Build Update 3 – Complete

I was quite proud with my progress detailed in the last post: Build Update 2 – Painting. With the motor/fin assembly now firmly bonded in place with copious amounts of epoxy, I shared a photo with a colleague of mine. He just so happens to have expert experience with high-powered model rocketry. Initially he seemed impressed. I then asked him about my plans to attach the cord for the parachute…

Oops…

Some context might be needed here. I have built and flown several low-powered rockets. In fact, the little foot-long Estes Alpha III, that I used in the photo above for comparison, was a birthday present from my wonderful parents, I think in 1998 (some 25 years ago now).

The tallest model I have built prior to undertaking GLYPO-001 is an Estes Mercury Redstone (above right), which stands at 2’5”. This rocket, however, uses C-class motors. The most powerful model that I have built is an Estes Big Daddy (above left), which at 1’8” is shorter but more stubby, and launches spectacularly slowly with D-class motors. With these low-powered models one simply glues a little elastic cord to the side of the cardboard body tube.

GLYPO-001 will fly with both G-class and H-class motors, making it a mid-to-high-powered rocket. I was not naive enough to imagine a simple elastic cord glued to the body tube would suffice. However, I thought I had a reasonable solution of a plywood anchor glued midway down the rocket body. My colleague’s reaction was not overwhelmingly positive. Unquestionably an expert in such matters, I had clearly made a mistake.

He explained that the common approach for these larger rockets is to anchor the recovery (parachute) cord on the motor mount at the bottom of the rocket. The motor mount is of course the strongest area of the vehicle, and this would need to be done before integration into the rocket body. At this point, I was wondering whether my ignorance had irreparably doomed the rocket.

An ‘engineering solution’

For the past 15 years I have worked as an aerospace engineer, the years prior as an aeronautical engineering student. I soon learnt the importance of the ‘engineering factor’ in my simulations. One would have to apply a factor to their numerics, bridging the gap between theory and reality. Those a little less kind, yet perhaps more honest, would call this a fudge factor.

Much the same is true of ‘engineering solutions’. Mathematicians and scientists would often devise a pure and beautiful solution, but neglect key details such as screws, bonds, rivets, and mounts, the very things that make the design practicable. ‘Engineering solution’ is not a dirty term, it is a realistic term. Not unsafe, not a bodge, just not pure.

At this point, I needed to fix my mistake, and was in dire need of just such a solution.

Thankfully I had a spare centring ring (Baltic birch plywood, from BlackCat Rocketry). I also had the cut-away ring that I was originally planning to use as an anchor (do not ask what I was thinking, in hindsight it is quite ridiculous).

I glued the two rings together with epoxy, drilling a hole and passing through the tubular Kevlar shock cord, which I tied in place. I also glued the cord for additional support and to prevent any fraying. This produced a solid anchor. The cut-out ring acted as a suitable spacer to account for the knot.

I then pushed this right down the body tube (using a bamboo pole) with generous amounts of high-quality epoxy, and it bonded well to the motor mount. The high surface-area contact should provide a high-strength bond, with the small mass penalty of the additional ring (12 g) and my cut-away ring (less than 12 g).

Recovery System

The first element of the recovery install was the anti-zipper kit. This sounds much more fancy than it is, it is a small foam ball and a Nomex sheath. For £5 it seemed worth a punt to protect the edge of the rocket body from damage caused by the cord during parachute deployment and descent.

The rest of the recovery system consists of a good length of 1/8” tubular Kevlar. This is approximately three to four body lengths long, which is a recommendation reiterated often online, for example in this article from the US National Association of Rocketry.

There is also a Nomex sheet to protect the parachute, and a 24” rip-stop nylon parachute of BlackCat Rocketry design and manufacture.

Mass and Centre of Gravity

With the recovery system installed, the rocket is finally complete. During the design process in OpenRocket I had been conservative with regards to mass distribution. I deliberately neglected some of the recovery system components in the hope that the absence of this mass further forward would shift the predicted centre of gravity further aft than reality.

The H128W motor has an assembled mass of 215 g (according to the Aerotech website). Wizard Rockets were happy to post the RMS 29/180 casing (which is inert aluminium), but for safety reasons would not post the reloadable propellant. The composite propellant is hazardous, thus many couriers will not permit transport. Additionally, Wizard will only hand over such a high-power motor to someone with the appropriate UKRA certification.

Consequently, I had to get creative when measuring the mass properties of the vehicle. As a keen baker, I had ceramic baking beans, useful for blind baking pastry, which are fairly dense and filled the casing with a uniform mass distribution.

In G-Class Configuration (G75J)

	As Modelled	As Built
Mass (with motor)	1,100 g	1,256 g
Centre of gravity (from nose tip)	81.1 cm	78 cm
Static margin*	2.25 cal	2.71 cal
Thrust:weight**	7.0:1	6.1:1

Assuming CoP is 99.5 cm, as calculated by OpenRocket
*Assuming average thrust of 75 N; integration of my thrust curve is 64 N

In H-Class Configuration (H128W)

	As Modelled	As Built
Mass (with motor)	1,082 g	1,243 g
Centre of gravity (from nose tip)	81.1 cm	77 cm
Static margin*	2.33 cal	2.84 cal
Thrust:weight**	12.1:1	10.5:1

Assuming CoP is 99.5 cm, as calculated by OpenRocket
*Assuming average thrust of 128 N; integration of my thrust curve is 122 N

Mass and Centre of Pressure Summary

The reality is that regardless of application, there is quite often a difference between as-envisaged and as-designed. Invariably there is a discrepancy between as-designed and as-manufactured. I am not shocked by any of the figures, though I was not expecting to be overweight by as much as 161 g.

I had under-accounted for my recovery system perhaps a little too aggressively. The upside is that this mass is further forward in the vehicle, which improves my static margin. As-built, it is very healthy. The rocket has plenty of thrust to fly safely, and I will of course update the simulations before flight.