Ten days ago I wrote that I was working on the last major puzzle (uprighting) on space capsule Tycho Deep Space II to make it all fit. That was apparently far from the truth. As things progressed another major problem suddenly emerged in SolidWorks.

The uprighting system is very much dependent on mass distribution. So, even though I knew it would not affect the final and very tolerant uprighting design, I began to add all subsystems in SolidWorks into the capsule to pinpoint the center of mass, which began to climb and climb upwards, leading to panic and despair.

So, what is the big deal if the uprighting system still works after a dramatic displacement of mass? Well, now the capsule is probably not stable during re-entry!

Anything flying or free falling (in a pressurized environment) will only have a stable flight if there is a correct balance between Cg (center of gravity) and Cp (center of pressure).

We all know how a dart-arrow works. The heavy mass (Cg) at the tip of the arrow turns towards the flying direction but only because the finned area in the back (Cp) is being forced the opposite direction, by drag. The dart arrow will fly stable without tumbling. The same principle goes for rockets and as a rule of thumb there must be at least 1-2 times the rocket diameter in distance between Cg and Cp, also knows as the stability margin.

On a dart arrow it is very obvious to spot how this works but it becomes slightly more difficult to see or even calculate on a blunt space capsule and when flying supersonic the fluid mechanics of airs changes, pushing Cp closer towards the Cg.

On the other hand, having Cg moving towards Cp due to propellant consumption, creating instability was the issue we faced during our LES test of Tycho Deep Space I. The change of Cg, due to propellant consumption, was of course known to us but identifying the exact spot for Cp, prior to flight, was difficult.

Right now I fear that Cg on Tycho Deep Space II is dangerously close to Cp or maybe they have even changed positions. If so, the capsule will be unstable and tumble during re-entry and all this must be fixed before moving any further by performing stability tests or perhaps even redesigning the capsule totally.

Tycho Deep Space II, center of mass (purple arrows) dangerously far from the heat shield likely to cause instability during re-entry. Image: Kristian von Bengtson

In the beginning of the US space program in the 1950s, NASA performed a series of scaled model tests to get wiser on these exact problems. They created 1/10 scale models of the Mercury capsule and tossed those into a vertical wind tunnel.

I aim to copy these tests even though it only gives me an indication of subsonic descent stability. It just so happen that such a facility is to be found close to Copenhagen, created for skydivers to practice their maneuverer and for plain fun. This vertical wind tunnel creates wind speeds of up to 230 km/h (142 mph) and they have agreed to support Copenhagen Suborbitals with letting us perform these tests. Thank you so much!

Instead of just letting the model capsule float around uncontrolled, Niels Foldager of Copenhagen Suborbitals suggested that we added a long stick into the side of the capsule creating a pivot point, working as the center of mass (Cg). The capsule will turn accordingly to the drag revealing at least if Cg is far enough from Cp, or the opposite. Since I would like to change Cg during several tests, we just move the stick/pivot point of the capsule.

It does not really matter how much the model weighs. As long as we can control the center of mass using a pivot point and the geometry of the model is correct, we will get some first indicators of the stability situation. We will bring at least a 1/10 scale model of the capsule and one extra added the future LES tower.

For now there is only one rule to rule them all: getting Cg, or the center of mass, as far down towards the heat-shield as possible.

Please enjoy these great old school videos from NASA on capsule stability (embedding unfortunately disabled). Soon, we will present similar test videos and hopefully be wiser on the subject.

NASA Langley, Re-entry Body Stability Tests

NASA Langley, Model Tests of the McDonnel Design of Project Mercury Capsule - Part 1

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Kristian von Bengtson