Orbital Math from Live Constants

The useful orbital numbers are already in /sim. Read them from the current parent, once, and leave the old 9.8 habit on the launchpad.

For an orbit with centre-to-vessel radius $r$, circular speed is $v_c = \sqrt{\mu/r}$. Here, mu is m³/s², radius is metres, and an apoapsis or periapsis is an altitude above the surface.

/sim (in guest)

parent=$(cat /sim/vessels/active/parent)
mu=$(cat "/sim/bodies/$parent/mu")
radius=$(cat "/sim/bodies/$parent/radius")
altitude=120000
awk "BEGIN { r=$radius + $altitude; print sqrt($mu / r) \" m/s\" }"

cat /sim/vessels/active/orbit/apoapsis
cat /sim/vessels/active/orbit/periapsis
cat /sim/vessels/active/orbit/period

HTTP (host)

parent=$(curl -s http://127.0.0.1:4242/v1/vessels/active/telemetry | jq -r .parent)
curl "http://127.0.0.1:4242/v1/bodies/$parent" | jq '{mu, mean_radius, soi_meters}'
curl http://127.0.0.1:4242/v1/vessels/active/telemetry | jq '.orbit'

The vis-viva equation is $v^2 = \mu(2/r - 1/a)$. Use metres and seconds throughout. Angles from the orbit files are degrees, so convert before feeding them to sin or cos.

Tip

Read body constants once at program startup. The planet is not changing its gravitational parameter between ticks. A vehicle’s engine set can change after staging, so engine data is different.

Next: turn a future UT and a CCI Δv into a scheduled burn.

Next: Schedule a burnHand an impulsive maneuver to KSA's flight computer.