Overview

This vehicle fits the bill. It is a 3 x 24mm cluster (upgradable to 5 x 24mm). The main body is for a conventional, motor-ejected, nylon parachute; the side pods can be used for experimental deployment systems, front or rear. Initially I plan to work on timers, so I'll deploy streamers from the side pods. As I gain confidence with a new systems, I intend to graduate to parachutes, and eventually I'll use longer motor delays so that the side ejection becomes primary and the conventional ejection charge is the back up. Since the main body has three ejection charges, something is going to deploy. However, since life is not perfect, this means that a late deployment is inevitable; hence the need for strength.

The basic materials can be had from three off the shelf kits: An Estes Phoenix and two Estes Bailouts. Indeed, a credible version of the whole rocket could be built from these alone, along with some balsa or plywood stock.

Scale fans should note that these two kits can be combined in a slightly different way to make a sport scale Ariane V as outlined to the right. The ratio of the two body tube sizes is .63 vs. the Ariane's .56, so the side pods are about two tenths of an inch too big. [BT56 tubes would be better, but a bit small]. The BT80 section would have to be shortened a few inches (depending on what size payload fairing you model) and the side pods would have to be trimmed just a hair. You'd need to carve the asymmetrical SRB nose cones and add a bit of putty to the Phoenix nose cone to get a better shape. Don't model the first Ariane V launch vehicle, it blew up!

Booster glider fans can alternatively attach two ARV-condor style parasite gliders as shown to the left. Watch your center(s) of gravity, though!

It is also likely that this kit could be made in a much more lightweight version suitable for a cluster of 18mm motors.

Let me know if you build it some other way.

Notes added after building:

It is amazing how much weight can be added with 12 fillets of epoxy! The ready-to-finish TT I built weighed in at 16.5 oz. empty (A Phoenix (8 oz.) and two Bailouts (3 oz. each) would be 14 oz, and the plywood fins and additional engine mounts would add a bit more; the difference in my ship is epoxy. The real problem, though, was that the additional weight was at the wrong end.

I thus added a 5" plug below the nosecone and some nose weight in order to get the C.G. back to where it needed to be, as computed by Barrowman and confirmed by swing testing. My version (call it the TT-A) is strong enough for HPR; it is perfectly possible to build one with minimal yellow glue and balsa and keep it under the Large Model Rocket Limit.

Specifications

Diameter: 2.6" primary, 1.637" side pods

Span: 12.6"

Weight: 14-16 oz. (20 oz.*)

Motors: 3 x D12-5, 5 x D12-5

C.G. Approx 11 inches from base (including 3 D12-3 motors)*.

C.P. Approx 5.67 inches from base (Barrowman Equations on simplest perspective). Swing testing indicates that the broad axis has a C.P. about 2 inches higher.

*as built

Construction

The finished rocket as of 2/21/98 is also shown.

Parts List (as built)

Construction Tips

After the motor mounts were installed in the bottom of the lower BT80, I added a BT80 coupler (actually, a slit piece of spare BT80) above it to strengthen the BT. I then made a stuffer tube assembly out of BT50, two RA5080 centering rings, and another BT80 coupler, and used it between the upper and lower BT80 body tubes. Another length of slit BT80 was added above the stuffer tube all the way to the base of the nose cone.

Since the fins are large and surface mounted, the sidepod bottoms need reinforcement. I used three centering rings, sandwiching a coupler between the lower two. Use epoxy to glue these assemblies into the side tubes to avoid the dreaded grab action of yellow glue.

Fins were rivet glued to the body tubes. There are two sizes of fins; the smaller ones (orange, in the drawing above) attach to the side pods. The paint scheme makes the fins seem symmetrical, but you'll only fool people looking straight at that side of the rocket.

I used epoxy filets for the BT joints and the fins.

I finished with 3 coats of Krylon sandable primer (the first two were mostly sanded off) followed by two coats of Sunshine Yellow over all, and two coats of Fluorescent Orange per the pattern. Lightly sanded with 400 grit after the first coat, 0000 Steel wool after the second. Finished with Krylon Clear Coat.

Flying

There are some safety considerations regarding a 3-motor cluster rocket and recovery delay selections--the design trick is to make sure the rocket is safe even if one or two of the motors does not ignite.

As it turns out, the increase of weight during construction was fortuitous, and I would have been compelled to add it either way.  At 20 oz, and using Estes D motors, the TT simulation reaches a bit over 400 feet if all three motors light.  The optimal delay is 4 seconds.  If two engines light, the rocket will make it to just over 200 feet with an optimal delay of just under 3 seconds.  If only one engine lights the TT will struggle less than 40 feet high (assuming it goes straight up). It won't have time to recover, but it won't be a safety hazard either--if the ground is soft enough, it might not even be damaged.

A lighter rocket goes higher, but is more likely to be destroyed if only one motor lights.  A heavier rocket would be truly underpowered.

Depending on finished weight, here are some additional simulation data, assuming drag coefficient of .75:
 
 

Using Aerotech E or F motors, the recovery margin should be better even though the motors are harder to reliably light.  The large frontal area slows down this rocket very quickly, so a 4 second delay remains a good choice. Sometime this year I'll try it on three E15-4Ws.

First flight:

After a bit of exertion, I was able to get the U.S. FAA to stop trying to disapprove of my LMR notification, and we were ready to fly.  Temperature was about 40 degrees F., winds were 5-10, ceiling was very high overcast.

Three Estes D12-3s were used.  Igniters were carefully selected, inserted, and individually tested for continuity.  A clipwhip was used to connect a 12 v gel cel.  With some trepidation I pushed the launch button.

All three motors lit up, and the rocket lifted off, faster than I expected.  There was modest weathercocking but no obvious wobbles or coning.  Altitude seemed every bit of 400'.  Ejection was before apogee, and it took a while for the  30" nylon hemispherical chute to drag itself open.  Recovery was within 20 yards of the pad, and the only damage was some minor paint scorching on the inboard ends of the side pods-consider aluminized paint there instead of yellow.

All in all, an excellent maiden flight!