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 chord 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’ve 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 chord 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 chord 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) chord on the motor mount at the bottom of the rocket. The motor mount is of course the strongest area of the vehicle, and this of course 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’ve 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’ isn’t a dirty term, it’s 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-way ring that I was originally planning to use as an anchor (don’t ask what I was thinking, in hindsight it’s quite ridiculous!).

I glued the two rings together with epoxy, drilling a hole and passing through the tubular Kevlar shock chord which I tied in place. I also glued the chord 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 bucket loads of high quality epoxy, and it bonded well to the motor mount. The high surface-area contact meant that this should provide a high-strength bond with the small mass penalty of the additional ring (12 g) and my cutaway 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’s a little foam ball and a Nomex sheath. For £5 it seems worth a punt to protect the edges of rocket body from damage caused by the chord during parachute deployment and descent.

The rest of the recovery system consists of a good length of 1/8″ tubular Kevlar. This is approximately 3-4 body lengths long, which is a recommendation I found reiterated often online, for example in this article on the US National Association of Rocketry. There is some Nomex sheet to protect the parachute, 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 the absence of this mass, further forward, will shift the predicted Centre of Gravy (CoG) further aft than reality.

The H128W motor has an assembled mass of 215 grammes (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 propellent. 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!) that 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

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

Mass and Centre or 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 wasn’t expecting to be overweight by as much 161 grammes.

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 improves my static margin, which as-built is very healthy. The rocket has plenty of thrust to fly safely, and I will of course update the simulations before flight.