Ride-shared atmospheric probes

[MAFman]

For probes similar to the one Galileo dropped into Jupiter, how would one implement communications without putting a gigantic antenna on the probe? I'm guessing the orbiter could be used as a relay.

[Rocket Witch]

Yes, the orbiter can act as relay. Other craft in the vicinity only need enough direct antenna power to reach the relay. They will need an external antenna to communicate with the local relay; the integrated internal antennae on command modules can't do this.

You can also control automated vessels without relays — and indeed without any line back to the KSC at all — if you instead have a nearby crewed craft that has both an RGU (the short cylindrical 1.25m and 2.5m probe cores) and a spare pilot (ie. two pilots total, as all vanilla command pods only require one pilot for control and I've never seen a modded pod require more). In this case I presume the controller and controllee only need direct-class external antennae, though it might even work on internals, all be it at extremely short range.

The probe(s) you detach will only be fully controllable while they have line-of-sight to the orbiter, which is obvious but I mention it as something to take special care with because both the probe and orbiter will be moving around quite fast at relatively close range, making the timing of your communications windows different to having a line straight to Kerbin. For example, atmospheric entry to certain places (eg. Tekto in OPM, Titan and Venus in RSS) can take a long time and the orbiter can pass over the horizon long before you land, potentially making you unable to prime chutes or landing gear if you didn't plan ahead.

Edited November 27, 2021 by Rocket Witch

[eddiew]

17 hours ago, Rocket Witch said:

The probe(s) you detach will only be fully controllable while they have line-of-sight to the orbiter, which is obvious but I mention it as something to take special care with because both the probe and orbiter will be moving around quite fast at relatively close range, making the timing of your communications windows different to having a line straight to Kerbin.

This is one of the biggest gotchas. My personal strat is to drop the atmo probe off in low (polar) orbit, then boost up the AP of the relay on the Kerbin-facing side of the planet. This way you just wait for a time when both vessels are coming from round the back of the planet and start the descent. Should have plenty of time with the comms relay accessible.

Bit harder for Jool due to it's scale and the fuel cost of changing your orbit. You may wish to establish the relay in high orbit as a separate vessel entirely.

[MAFman]

4 hours ago, eddiew said:

This is one of the biggest gotchas. My personal strat is to drop the atmo probe off in low (polar) orbit, then boost up the AP of the relay on the Kerbin-facing side of the planet. This way you just wait for a time when both vessels are coming from round the back of the planet and start the descent. Should have plenty of time with the comms relay accessible.

Bit harder for Jool due to it's scale and the fuel cost of changing your orbit. You may wish to establish the relay in high orbit as a separate vessel entirely.

IIRC, didn't Galileo release the probe before it even entered Jupiter orbit?

[eddiew]

1 hour ago, MAFman said:

IIRC, didn't Galileo release the probe before it even entered Jupiter orbit?

If you want to try doing that, feel free   My gut feeling is that you'd have to be almost skimming the atmosphere when you drop the probe, and have it decelerate really hard so's to be in the clouds very quickly.

You'd also likely end up doing this either at the daylight terminator, or into the night side, because you're coming from close to the sun and would reach Joolian periapsis around the back of it. This means you have no control, because Kerbin has got to be on the sunward side.

Although I guess the Galileo probe had no concept of remote control, so it might well have just dumped all it's data to the orbiter for later transmission.