Wednesday, March 28, 2007

Dynamic Stability and Launch Rails

A quick update on our wind tunnel testing and the consequences for the launch rail/launch windows:

So far we have been planning passive guidance for the rocket (read: launch rail and fins). This is largely because time and personnel constraints - once we have the initial rocket built, we plan to add a gimbal for thrust vector control at some point in the future.

The general concept is that at low velocities, the rocket is not very stable in a crosswind. By providing a rail to guide the rocket during the first few seconds of acceleration, we help keep it flying straight until it has enough velocity to remain stable on its own.

As posted earlier, our team has been lucky enough to have a large truss donated to us from Solid Rock Steel for use as our launch rail. The truss is about 14m long and capable of supporting far more than the rocket will weigh.

Stability is dependent on the relative positions of the Cg and the Cp, which we have estimated using Unigraphics for mass estimates and Rocksim for static Cp. However, we have also been trying to determine the relationship between stability and effective angle of attack, called dynamic stability. To do this, we have been testing a scale model in UBC's wind tunnel.

The (incomplete) data right now looks like there's a marginal stability angle of about 12 degrees, although we need to do more tests next week.

Once we have the marginal stability angle in hand, using the known thrust-to-weight ratio and the known launch rail length, we will know what our crosswind limits are for launch. This will allow us to develop "launch windows" and a set of conditions under which we can fly.

One difficulty with passive guidance is that Aurora-1 has a low thrust to weight ratio compared to solid propellant rockets (the T/W ratio depends on how much rocket propellant gets pumped in - therefore lower altitude flights actually have a higher T/W ratio). This means that it either needs a long launch rail, low crosswinds, or both. For instance, if the angle was actually 12 degrees, it looks like we would be limited to crosswinds under about 17km/hr for our first planned launch to 30,000ft.

Edit: Whoops, I made an error. The angle we're looking at is more like 27 degrees, not 12. This would allow the rocket, partially fueled for 30,000 ft flight, to launch in something more like 28km/hr crosswinds. Fully fueled for 100km it would only be able to launch in 18km winds. This assumes use of the entire launch rail length, however, so in reality we will have to knock down those critical wind levels by a bit to compensate for the fact that the rocket will be able to rotate near the end of the rail.

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