Operation Bullfinch

[Hotel26]

Whereas, I prefer to use NERV-propulsion for most orbital and deep space operations, the length of LKO maneuvers to inject a transfer even to Mun and Minmus can be rather long.

So I built a Thud-based tug, Bull (pictured left), as a mundane tug for orbital operations but with an auxiliary function to provide that quicker transfer injection to lighter payloads.

       

The mainstay of my tug fleet is Finch (pictured right).  It usually launches from the pad as a second stage and, though it is fully recoverable, often continues on into extended service in orbit/space.

At the end of the quick project to build Bull, I discovered a fun, little game to use Bull to launch a payload to the Mun and then separate to coast back to aerobrake before returning to a home station to refuel, ready for next use.

Meanwhile, a Finch, waiting in orbit at the Mun (or potentially Minmus), is able to plot an intercept with the separated payload and capture it for delivery to LMO.  It is surprisingly fun to do...

Please see also: Hitchhiking to Moho.

More details follow in the next post...

Edited April 30, 2023 by Hotel26

[Hotel26]

So here is a worked example:  [click + arrows => slide show]

          

             

 

1. A Bull has picked up a Hitchhiker payload containing 4 tourists.  It will perform a transfer injection to the Mun.  That will give it a Mun encounter plus a Mun periapsis.
2. Switching to the Finch waiting in low Mun orbit, we make the Bull a target and plot an intercept transfer, which gives us the Bull periapsis at the Mun (686,216m), it's arrival time (1d 1h 30m), our time to reach the maneuver point (17m 20s).  [Slide shown is several minutes stale, so numbers below were taken slightly earlier.]  We also note our orbital period (per KER (LHS HUD): 42m 31.7s).  Now run the arithmetic:
   >meet 1d1h33m 1h42m23s 19m31 42m32
   Orbit for 0d5h17m15.000s, then extend final orbit to 0d0h52m53.000ss; then intercept
   >
3. After loitering in orbit for 5h 17m (N - 1 orbits), we approach the final orbit but execute a short burn to extend its period to 52m 53s.  This will kill the remainder of the wait time.
4. After that final orbit, execute the intercept transfer and we find an expected 298m rendez-vous 1h 20m 13s hence.
5. After the Target Retro burn, actual distance is 302m
6. Finch is about to take the baton (Hitchhiker) from Bull.  All are still moving on an escape trajectory for a coast back to Bull's starting point.

After the payload transfer, an alarm is set on Bull's Mun escape.  Finch burns retro to lower its orbit back to Mun space station altitude.  After Bull escape, it performs a short retro to lower its altitude to 50km, inside Kerbin's atmosphere.

NOTE BENE: for Bull re-entry, I like to adopt an ORB Normal attitude and then spin up the craft to about 2 revolutions per second.  This gives it gyroscopic stability in a broadside attitude that incurs some heavy drag, while somewhat distributing the heat.  Three aero-braking sessions is about as much as I can stand and I burn fuel on the last to begin the rendez-vous with an LKO fuel station.

Edited April 30, 2023 by Hotel26

[Hotel26]

Referring to (the slightly stale) slide 2 in the previous, we observe that the payload will arrive in 1d 1h 33m (27180s) but we are ready to get there in 1h 42m 23s (6143s), which is way too early.  We need to kill 27180 - 6143 seconds (21037s total) in waiting orbit.  Our orbital period is 42m32s (2552s), so the wait time is 21037 / 2552: 8.24 orbits (Norb).

Therefore, our plan is going to be to spin for 7 orbits (Norb - 1).  Then extend the period of the final (8th) orbit by 0.24 orbits.  The easiest and most practical way to do this (after setting an alarm to take us into the penultimate orbit) is to replot the intercept transfer and  compute the arrival discrepancy between payload and interceptor.  In this case it will be 21037 - 8 * 2552: 621 seconds.  We extend our last orbit to take 2552 + 621: 3172 seconds.

As soon as we have made that burn and are executing our last orbit, we can now replot the intercept transfer.  A good tip is to make the interceptor apoapsis altitude identical to the payload periapsis altitude.

In pseudo-code:

given Tpay, Tcat, Tman and Torb:

Spoiler

  Tidle = Tpay - Tcat
  Norb = int(Tidle / float(Torb))
  Tlast = Tidle - (Norb - 1) * Torb

  print(
    "Orbit for {0}, then extend final orbit to {1}s; then intercept"
    .format(KSP_time.fromSeconds((Norb - 1) * Torb + Tman), KSP_time.fromSeconds(Tlast)))

I think that, with the outline above, it is simple enough to do with a calculator (and no program is required):

1. set an alarm for N - 1 orbits.  (Go away and do other things.)
2. Plot the intercept just before the last orbit and extend its period to match the difference between payload and interceptor arrivals.
3. Finally, execute the intercept as one would usually do.

 

And a note to the cognoscenti who may read this:

Spoiler

All of the above (pertaining to Operation Bullfinch) may/will seem painfully obvious and trivial but I invite your patience and certainly also welcome insights, advice, improvements, objections, error reports.

In my experience, precisely half of KSP rocketeers know much more than I do but, most curiously, almost never publish any details of what I -- and I expect the other half of people, who neither have discovered all of the insights with which KSP endlessly entertains -- do not know and have yet to discover.

Thus it is for myself and that latter half that I find amusement in occasionally documenting some of the joy that KSP brings.

All in the name of: "It is surprisingly fun to do... ".

 

Edited April 30, 2023 by Hotel26