Thursday, September 04, 2008

Solar Sails in Orbiter

I’m heading home Friday after a busy week in Glasgow, and with this very flaky hotel internet connection, I haven’t had much of a chance for blog reading or writing. But I did manage to read a great post on solar sails tonight at Centauri Dreams. Paul Gilster reviews an interesting new book, Solar Sails: A Novel Approach to Interplanetary Travel.

This prompted me to see what Orbiter add-on makers have to offer in the solar sail department, and was happy to find that a 2002 add-on by “ObiWan” (Felipe Comparini) had been updated in 2007 for the current Orbiter version. I had to play around with installation folders to get it to work, but once I did, it worked well (I moved some .cfg and .ini files to the main /config folder - note that the orbiter.log file can be very informative when Orbiter crashes due to missing or misplaced files).

The solar sail configuration file says its area is 2800 (presumably square meters, about 53 meters square). This looks about right compared to the shuttle I inserted in the second picture for scale (shuttle length is about 37 meters). So this is not an interstellar sail, but it lets you play around to see how these things work. The included scenario starts you in a near-circular Earth orbit at about 1220 km altitude where you can separate from an upper stage (J), wait a few minutes for clearance, and deploy the sail (D). The animation is pretty cool.

This new version includes an autopilot which takes care of orienting the sail so the solar flux raises your apogee over a series of many orbits (turning edge-on to the sun on the “upper” part of each orbit). Normal and transverse forces are reported at the bottom of the screen, along with the sail angle (maximum normal force is about 0.35 Newtons or about 0.078 pounds-force). I used up to 1000x time acceleration and got the apogee up to about 7000 km before saving my scenario and quitting to write this.

ObiWan includes very brief instructions and the solar radiation pressure equations he used to implement this add-on. It always impresses me that Orbiter’s open architecture allows things like space elevators, rail guns, and solar sails in addition to any kind of rocket-like vehicle, and that add-on developers use these features in their add-ons – all for free.

P.S. The force values reported by this add-on seem substantially too high. The solar radiation pressure at 1 AU is about 4.5x10-6 N/m^2 so for a perfectly reflecting surface at normal incidence, the force would be 2*(4.5x10-6 N/m^2)*(2800 m^2) or about 0.025 newton. To get 0.35 N would take almost 40,000 square meters.


Unknown said...

Bruce, the Orbiter sail looks great, and the design should allow some experimentation with the concept. I have to get involved with Orbiter one of these days!

Felipe Comparini said...

I will check the forces, but the size of the sail visible is not the size that you can asign on the config file. As long as I remember, it was much bigger on the config than the visual.