Background
Some time ago, there were several articles on rec.models.rockets in which people asked about "strap on boosters". G. Harry Stine mentioned the technique in "The Handbook of Model Rocketry", although he called it parallel staging, and therefore discussed it in the chapter on staging. However, apart from a brief mention, he did not say much about it, concentrating the chapter mostly on the more common series staging. So I decided to try to build a parallel staged rocket. The object of the exercise was to make it as simple as possible, but also as safe as possible - at least as safe as clustering or series staging, which are widely accepted techniques in model rocketry.

Flight History

Titania made its first flight at IRW99 on 21st August, when few people had yet arrived. For this trial run, the boosters were loaded with B6-0s and the sustainer with a C6-5 in an adaptor made from an empty D12 casing. The flight was successful, with both boosters detaching as intended and the sustainer continuing on a straight flight. Two days later, Titania was loaded to full power - C6-0s in the booster and a D12-7 in the sustainer. This flight was also successful, which was just as well because there were more spectators, including a reporter from BBC Radio who was visiting IRW to collect footage for a programme called "Selling Space". The boosters detached and dropped into the field. As for the sustainer, its Estes parachute shredded, which meant Titania landed safely in the field rather than drifting off into some nearby trees.

Titania's next flight was at Boglob. The boosters were not used because they would have got lost in long grass.

On 30th July Titania was one of the rockets launched as part of the Clarecraft Discworld Event, a sort of open air convention for fans of the writer Terry Pratchett. The photo on the left was taken prior to its launch, witnessed by Terry Pratchett himself. Again the flight was good, with the boosters detaching and dropping into the field. The sustainer weathercocked a little in the wind, did not drift back far enough, and landed rather close to some cars, but managed to avoid hitting them.



Parts
Nose cone=Estes PNC-60AH
Upper body=1 complete Estes BT-60.
Lower body=1 complete Estes BT-55.
Transition=custom balsa BT-60 - BT-55 adapter. Cone=40mm long, extensions=30mm long.
Booster body=150mm of Estes BT-20. (2 of)
Booster cone=matched pair from Estes NC20 pack.
Booster coupler=20mm cut from 18mm casing. (2 of)
Fins=3mm thick balsa.
Screw eye=from hardware shop.
Motor block (sustainer)=5mm cut from empty D12 casing.
Motor block (booster)=5mm cut from empty 18mm casing. (2 of)
Motor mount tube (sustainer)=70mm of Estes BT-50.
Motor retaining clip=standard Estes type.
Centring rings=cut from cardboard.
Shock cord (sustainer)=sewing elastic, 1m x 6mm.
Shock cord (boosters)=450mm of Kevlar. (2 of)
Booster mount point base=4mm thick balsa.
Booster attachment tubes=2.5mm and 4mm diameter brass rod.

Construction
Tube Marking
Either print out the supplied template (if you can get the printer to make it the right size); or make your own (a rectangle to wrap around a BT-55, marked in 1/8ths, one line missing); or mark up the BT-55 by whatever other means you prefer. There should be four lines, equally spaced; the fins will go on these. Midway between two adjacent fin lines is another line, which is where a booster mount point will go; a second such line should be diametrically opposite it. Finally, another line, midway between a pair of fins without a booster mount point between them, is the line for the launch lug, which should be extended 200mm from the rear end of the tube.

Motor Mount
The motor mount is the basic Estes style for putting a 24mm motor in a BT-55. Make a slit in the mount tube 5mm from the front end and insert one end of the retaining clip into it, then glue the motor block (the front 5mm of a spent D12 casing) into the front end. Make two centring rings out of cardboard, inner diameter 25mm, outer diameter 32.5mm. Glue them to the mount tube, one 10mm from the front end, the other 50mm from the front end. Reinforce the joints with glue fillets. While all this is drying, why not fit the...

Shock Cord
Again, basic Estes stuff. Cut out a paper trapezoid and fold it in three. Spread glue over the narrow end, put one end of the shock cord onto it so that the end touches the fold nearest this end, then fold the paper. Spread glue over the second section of the mount and fold it again. Clamp it until it's dry. Then glue it into the front end of the BT-55, at least 35mm down. Leave it to dry, and in the meantime you can make the...

Upper Body
Glue the transition into one end of the BT-60. You can also glue the nose-cone into the other end; or you can wrap some tape round the nose-cone's shoulder to make it fit tightly, then you get to use the BT-60 as a payload bay. Screw the screw eye into the transition, remove it, put a little glue into the hole it made, then screw it back in and leave it.

Boosters
Take 150mm of BT-20. Cut 40mm off one end; this cut off piece will hereafter be known as the front tube, the remaining 110mm is the rear tube. Cut about 20mm off the front end of a spent 18mm casing and glue it into the front tube so that about 10mm sticks out; this will be the coupler.

Cut about 5mm off the front end of a spent 18mm casing (maybe the same one which donated the coupler). This will be the booster's motor block. Take 450mm of Kevlar thread and tie it round the motor block, then glue the motor block 65mm into the rear tube. (You can use another spent 18mm casing to position it. Put a mark 5mm from the rear end of the casing, then use the casing to push the motor block into the booster's rear tube. When the mark is level with the end of the tube, stop pushing and remove the motor casing.) Pass the free end of the Kevlar through the coupler and right through the front tube, tie it onto a nose-cone, then glue the nose-cone into the front tube.

Now do all that again to make the second booster.

Booster Attachments


From 4mm thick balsa, cut three rectangles:
51mm x 10mm (to become a booster mount base which will be attached to the sustainer);
9mm x 8.5mm (to become the forward booster attachment);
12mm x 8.5mm (to become the aft booster attachment).

From the 2.5mm diameter brass rod/tube, cut four pieces 61mm long.
From the 4mm diameter brass tube, cut four pieces 6mm long and four pieces 11mm long.

Glue a 61mm x 2.5mm tube to either side of the booster mount base, so that the tubes extend 4mm past one end of the base and 6mm past the other. Be very careful not to get any glue onto these extensions! Don't run the glue the whole length of the base - leave about 2-3mm unglued at each end of each side. Repeat the procedure to make a second booster mount, then leave both aside until they're dry. Leave them on a flat surface. The tubes and the bottoms of the booster mount bases should all be able to touch this surface; the tops of the booster mount bases should stick out above the tubes.

When the booster mounts are dry, slide a 6mm x 4mm tube onto each of the 4mm extensions, and an 11mm x 4mm tube onto each of the 6mm extensions. Glue a 9mm x 8.5mm block of balsa between the 6mm x 4mm tubes, and a 12mm x 8.5mm block between the 11mm tubes. These blocks, with their small tubes attached, will be the booster attachments. The blocks should all touch the booster mount base, but none of the small tubes should do so. Again, be very careful not to get any glue onto the booster mount tube ends, otherwise the booster attachments will have trouble separating from the booster mounts. Leave all this to dry solid.

Slide the booster attachments off a booster mount. Turn the booster mount upside down, then slide the booster attachments back on. The bases of the booster attachments should now not be level with the base of the booster mount. Glue the booster attachments to a booster, so that the smaller one is glued to the forward part of the booster's body tube. Set the assembly to rest on the booster mount base. Check that the attachment and mount point tubes are in line with the booster's body. As usual, make sure you don't get glue on the attachment and mount point tubes. Do the same to the other booster, attachments and mount point.

If you've done all this correctly, you can pull the booster apart and the front part, having smaller attachment tubes, should come away from the mount point slightly more readily than the aft part. Remove both boosters from the mount points. Draw a line down the middle of each mount point. Glue a mount point to the aft sustainer body tube; align the lines drawn on the mount point with the lines marked on the tube for the booster mount point. Fit the booster onto the mount point, then you can move the mount point to the correct distance up the tube such that the aft end of the booster is level with the aft end of the sustainer body tube. Remove the booster again, check that the mount point is still properly lined up, then leave it to dry. Repeat the procedure to glue the other booster mount point to the opposite side of the sustainer.

Fins and Launch Lug
The template below shows the correct shape and size of the fins, which should be made from balsa 3mm thick. If, like me, you do not have wide enough balsa sheets to make each fin out as a single unit, you can cut the template as indicated, on a line parallel to the leading edge, to make two pieces for each fin. Make four fins and glue them to the appropriate lines on the lower sustainer body tube.

The launch lug can be made from a drinking straw. Cut 45mm from a straw wide enough to accept a 5mm launch rod. Angle the cut at 45 degrees, so that the leading edge of the lug is swept back. (I don't know if this does much for the rocket's performance but it does improve its appearance a little!) To allow the rod to clear the upper body tube, the launch lug needs a standoff made from a 45mm x 10mm rectangle of 4mm thick balsa. Again, the leading edge can be angled to match the launch lug. Glue the standoff to the body tube 155mm from the rear end, and glue the launch lug to the standoff.

Finishing
To get a decent finish on the fins, I used the traditional method - sanding sealer and sanding block, applied three times. Fillets on the fins, launch lug standoff and booster attachments are epoxy. The lower sustainer body and fins are white; the upper booster body, nose-cone and booster rear bodies are red; the booster front bodies and nose-cones are gold. All paints are car sprays. Of course, you are free to use your own favourite ingredients for this stage!

One thing is important, though. Mask the booster mount point tubes when spraying the sustainer body, and mask the booster attachment tubes when spraying the boosters. You do not want to get paint on the ends of the booster mount tubes or the insides of the booster attachment tubes. They are quite close fitting to prevent the booster from wobbling, and if they get painted, they may not separate.

Flight Manual

Recommended Motors
Sustainer: C6-5 (first flight), D12-5, D12-7
Boosters: B6-0 (first flight), C6-0

The D12-7 and B6-0 have now been discontinued, unfortunately. A D12-5 may be used for the sustainer instead. To use a C6-5 in the sustainer, an adaptor must be made. A simple way is to knock the clay nozzle out of a used D12 motor; the empty casing will act as an adaptor. The sustainer will also fly on a D12-5 without the boosters.

All motors above are Estes. I do not know for sure, but I've read that Quest boosters do not provide the pressure at burn-out which Estes boosters do, and while that pressure is bad for series staging, it is necessary for parallel staging. Quest boosters are therefore not recommended. I have no experience with other motor manufacturers, but would guess that Aerotech motors could be used in the sustainer.

Recovery Systems

For recovery, Titania can use a 12" parachute. Mine has used stock Estes parachutes, and once even landed safely on an Estes 12" parachute with a spill hole cut out. These landings were all on relatively soft, grassy ground. The boosters are fitted with 12" x 1" streamers made from mylar (cut from a "space blanket", to be precise).

The sustainer parachute should be protected by 4 or 5 pieces of wadding. Each booster should also be given one piece to protect the streamers.

Ignition
If you have a good launch controller, preferably powered by a 12V battery, you can put a separate igniter in each motor. This is the method I use. It should also be possible to use fast pyrotechnic fuse such as Quickmatch to fire all three motors from one igniter. One method, which I have not personally tried but which may be worth trying, is to fit an igniter into the sustainer's motor, and have Quickmatch running from its nozzle to the boosters. This has the disadvantage that the boosters will ignite after the sustainer, but has the advantage that the boosters will not ignite if the sustainer fails to ignite.

Templates

Click to Enlarge

A scale of 5cm is provided. If you can get this to print out to the correct size, you can cut out the fin and alignment templates. If not, the diagram shows the sizes of the fin's root, tip and trailing edges, as well as the leading edge sweep.