Hitchhiker Server Sky Test

Testing the first server sky thinsat arrays may be challenging - they don't belong in a common orbit, they won't last long below 1000 km altitude due to ram drag. It would be nice to deploy the first tests at M288, but that will require a custom launch.

If we dispensed thinsats from a GTO transfer orbit with a 622 km perigee ( 7000 km radius ), the atmospheric density could be as high as 4E-13 kg/m³ at perigee. Lets assume that we orient the thinsats edge-on to the stream as it passes through perigee, with a ram area of perhaps 20 cm². The perigee velocity is 9.88 km/s, so the drag is 2e-3 * 4e-13 * 9880³ or 7.7 e-4 N, 0.15 m/s² decelleration of a 5 gram thinsat. If that level drag occurs within 50 kilometers above perigee ( r_1 =7050 km), then the orbital angle is computed from:

\large r_1 = a \Large { {1-e^2} \over { 1 + e \cos(\theta) } }

\large \cos(\theta) = { { \Large { { a ~ (1-e^2) } \over { r_1 } } - 1 } \over e }

\large \cos(\theta) = { { \Large { { 24582 ~ (1-0.7152^2) } \over 7050 } - 1 } \over 0.7152 } = 0.183 radians

The seconds per radian is r_p / v_p = 7000 / 9.88 = 708.3 s / radian, so the time spent in the high drag region is 2 * 0.183 * 708.3 = 260 seconds, and the velocity lost for the first pass is about 40 m/s, 9880 to 9840 m/s. For the second pass, the velocity and drag will be smaller, but apogee will drop significantly and more time will be spent in the drag zone. After about 50 orbits, the orbit decays sufficiently that a thinsat will start a fast spiral to burnup, lasting less than a day. If the thinsat loses control, it will decay in perhaps 10 orbits. The experiment will stay in orbit for perhaps 2 weeks.

This would be a good first test for the first maneuverable thinsat, if we can hitch a 5 gram ride on a GTO transfer vehicle. However, this is far too much drag and buffeting to keep an array assembled by light pressure intact.

24 GHz instead of 70 GHz

The 1.25 cm amateur radio band extends from 24.0 GHz to 24.05 GHz, and is barely used. This is near the 23 GHz water peak, perhaps .18 dB/km at 7.5g/m³ water saturation. At 10 degrees elevation and 7 km lapse rate, that is 7.3 dB attenuation; a factor of 5.3, which might be lousy for ordinary high bandwidth satellite communication, but perfectly adequate for low bandwidth telemetry.

The $20 Toy Rocket Motor Trick

Let's raise perigee just enough to make 92 each 5 gram thinsats (V = 3 array) last a couple of months - lets assume that is 922 km altitude perigee (7300 km radius). Assume a carrier that brings the total mass up to a kilogram. Start with a GTO transfer orbit with a perigee of 622 km altitude (7000 km radius), with an apogee velocity of 1640.7 m/s. The test orbit has an apogee velocity of 1692.4 m/s, for a delta V of 29.5 meters per second for one kilogram, the impulse of 30 Newton-seconds.

That is similar to one Estes E9-4 model rocket engine (30 N-s, 57 grams, 2.8 seconds).

Perigee velocity is 5.53 km/s. Let's assume that we are edge on as before. The average air density at 922 km altitude is about 4e-15 kg/m³ and that drag occurs within 100 km of perigee; a 30 degree arc, 0.52 radians, 390 seconds of drag time.

The normal air density is 4e-15 kg/m³. The edge-on drag through perigee is A \rho V^3 , or $ 0.002 * 4e-15 * 9650³ or 2.7e-5 N, decellerating the 5 gram thinsat by 5.4e-3 m/s², or 2 m/s per orbit.

Circular orbit velocity at that altitude is 7390 m/s, 2260 m/s less than our starting orbit perige, so we would get about 1000 experimental orbits before the altitude starts decaying fast.

We will use three more toy rocket motors on the 500 gram carrier at apogee, after thinsat release, to bring the perigee down to 0 km altitude, for reentry. That requires 90 m/s delta V, 45 N-s impulse. Three is overkill, but means we can deorbit the whole predeployment assembly if something goes wrong.