This article describes an unusual kit-bash, of a Vaughn Brothers BoB into a lightweight 29mm 2-stage rocket. The extra parts required are minimal and except for the necessary electronics the total cost of the conversion is very low. I added PML ‘chutes and AeroPack motor retainers because of their functionality and the very "professional" touch they add to the rocket.

I have always been interested in multi-stage rockets, but the kits that are available are all quite large and fairly heavy, and our local launch area only has a 2000 foot waiver. I couldn’t find one which could be safely flown to such a low altitude, and it seemed a bit pointless to buy an expensive kit that I’d only fly once or twice a year at a remote site.

The VB BoB seemed like a good starting point for a 2-stage rocket, since in completed single-stage form it weighs only 18oz, it has a lot of airframe tubing to play with, and being only 1.9" diameter meant it should fly well on relatively small motors. As it turned out, the finished "Avenger" weighed in at 34oz, less than twice the weight of BoB, including all electronics and batteries, but without motors, wadding or ejection charge. I was happy!

For those who are not familiar with the BoB ["Ballistic or Bust"], it is a 29mm sport rocket, somewhat in the spirit of the Mean Machine, featuring .063" G10 fins and 1.9" airframe tubing. The kit comes as follows:

• 1 x Plastic nosecone
• 2 x 34" Body tube 1.9"o.d.
• 3 x G10 fins
• 1 x 29mm x 12" MMT
• 3 x 1/8 CR
• 1 x Bulkhead assembly
• Chute, launch lugs, shock cord, instructions

To create a 2-stage vehicle, I used everything except the parachute [but that was personal preference; you could use the stock one for the booster, and since you’ll need another one for the sustainer, choose a design that will minimize drift.]

• 1 x 29mm MMT 12" long
• 1 x 1.9 coupler 5" long
• 1 x 1.9 coupler
• 1 x 38mm MMT 5" long
• 3 x 1.9x29mm CRs
• 1/8" aircraft ply for bulkheads
• extra ¼" launch lugs
• Parachute, kevlar shock cord, screw eye, small countersunk head wood screws
• Fins [see text for dimensions] cut from 12" x 4" x .063" G10 sheet

ELECTRONICS

Electronics are necessary in a composite-motor multi-stage rocket to ignite the upper stages in flight. I chose a Transolve staging timer because it is very compact and reasonably priced. For the sustainer I chose a Transolve P6 altimeter; in principle you could run the upper stage without any electronics, but I think this is just asking for trouble - if the motor for some reason doesn’t ignite, prepare for "shovel recovery". So the altimeter performs three functions, recording the peak altitude, deploying the chute at apogee [with motor ejection as a backup], and allowing the rocket to be safely recovered even if the upper stage becomes an impromptu boosted dart.

CONSTRUCTION

First you will need to take the two 34" body tubes and cut them to the following lengths:

• 24" - Booster airframe
• 22" - Sustainer airframe
• 10" - Sustainer payload bay
• 2" - Interstage coupler sleeve

This leaves you with a leftover 10" section. In hindsight if I were to convert another BoB into an Avenger I would make both airframes the same length, maybe 26" each, with an 8" payload. Space is pretty tight in the sustainer for an ejection charge, wadding, and parachute, and the weight difference would be negligible.

BOOSTER

I have my own favorite method of shock cord mounting in rockets too small and/or light to use eyebolts or u-bolts.

I drill a small hole in the top centering ring to suit the size of the cord, tie a double overhand knot on the aft side of the ring, and apply a generous amount of epoxy to secure the tail and knot of the cord to the motor tube/centering ring area.

This can be seen in the photo of the sustainer MMT. Apply generous fin fillets, attach the launch lug 7" from the bottom and that’s about it!

SUSTAINER

The fin shape for the sustainer was designed using Rocksim 4.0, using the following criteria:

• minimum size to achieve adequate stability with heaviest 29mm motor [I200W]
• as the interstage coupler fits approximately 2" into the sustainer, the fins had to have a fairly long root chord for strength, since the fin tabs would not be full length
• if possible all three fins should be cut from a single 12"x4" sheet of G10 [because I already had one!]
• the sweep/span dimensions should create a right angle, so the leading and trailing edges can be formed by the exactly square corners of the sheet.

This was accomplished with the following fin dimensions:

• Root chord 7-1/16"
• Span 2-1/2"
• Leading edge sweep 6"
• Fin tab approx. 0.35" [sand to an exact fit],tabs extend only 4-3/4 from tip

As far as fitting them all on one 12"x4" sheet, it can be done if you are prepared to accept some small notches in the fin tabs which will not affect the overall integrity of the structure.

Cut three 1/16" slots in the sustainer airframe, starting 2-1/8" from the aft end, and 4-3/4" long.

Here is where it gets tricky. If you have a long, thin applicator and a steady hand it’s possible to apply epoxy to the inside of the airframe tubing whilst inserting the MMT so that all three rings get bonded. Having said that I’m not even going to attempt to describe exactly how I managed, only that it was extremely fiddly!

As soon as the MMT is in place, look through the slots and using a suitably shaped implement [such as a popsicle stick cut on an angle], scrape away any stray epoxy which will impede fitting the fin tabs flush with the motor tube.

Apply epoxy to the entire root edge of the fins and fix them in place. When filleting the fins, be generous as there are no internal fillets. Maybe 24-hour epoxy would be beneficial here.

Attach the two 1" launch lugs, one at the bottom of the airframe, the other 10" up.

PAYLOAD

Use the 10" [or 8" – see above] section of body tube, and mark it for three equidistant holes 2" from one end. These will be the static pressure ports for the altimeter.

I have found that drilling nice-looking holes in cardboard is not that hard if you drill small pilot holes, reinforce with low-viscosity CA, trim off the edges, re-drill, trim, etc., and gradually work up to the finished size when you can sand both sides of the holes for a smooth finish. This is why I suggest drilling the holes before fitting the coupler-bulkhead assembly so you have better access to the inside of such a small tube.

Epoxy the BHA in place, not forgetting to drill a small hole for the wiring to pass through for the ejection charge. I used an eyebolt with nuts and washers for the payload bay, and kept the wood-screw eyebolt supplied with the kit for the rear closure of the staging coupler/electronics bay.

STAGE COUPLER/ELECTRONICS BAY

This part of the rocket is the key to the whole concept, and until I received the Transolve timer, I wasn’t too sure exactly what it was going to look like. I finally arrived at a compact, neat-looking design that is easy to build and very robust using easily available parts.

• a 1.9" coupler, 5" long [NOTE – not all couplers are the same wall thickness!]
• the plywood plug which forms the aft closure of the bay [I didn’t have any 3/8" ply so I glued 3 small pieces of 1/8" together and finished them on a belt sander]
• a 5" section of 38mm MMT, which is an exact fit in the coupler
• a 1/8 ply bulkhead for the forward end of the bay, with a hole for the ignition wiring
• a 1" long section of coupler
• the 2" long section of body tube

This bay was constructed specifically for the Transolve timer, with a little room left over for packing and wiring, so check the dimensions of the timer you intend to use as it might not fit.

The plywood plug [photo at right] is ½ " thick, made from a 1-1/2" diameter 3/8" thick disc epoxied to a 1.8" x 1/8" thick disc. It is drilled in the center for the screw eye, and around the perimeter for the three countersunk retaining screws.

Epoxy the 2" long section of body tube to the 5" coupler, 1-3/4" from one end. Mark the body tube for three 1/8" holes equally spaced in the center. These holes are to allow pressure built up during igniter firing to be vented safely overboard without prematurely separating the stages and possibly yanking the igniter out of the motor before it is fully lit. See photo c)

Epoxy the 1/8" thick bulkhead into the 38mm MMT, opposite to the removable plug end.

This is where the wiring for the sustainer igniter comes through, and will be protected with wadding when you prep the rocket for flight.

Epoxy the electronics bay into the coupler with about 2-1/2" of 38mm tube exposed, as seen in the photo below, which also shows the screw eye installed [I like to use epoxy on this too].

ASSEMBLY

NOTE – Make SURE the nosecone is a TIGHT fit or somehow POSITIVELY RETAINED or you could spend a lot of time looking for your altimeter if the deployment speed is too high!

You will need to run some suitably sized wiring out of the holes in both bays; I sealed around the holes with silicone sealant but I have also found that Blu-Tack works just as well. It’s also a good idea to tie a knot in the wiring on the inside of the bulkhead, for obvious reasons.

The AeroPack retainer is an easy fit in the stage coupler with plenty of room to spare.

Each friction fit in the rocket should have enough "stiction" to support the mass of whatever is beneath it, or you may suffer drag-induced premature separation.

It is of course critical that the igniter in the upper stage does not become dislodged due to the g-forces encountered in the boost phase!

When installing an igniter in the sustainer motor, I used orthodontic elastic bands, as they are very strong and durable, and so small [about 5/16" i.d.] you don’t have to double them up.

Also, don’t pack the protective wadding in the interstage so tightly that it blocks the pressure relief holes.

FLIGHT

It is extremely important to use a program such as Wrasp or Rocksim [or whatever your favorite is] to simulate flight performance of a multi-stage composite-motor rocket. Otherwise you are just guessing when it comes to sustainer ignition delay.

For the first flight I chose a G80-4 for the booster [good] and a D15-7 for the sustainer [not so good – it looked okay on the computer but lacked grunt against the wind on launch day].

[A note on propellant types – play it safe at first and use easy-to-light Blue Thunder for the upper stage, at least until you have a feel for how the rocket stages, or your delay time might end up a lot longer than you planned!]

The first flight of the Avenger was a success. I set the delay as short as I thought I could go without igniting during the booster burn, and the timer worked perfectly. The rocket staged clean and fast, like a blackpowder 2-stage kit! It was a joy to see! This was also my first use of an altimeter to deploy a parachute, and the P6 fired at apogee. It gave me a feeling of real satisfaction to watch both stages coming down safely on identical chutes from very different altitudes.

For those who haven’t tried it, building and flying a multi-stage rocket is great fun and gives a feeling of technical achievement different from a single-stage kit, all the more so if you have a major hand in the design. Go for it!

WHAT NEXT?

An obvious question, perhaps. Being fairly nervous I did not photograph the maiden flight, but I intend to rectify that omission at our next meet in March.

I think the performance could be usefully increased by basing the rocket on, say, a VB Extreme 38 with some extra airframe tubing. Packaging the electronics in the coupler might then become a real challenge!

Or alternatively, a VB Extreme 54 [plus more tubing] with a booster long enough to take a Hypertek J motor, and a sustainer that will take the biggest 38mm reload, the J570? Acme fin canisters would simplify construction in both sizes, too.

All of these approaches allow a less wallet-draining introduction to multi-stage rocketry and plenty of room for creative engineering solutions. After all, even if you don’t have an altimeter [you were going to buy one, right?] you can start off by designing both stages so that they can be flown independently, so you’ve made two rockets out of one; just swap the nosecone/payload sections!