Mike's Level 3 Project

I'm scheming for a L3 certification. Preferably with a Hybrid. Why a hybrid? Partly because I can get a motor and a reload for the cost of one solid "M" reload without the motor case. So it's less expensive. I'm also comitted to showing those in the area that Hybrids are a viable alternative for all sizes of rockets, including the big ones. In that light I'd like to do a Hybrid L3 as well as at some point some large experimental hybrid motor work and flights.

The current motor I'm thinking of is the RATT L/M-900 motor combo. The only other contenders are the HyperTek 98mm M motors, An advantage of the RATT is the per-flight costs are cheaper, largely due to the simpler and less costly grain (a slightly milled pipe insert) vs the more expensive HyperTek reloads, which are specially cast motor/thrust chamber assemblies. The HyperTek also has higher thrust profiles than the RATT, allowing for more weight flexibility in the rocket design.

A disadvantage of the RATT is the lower average thrust and only one thrust profile, at average 900ns. It's also very long at 78" for the M assembly due to the 64mm tube. So we're looking at a 30 (preferable) to 40 (marginal in winds) rocket. The Hypertek at 98mm is only about 40" long so for my target 6" rocket can be about 38" shorter, allowing for an easier build to weight.

The overall design I like and want to build to is an AGM-78, commonly known as a Standard Arm. There is a nice 6" design starting point RockSim file on EMRR. The long strakes may also allow for lighter construction of the BT as it naturally reinforces it.

Ideally, I can also fly my EX solids in this motor. Offhand, I'd like to be able to accomodate at leasta a Baby M solid and that of average to low average thrust (say the AT M1319 as a max average thrust to design for). But the primary design is for the hybrids, so I will sacrifice the higher end of solids for solid L/M hybrid flights in my design. And get that approved by my TAP :)

The shortest length Hybrid MMT would, I think, be about 40" long 98mm for the smallest HyperTek M. Starting with that I would already have to lengthen the "starting point" AGM-78 mmt by about 15" or so. And maybe upsize the chutes, requiring uplengthed payload bay ...

With that as a new baseline, I could also fly at least fly a 30" 75mm Loki Baby M motor, so I'm in range of my solids compatibility.

Aside from a 40" HyperTek in 98mm, I'm also considering the RATT Works L/M-900 motor combo. It's a 64mm (2.5") motor so is 42" long for the L and 78" long for the M configuration. If I designed for this combination I would build the rocket as a stretched AGM-78 with a 42" 75mm MMT for L hybrid and M solids flights and have an attachable booster extension (semi-scale to the AGM-78 booster), that would provide the length to accomodate the 78" long M tube for M flights. The only disadvantge here is that the rocket that would accomodate a M HyperTek would only accomodate an L RATT. With the RATT L & M having the same average thrust, I'd need to conserve weight to allow for the booster weight and still stay under design weight.

For the RATT Works L/M-900 I'm told that a 30 lb loaded rocket is great and that 40lbs will work on calm days. I'd prefer to not worry about calm days at Whitakers. And with a 13 lb loaded motor for the RATT, that leaves only a 17lb budget for the rest of the rocket. With the RATT that must acccomodate the extra 36" of booster extension.

I am otherwise told, not by my TAP, but by others that fly the RATT L/M, that straight up PML 6" BT is more than adequeate for the thrust and wouldn't need to be strengthened for that reason. So I could consider skipping glassing the BT. I'm also told that many fly the RATT L/M in 4" rockets to further help keep weight down. That seems to yield rockets about 12' long.

The RATT would also be flyable anywhere. Since I mostly fly at Whitakers, that's less of an issue, but still exists. Even though Whitakers has GSE, I might need the drop down stem mods for the M, so that would be an additional cost.

So I'm not sure about the motor. The HyperTek allows me to build one rocket, and a shorter one at that, but at the expense of higher per-flight cost.

Once nice advantage of the AGM-78 is the already designed in strakes. While they add weight, they do stiffen the rocket and would allow more flexibility in what I built up for the body tube (perhaps one less wrap of fiberglass or CF).

I'm considering "true composite construction" which mean fins, for example, will be surface mounted to the airframe, not TTW. For "true composite airframes" (as I'm told by Dave Triano of Shadow Aero), cutting your composite structure weakens, but proper surface mounting is actually stronger. Emphasis on "proper".

I think the strakes will go a long way towards any "first time composite" issues wrt strengthening, esp as single piece fins.

The challenge then is transferring motor force to the body ... but I'm getting ahead ... as that's assuming the motor inserts are full rocket diameter.

I have a new page on Composite Structure Evaluations - Composite Airframes.

Two stage? Not for the L3 cert flight, but with some careful design, It could be built in such a way as to be the sustainer ... hmm ...