to One Model Rocket (a modified Tres):
Out of the three motor tubes, one is replaced with a longer piece of
BT-20 ducted to a long piece of BT-50, which has a pair of centering
rings to fit it into the Tres' lower body. Notches cut out of these
rings turn them into a baffle. When the rocket is painted, the tube
which is ducted to the extension is painted distinctively as it will
need a booster motor while the other two will need regular motors with
short delay charges - I painted the whole booster blue and put a gold
ring round the ducted motor tube.
A piston is made from a piece of BT-20 sliced lengthwise, expanded so it
will slide freely over unmodified BT-20, and sealed with paper. The
Tres' lower body tube is cut just above where the fin extensions end.
The modified motor mount is glued into the lower part of the tube, the
piston is glued into the upper part, so that the two parts will mate but
will separate when the two motors that aren't ducted into the BT-50 fire
their ejection charges. A shock cord joins the two parts, and a
streamer is fitted for recovery.
The solid balsa coupler is replaced with a scratch-built coupler made
from tubing and paper, which is vented. Vent holes are also cut into
the upper part of the lower body tube to match the holes in the coupler.
The entire lower part of the Tres thus becomes a booster. The upper
part gets a standard 18mm motor mount and becomes a sustainer. Ignition
of the sustainer through such a long booster requires the use of
quickmatch fuse, which does not require a license in the UK. Three To
One has been successfully launched with a full set of motors - B6-0 and
two B6-2 in the booster, B6-4 in the sustainer. All lit, and both
booster and sustainer were recovered intact.