Timing Atmospheric probe and orbiter for radio coverage.

[GarrisonChisholm]

Good day one and all.  I am embarking upon an exploration of the gas giants (out planets mod) and my plan calls for a probe stage in three parts.  A gas giant entry probe, to be detached after entering SOI, an orbital insertion stage that when decoupled from the orbiter itself will serve as a potential relay, and then the orbiter itself.

My issue is, as I start to think about it, how to ensure that the entry probe stays in communication with the orbiter/insertion-stage as it enters the planet's atmosphere.  Logically the probe would seem to need to slow down after separation, or else the entry probe might circle around the planet and depart line of sight and lose data transmission.

Does anyone have a "method" to ensure this works out, or do I just have to play with varying angles and try to make it work?

[Zhetaan]

I can think of two ways:

1. Timing.  Follow the entry probe with the insertion-orbiter in a nearly identical track (or it can be exactly identical if you're using aerobraking or aerocapture).  This would probably be best done by use of a radial burn after the orbiter burns for a near-but-not-quite-atmospheric interface.  There will be drift but you should be able to keep line of sight.  However, you may find that you must release the entry probe at a point well after entry to the sphere of influence.
2. Sequence of stage separation.  You may have a better time if you decide to capture to a high elliptical orbit, release the entry probe, and then complete your orbit insertion.  If you release the insertion stage and make any other manoeuvre, then your orbital tracks will give better-than-half radio coverage for when your entry probe returns.  This will also depend on timing and you'll lose some delta-V from keeping the probe attached, but if need be, you can take fuel from the probe--unless you mean to say that the probe is purely kinetic?

[GarrisonChisholm]

The probe has some Dv, yes, though only enough to change course.  Maybe I'll sketch up a diagram and get some screen-shots when I'm home later, but the Orbiter was going to use Dv insertion and so not touch the atmosphere, but the entry probe would be on a closer orbit to intersect of course, so it would be out-pacing the orbiter until the atmosphere started to slow it down.  I can't put a big transmitter on the probe because entry would tear then off, so I need to stay within 1,5m M and LoS to make sure i get the data.

[Zhetaan]

This fits a sort of combination of the above, but you could perhaps try setting up a resonant orbit to ensure that both vessels are on the same side of the planet at atmospheric entry.  This would also allow you the flexibility to build a sort of delay-spacing into the flight plan so that the probe doesn't fall behind the orbiter so much as it falls back to the orbiter.  This would require keeping everything together for insertion and capture:  you'll still want to have a low, almost atmosphere-grazing periapsis, but also to keep a high apoapsis for ease of setting up the atmospheric entry later.  Release the probe (if you used decouplers or separators and didn't turn down the decoupler force, you'll have to chase it to keep your velocities matched--consider releasing the probe in a retrograde direction) and at the next periapsis, burn retrograde with the orbiter so that the orbital periods are some simple rational resonance.  7:8 requires less delta-V, but 3:4 requires less time.  It's up to you.

Let's assume that you decide to use 7:8.  This means that the orbital period of the orbiter is one-eighth less than the orbital period of the probe, which is left in the larger orbit.  In other words, for every eight orbits of the (inner, thus faster) orbiter, you get seven orbits of the probe to put them both back in the same place (at periapsis).  Control the probe and wait for six and one-half orbits:  this will put you at the apoapsis, where your tiny amount of delta-V will get the most effect.  Assuming that you kept your periapsis low, it won't take much to drop it into the atmosphere, and the total change to orbital period is so small that the orbiter will be literally right above the probe when it interfaces with the atmosphere.  Depending on how quickly the probe decelerates, that may mean that your orbiter will move too quickly anyway, thus pulling ahead and leaving communication range too early.  In that case you will need to perform the deceleration burn on the five and one-half mark, which will put the orbiter back by an eighth-orbit, which will give you more time to transmit when you get to that point.

Of course, if you want finer control over the timing, then you need to span it to take more time so that you can more easily divide the orbit into finer fractions.  That would imply resonances of 11:12 or 14:15 or whatever you like.  You may need to become very familiar with quicksave.

Edited January 31, 2019 by Zhetaan

[GarrisonChisholm]

That's very insightful, I appreciate the input.  I think I'll have to "simulate" it and then make my best effort.

This is the probe, so as designed the Entry Probe needs to detach before the Insertion Stage can fire to break the Orbiter into orbit.

I'll see how it goes!

[TheFlyingKerman]

In my experience with Jool, the entry probe stops soon after entering the atmosphere, so you just need to make sure you entry point is facing home and the relay has LOS and connection during that period of time.

[MrSystems]

There's a formula for orbital period you can use:
T = 2 pi SQRT( a^3 / mu)
a = (mu (T/2 pi)²)^(1/3)
T is in seconds, a is the semi-major axis in meters ((apoapsis + periapsis)/2 + body_radius), and mu is the gravitational parameter for the body. For modded systems you can get mu and radius from the .cfg files (OPM's are kept in the KopernicusConfigs subfolder & reproduced in the table below).

Then devise two orbits for your probe: a lower orbit that doesn't enter the atmosphere, and an entry orbit. You'll let the probe go around the lower orbit two full revolutions before doing the entry burn. Time it so that the orbiting relay goes around the body 3/2 or 5/2  or 7/2 times in the amount of time it takes the probe to go around twice or thrice at the lower orbit plus 1/2 a time on the entry.

Example with Sarnus:

Relay orbiting at apoapsis 20Mm, periapsis 1Mm; orbital period = 43,546s (2d 0hr 5min)
3/2 orbits = 65,319s, 5/2 orbits = 108,865s, 7/2 orbits = 152,411s
Planned entry orbit at apoapsis 1Mm periapsis 200km; orbital period = 9,937s (2hr 45min); 1/2 period = 4,968s
Remaining time = 60,350, 103,896, or 147,442s.
Intermediate orbit needs to have a period of one of those numbers divided by a whole number. 60,350/2=30,175s (1d 2hr 22min) is a good choice.
Calculate the semi-major axis of an orbit with that period:12,372,562m
Keeping one apsis at 1Mm, calculate the opposite apsis for an orbit with SMA : 2*(SMA - body_radius) - 1,000,000m = 13,145,123m

So you would detach your probe at periapsis, burn with your probe to an orbit at periapsis 1Mm apoapsis 13Mm, then let the probe go around one revolution before performing the entry burn.  Vis-viva gives the delta-v for the two burns of 160.4m/s for the first one and 415.3m/s for the second.

Mu and radii for OPM gas giants

PLANET	MU (m3/s2)	RADIUS (m)
Sarnus	8.2117744E+13	5,300,000
Urlum	1.1948654E+13	2,177,000
Neidon	1.4172721E+13	2,145,000