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.

Instead, let's dispense as a co-payload from a GTO transfer orbit. We will test in a highly inclined orbit, in three steps:

• 1) Deploy: use a cold gas thruster to slightly lower apogee and significantly raise perigee. This will be the orbit for our experiment. The inclination of this orbit will be a function of launch latitude, and the ascending node will be on the equatorial plane.

• 2) Dispense approximately 100 thinsats near apogee. The subsequent fate of the dispenser is undefined; perhaps we can use it later for collision experiments.

• 3) Experiment: the thinsats maneuver into arrays and begin the test. The perigee drag is high enough that the apogee will decay significantly with each orbit. Thinsats will orient for minimum drag, but because they are curved the drag will be significant.

• 4) Deorbit: if damage has not set the thinsats tumbling, lock the optical thrusters to produce a rapid yaw tumble. This will greatly increase perigee drag.

• 5) Reentry: after apogee decays to perigee, the orbit will rapidly spiral in. When tumbling thinsats reach ISS altitude, they will reenter one or two orbits later.

The starting GTO transfer orbit will have a perigee radius of r_{P-GTO} (perhaps 7000 km) and an apogee radius of r_{A-GTO} (perhaps 42,100 km ) one or two orbits after release from the GTO upper stage. Using these numbers and the gravitational coefficient \mu , we can compute the perigee and apogee velocities v_{P-GTO} and p_{A-GTO} , the semimajor axis a , the eccentricity \epsilon , and the characteristic velocity v_0 .

Computing the experimental orbits will require numerical integration, with estimated perturbations telling us how difficult it will be to keep arrays together. We can approximate all thinsats in an array in one trajectory in two regimes: (1) highly elliptical, dominated by perigee drag on a thinsat oriented for minimum drag, and (2) a spiral, with decay increasing rapidly as thermospheric density increases exponentially and the thinsats tumble. It may be possible to arrange and observe a collision between one or two thinsats and the heavier and slow-orbit-decay dispenser, which will still be in a high velocity, high eccentricity orbit.

Gas density will low during quiet sun periods around 2016, and maximum about 5 years later in 2021. If possible, we should schedule the test so that all probably-disabled thinsats will reenter during that solar maximum.

Computing the Experimental orbit

The initial orbit

Starting with r_{A-GTO} and r_{P-GTO} , we can compute the semimajor axis a and other parameters:

\large a ~ = ~ 0.5 * ( r_{A-GTO} + r_{P-GTO} ) ~ ~ ~ ~ ~ ~ ~ semimajor axis

\large \epsilon ~ = ~ ( r_{A-GTO} - r_{P-GTO} ) / ( r_{A-GTO} + r_{P-GTO} ~ ~ ~ ~ ~ eccentricity

\large T ~ = ~ 2\pi \sqrt{ a^3/\mu } ~ ~ ~ ~ ~ orbital period (sidereal)

\large v_0 ~ = ~ \sqrt{ \mu / ( a (1 - {\epsilon}^2) ) } ~ ~ ~ ~ characteristic velocity

\large v_a ~ = ~ v_0 ( 1 - {\epsilon} ) ~ ~ ~ ~ ~ apogee velocity

\large v_p ~ = ~ v_0 ( 1 + {\epsilon} ) ~ ~ ~ ~ ~ perigee velocity

For the first approximation, ignore J_2 and other complexities in the gravitational field, but include them in the numerical model. They will cause the elliptical orbit to precess towards the east.

Atmospheric Drag

The atmosphere becomes exponentially thinner with altitude until it is as low as the density of interplanetary space. The density can be approximated by

\large \rho ( r ) ~ = ~ {\rho}_0 e^{ - ( r - r_0 ) / L }

where {\rho}_0 is the density at altitude r_0 , a function of temperature, solar activity, and longitude, which vary with time. L is the lapse rate, the distance over which the atmosphere density decays by a factor of e = 2.718 . Solar activity can triple temperature, and greatly increase the density of the atmosphere at very high altitude. Most of the gas at these high altitudes is hydrogen and helium. Because the sun heats the atmosphere during the day, the temperature and density peaks in the afternoon 1400 hours position. The air is coldest at before dawn, at the 0400 hours position. The mean free path of the air molecules above 1000 km is larger than the lapse rate, so the atoms rarely collide.

When an object passes through this thin gas at very high orbital speeds, it collides with the gas molecules, and experiences a force proportional to the gas density \rho times the area A times the velocity squared . The gas is thickest at perigee, a density of {\rho}_p , where the lapse rate (which changes with altitude) is L_p . The modified density equation is:

\large \rho ( r ) ~ = ~ {\rho}_p e^{ - ( r - r_p ) / L_p } ~ ~ ~ ~ density near perigee

A small change in radius makes a large change in drag, and the velocity is inversely proportional to radius. We will assume the velocity near perigee is a constant v_p . So, the drag force for area A (the area perpendicular to the velocity) is

\large F ~ = ~ \rho A { v_p}^2 ~ ~ ~ ~ drag force

The acceleration for a thinsat with mass M is:

\large a ~ = ~{ \Large { \rho A } \over M } { v_p}^2 ~ ~ ~ ~ acceleration

The radial velocity v_r is zero at perigee, and increases in magnitude at angles to the side:

\large v_r ~ = ~ \epsilon v_0 \sin( \theta ) ~ ~ ~ ~ radial velocity at orbit angle \theta

v_0 = v_p / ( 1 + \epsilon )

\large v_r ~ = ~ { \Large \epsilon \over { ( 1 + \epsilon ) } } v_p \theta ~ ~ ~ ~ radial velocity near perigee for small \theta

\Large { { d \theta } \over { d t } } ~ \large \approx ~ \Large { { v_p } \over { r_p } } ~ ~ ~ ~ angular velocity near perigee

Define y = r - r_p

\Large { dy \over dt } \large ~ = ~ \Large { dr \over dt } \large ~ = ~ { \Large \epsilon \over { ( 1 + \epsilon ) } } v_p \theta ~ ~ ~ ~ radial velocity

Assuming angular velocity is approximately constant, and starting with t = 0 at perigee:

\theta ~ = ~ ( v_p / r_p ) t

so

\Large { dy \over dt } \large ~ = ~ { \Large \epsilon \over { ( 1 + \epsilon ) } } { {v_p}^2 \over { r_p } } ~ t

Integrating:

\large y ~ = ~ { \Large \epsilon \over { 2 ( 1 + \epsilon ) } } { {v_p}^2 \over { r_p } } ~ t^2

Solving for t as a function of y:

\large t ~ = ~ { \Large \sqrt{ { { 2 ( 1 + \epsilon ) } \over \epsilon } { { r_p } \over {v_p}^2 } \large y } }

derivative:

\large dt ~ = ~ { \Large \sqrt{ \left( 1 + { 1 \over \epsilon } \right) { { r_p } \over { {v_p}^2 y } } } } ~ dy

The decrease of v_p during one perigee pass is the integral of the acceleration over time :

\large \Delta v_p ~ = ~ { \LARGE \int_{-\infty}^{\infty} } a ~ dt

From symmetry:

\large \Delta v_p ~ = ~ 2 { \LARGE \int_0^{\infty} } a ~ dt

\large \Delta v_p ~ = ~ 2 { \LARGE \int_0^{\infty} } { { {\rho}_0 A } \over M } {v_p}^2 e^{-y / L } \large ~ dt

Recasting as an integral over y:

\large \Delta v_p ~ = ~ 2 { \LARGE \int_0^{\infty} } { { {\rho}_0 A } \over M } {v_p}^2 e^{-y / L } ~ { \sqrt{ \left( 1 + { 1 \over \epsilon } \right) { { r_p } \over {v_p}^2 y } } } \large ~ dy

Moving and combining constants out of the integral:

\large \Delta v_p ~ = ~ { { 2 {\rho}_0 ~ A ~ v_p } \over M } \sqrt{\left( 1 + { 1 \over \epsilon } \right) r_p } { \LARGE \int_0^{\infty} } e^{-y / L } \sqrt{ 1 / y } ~ dy ~ ~ ~

The integral can be solved with a change in variables, let ~ y = L b^2 ~ so that ~ dy = 2 L b ~ db ~ and ~ ~ \sqrt{ 1 / y } = b \sqrt{ 1 / L } ~E ~ ~ :

{ \LARGE \int_0^{\infty} } e^{-y / L } \sqrt{ 1 / y } ~ dy ~ = ~ 2 \sqrt{ L } { \LARGE \int_0^{\infty} } e^{-b^2} ~ db = \sqrt{ \pi L } ~ ~ ~

So:

\large \Delta v_p ~ = ~ { { 2 {\rho}_0 ~ A ~ v_p } \over M } \sqrt{ \left( 1 + { 1 \over \epsilon } \right) \pi ~ r_p ~ L } ~ ~ ~ Equation S1

Find \epsilon from r_p , v_p and the constant \mu :

\large r_p ~ = ~ ( 1 - \epsilon ) a ~ ~ ~

\large ( 1 - \epsilon ) ~ = ~ \Large { r_p \over a } ~ ~ ~

\large v_p ~ = ~ \LARGE \sqrt{{{2\mu}\over{r_p}}-{{\mu}\over{a}}} ~ ~ ~

\Large { v_p^2 \over \mu } ~ = ~ { 2 \over r_p } - { 1 \over a } ~ ~ ~

\Large { 1 \over a } ~ = ~ { 2 \over r_p } - { v_p^2 \over \mu } ~ ~ ~

\large ( 1 - \epsilon ) ~ = ~ 2 - \Large { { r_p v_p^2 } \over \mu } ~ ~ ~

\large \epsilon ~ = ~ \Large { { r_p v_p^2 } \over \mu } - 1 ~ ~ ~ Equation S2

So we can remove \epsilon :

\large 1 + \Large { 1 \over \epsilon } ~ = ~ { 1 \over { 1 - \mu / r_p v_p^2 } } ~ ~ ~

\large \Delta v_p ~ = ~ { { 2 {\rho}_0 ~ A ~ v_p } \over M } \sqrt{ \left( 1 \over { 1 - \mu / r_p v_p^2 } \right) \pi ~ r_p ~ L } ~ ~ ~ Equation S3

So

MoreLater

Old stuff, to be reworked

\\ 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.