to the rocket is well founded, but when you think of airframe think of the internal structure unit that comprises the fins, bulkplates, nosecone weight and attachment, etc. all of which you have to get moving upward. We integrated the fins and motor mount assembly and attached the structure to the airframe the entire length of each fin root along with internal stringers going all the way from the motor mount structure to the nosecone. Think lots and lots of surface area for attach points. Construction adhesive and long screws work much better on wood parts than epoxy, is cheaper, and is easier to work with.
- If you make the thing big enough and heavy enough you can use plywood for the fins. They will be heavy, but at speeds of 500-600 mph they won't be compromised during flight. Landing maybe, but not flight. I mentioned above, we used surplus 1" honeycomb core skinned with fiberglass and edged with moulding from the lumberyard.. Ours were pretty light but extremely rigid.
- You're not only going to need a team to build the rocket, but consider transport and prep at the field. We had a team of about 8-10 people who helped during the last build cycle and about that many at the field helping with final assembly/prep and raising the thing to vertical. One project lead is important, I think. You can have sub-teams for fin structure, airframe structure, recovery, electronics, propulsion, etc. but you need a leader who helps talk through disagreement and who can spend money. Oh, the money. This sort of effort costs a lot. The leader must control the purse as well.
- There are a ton of other things to consider. A ton. Get a team together and start some serious design work. What skills do you have available? What materials? What tools/workspace? etc. It took from 2000 to 2015 for the KLOUDBuster Max to go from glassing the first piece of Sonotube to launch.
I'll probably post more later.
OK Lance great post I’ve had a chance to mull over your guidance.
The following is just shooting from the hip and is a call to everyone for input:
I’d like to hear about the ejection idea using explosive bolts. Also gave consideration to an airbag or two just to get the nosecone off. Have the drogue be a large chute to pull an even larger cargo chute out. Surplus cargo chutes up to 100’ are available.
I’m going to go to a custom wood crafter to have him build the bottom plate where the motor thrust ring will push. It will be beefy.
He can also craft a forward thrust plate so the motor can push against that. Both forward and aft thrust plates will be glued and bolted in.
He can also build the fins out of wood. You mentioned plywood. Should they be glassed? What if I went with something like oak? Expensive but strong. Could be beveled as needed.
Fins attached to the airframe with zinc mounting plates that are bolted through the airframe to 2x4’s that are perhaps shaped to the circumference of the airframe.
Are internal stringers necessary? Could I really lay on the fiberglass or even carbon to the sonotube for external strength?
I’ve got a good budget and a patient wife.
Am going to need LOTS of help at the launch site. I know there are many good people out there who will be willing to help.
I’ve got a 30x50 rocket shed.
I’m not an expert with RockSim or Open Rocket. Would like someone to plug a 30” and 20-30 ft rocket in with a heavy tail in for me.
Tell me the best fin design. What size should they be?
Please Lance you and everyone else tear into these rough plans and tell me the strengths and weaknesses.
My job is such that I’m gone for 3 days but then have 3 full days to work on this. I’m pretty dedicated when faced with a challenge. This can be ready for LDRS 2019.
Thanks!
Chuck C.
