Golden Crypt

by Geoff Canham, LUNAR #493

The Gold level of NARTREK (NAR Training Rocketeers for Experience and Knowledge) involved designing a rocket, calculating its stability and performance, and then flying it six times to compare actual performance with predicted. Also, the rocket had to be a payload model, or a competition rocket. "Competition" could mean anything from a rocket powered by an A motor with a streamer recovery, on upwards. But when I saw the notice of the new Future/Fiction Scale Competition (Vol. 5, No. 2), it opened the door for me to build a rocket that I already intended to build sometime, and complete the Gold level at the same time.

The Eagle comic was first published in Britain in 1950, and continued through to 1969, and the cover story was titled "Dan Dare - Pilot of the Future." He was an astronaut in Spacefleet, obviously the forerunner of Star Fleet. One of the storylines (called "The Man from Nowhere") featured a very stylish interstellar ship from the planet Cryptos, and it was the Crypt Ship that was to become the rocket I would design and build for the NARTREK Gold qualification.

I work in the building industry, and frequently receive sets of construction drawings that come rolled around cardboard tubes. These tubes make ideal body tubes, and the one I used for this rocket had an outside diameter of 2.25" and an inside diameter of 2.125". It resulted in a fairly heavy rocket, but it also gives a fairly stately flight. The nosecone and boattail were constructed from thin card (similar to the cardboard on your breakfast cereal packet). On the accompanying drawing, the body tube is labeled "B" and is 38.58 cm long. The nosecone is 18.41 cm long and consists of parts A (also cut from the same 2.25" diameter tube) and E, D and C which are formed from the thin cardboard.

(GIF, 11KB)
Construction drawing of the Crypt Ship.

The outline of the components is shown on the attached drawing, and I apologize for the poor quality, but the software used is more for measuring than drawing. The grid shown over part of the drawing is at 25mm spacing. Along the bottom is an elevation of the main body, including nosecone and boattail, immediately above that, on the right side, are the outlines of parts C and D, and above those are parts E, F, G and H.

The boattail was constructed from parts F, G and H, all glued together, then glued to the base of the body tube, part B. The nosecone and boattail then had spackling applied to it, to make them more like a curved shape than a segmented one.

Actually, before the boattail was attached to the body tube, the engine mount had to be constructed and glued inside the boattail. The engine mount consisted of a cardboard tube, constructed from part J, to take 24mm diameter motors (the rocket was designed to fly on Aerotech E15-4 and E30-4 motors), and part of an old Estes D12-3 motor casing was glued inside it as a stop. Two rings, as shown as part K, were cut out and glued to the outside of the engine mount tube to support the mount inside the body tube. The serrated end to part J is actually triangular shaped glue-tabs, used to fix the lower end of the engine mount inside the base of the boattail.

The three fins were constructed primarily out of 1/8" balsa, and are of through-the-wall construction. The outline is shown hatched as part M, and you can see that the part extending into the body tube is notched around one of the engine mount supports. The mock-engines on the fins are made from 1/2" thick balsa glued to either side of the 1/8" thick fins. The drawing shows a vertical and a horizontal section through the completed mock-engine, and L shows the template used to add the detail. The hatched areas are triangular grooves cut in the face and painted black. The horizontal line immediately above the cut-outs is also a groove cut in the face, and the line above that is a break line in the painting. Above the line it is painted red, below it silver/chrome.

The section of fin on the body tube side of the mock-engine is faced on either side with an additional layer of 1/16" thick balsa, so the total thickness of fin adjacent to, and extending into, the body tube is 1/4". The leading and trailing edges of the fins were then sanded off to an aerofoil type shape.

The nosecone had another cardboard tube glued inside it, the other end of the tube fitting easily into the top of the body tube to hold the two parts together. Spackle was added inside the nosecone to ensure stability in flight by adding weight, and elastic cord was glued inside the nosecone and body tube to act as a shockcord. The launch lug was cut from a drinking straw (do I believe in recycling, or what?).

The parachute was made out of plastic sheeting provided by a local nursery to protect the carpet in my car when I bought some plants for the garden. The parachute itself was cut out as a 20" square with the corners removed. The parachute cords were made from dental floss that the dentist insists on giving me each time he drills a few holes in my head. The plastic sheet was reinforced at attachment points by squares cut from a self adhesive label, and after three cords pulled loose on the first flight, the attachments were further reinforced with mailing tape.

However, that first flight was not typical (happily). The wind was very strong down at the Berkeley Marina on that Saturday morning, and the rocket weathercocked badly, rising not much more than 100' and most of its flight was almost horizontal. It was down to about 20' when the parachute deployed, and I didn't really notice much reduction in speed before it was down. My mind didn't want to see the remains, but my feet broke out into a jog. But the only damage from landing was a minor dent in the nosecone! There was damage to the fins, but that had occurred before take-off. After ignition, the motor had sent out tongues of flame for a second or two, which had licked a lot of the paint off the base of the rocket, before the motor suddenly developed the thrust to lift off. It seems likely that the igniter was not fully inserted into the motor.

The second flight of the Crypt Ship was two weeks later, after a touch-up of the paintwork. This was the LUNAR meet in May, 1998 (Vol. 5, No. 4), that was also the first try-out of the rules for the Future/Fiction Scale Contest. It was also a similarly windy day, like on the first flight. To overcome the potential for weathercocking, I had decided to use an E30-4 motor this time, to try and get it moving up and away faster, so the wind would have less opportunity to change the flight path. And it worked. The flight was straight up, the parachute deployed just after apogee, and I and the contest judge, Jack Hagerty, started walking over to where it was landing. A younger recovery team beat us to the landing site, and one of them came rushing back saying "The rocket exploded!" That at least was consistent with the story the rocket comes from, where it either explodes or gets shot down at the end of the two flights. It seems that what had happened was that the rocket had landed on one fin, and 1/8" balsa is not that good at supporting just over a pound in weight, even when assisted somewhat by a parachute. The fin must have buckled until it (the part beyond the mock-motor) shattered into about six parts. Otherwise the rocket was fine, and the parts of the fin were reassembled a few days later, and repainted within a week. It's third flight was at the June LUNAR launch, where it flew well on an E15-4 and everything was intact after touching down alongside the low-power safety-check line.

That got me half way through the number of flights needed to meet the NARTREK Gold requirements. It has to fly three times on each of the two motors it is designed for, and the results of the flight compared with the predicted flight characteristics.

Information date: September 18, 1998 lk