Minimal Mun/return for kids

[Hotel26]

My son has started to show interest in KSP...

So I thought I'd put together a mission to the Mun and return that embodies rendez-vous and docking.  It's been a challenge to dust off some cobwebs!

My goals are: 1 Kerbal.  Plant a flag.  Anywhere on the Mun.  Return.  Splash down within 100km of KSC.  Or land on the Green...    Necessitate a rendez-vous and docking.  This is really important because this is a pedagogical mission!  It can't be too close on the budget because this is for kids.

So I have a working prototype and of course I'm wondering how much the scalpel could take off this mission now.  It feels like it's pretty fat...

Here's a shot of my first cut at this:

which is:

junior dock
z200 battery
flr10 mp
mk1 pod communotron 16S
t400 with 3 mk2r chutes and 3 rv105 rcs and 3 lt1landing struts
lv090 terrier
junior dock
fla10 adapter
2 T800 tanks
dart engine
tr18A stack decoupler
t800
rocko adapter
jumbo64 with 3 gantries and 3 r8 winglets
mainsail engine

 

 

Edited August 1, 2018 by Hotel26

[mk1980]

i like the idea in general, but in my opinion it would make more sense to (for example) launch 2 separate vehicles - one bringing the lander can to mun orbit, the other bringing the crewed capsule that will later return.

then you'd have an actual reason to do a rendezvous & docking to get the kerbal into the lander (and a reason to get him back into his "mothership" capsule afterwards)

when i was a kid, i was curious and didn't shy away from asking questions. i think i would have asked something like "ok dad, i get it, but why don't we just land on the mun and return from there? what's the point of the docking afterwards?"

if you split it into 2 separate vessels, you can trim the lander down (only needs the lightweight lander can), probably also only the tiny starter landing struts with a spark engine on the bottom and a fuel tank on top of the lander can (and docking port on top of it?)

and you can add a heatshield to the capsule and point out that the capsule + heatshield + parachutes is maybe twice the weight of the lander can, so it would need much more fuel and maybe also a bigger engine - which would be heavier and require even more fuel and requires longer landing struts to not hit the ground - which are also heavier etc. etc.

 

just a few thoughts. hope your son likes the game and likes your lessons :)

[TheFlyingKerman]

It would make more sense if the munar lander does not re-enter the atmosphere. This design is closer than a direct ascend one.

As for cutting things down:

I'd discard the Terrier before entering the atmosphere. Then one parachute is enough.

There is no need for the mono-propellant tank because the pod carries some.

In general the engines are a bit overkill. Maybe spark for the lander, Terrier for the upper/transfer stage and Skipper + SRB for the boaster stage?

[vyznev]

The way I see it, "can't be too close on the delta-v budget" and "requires Mun orbit rendez-vous and docking" are kind of incompatible in stock KSP. The problem is that returning from low Mun orbit to Kerbin takes only about 300 m/s of delta-v, give or take a few dozen, so pretty much any Mun ascent vehicle with more spare delta-v than that can just return to Kerbin directly. (Of course, the vehicle also needs to survive re-entry into Kerbin's atmosphere, but the default re-entry heating settings are pretty forgiving, too.)

That said, I basically see two ways to go about doing this. One would be to try to recreate the real-life Apollo mission constraints as faithfully as possible:

• Must send multiple kerbals to the Mun, and leave at least one in orbit while the others land.
• No probe cores (at least, nothing more advanced than a Stayputnik).
• No EVAs except when landed, and no command seats. (Career mode will actually enforce this, if you haven't unlocked the necessary tech and building upgrades.)
• Optional: all kerballed vehicles occupied for more than, say, 15 minutes must carry some dead weight (e.g. ore tanks) to simulate life support equipment. The amount to be carried should be roughly proportional to the maximum kerbal-hours spent in the vehicle. (Or install an actual life-support mod, although most of them also tend to be pretty generous with the survival time / mass ratio.)
• Require an actual command pod for Kerbin re-entry. (No slapping a heat shield on a lander can or just riding a command seat down!)

Hopefully, all these constraints should actually make leaving a proper manned return vehicle in Mun orbit and docking with it both necessary and practical.

The other option would be the Kerbal approach: just cut down all safety margins to the bare minimum and, in particular, make the Mun lander so flimsy and lightweight that it actually cannot return to Kerbin on its own. Obviously, that's the approach I chose to go with. Here's my first (successful) try, the Krapollo 14:

It carries a total of 9,200 kg of fuel and oxidizer at launch, which actually turned out to be way overkill; I'd read the delta-v chart wrong and accidentally allocated about 1 km/s more delta-v for the Kerbin-Mun-Kerbin transfer than I actually needed. Oops.  Still, it's at least a decent proof of concept.

Spoiler

The monopropellant, which I forgot to remove from the lander can, is not used. In fact, the lander can never leaves the launch pad; before launch, you'll need to EVA the pilot and transfer them into the command seat inside the fairing, conveniently located right next to the lander can hatch. (Tip: if you want to make use of prograde hold, also make sure to select the lower docking port and click "control from here" before takeoff to get the navball oriented correctly.)

With careful flying and a fairly shallow ascent profile, the bottom stage (with two FL-T800 tanks and a swivel engine) can bring the upper stage almost to LKO, with only a few hundred m/s of delta-v from the transfer stage needed to finish the job. Actually, with almost twice as much fuel on the transfer stage as needed, it really doesn't matter how you fly the ascent. Just launch straight up if you want, it should be fine. 

Once in orbit, just do a standard Mun transfer:

After reaching low Mun orbit, decouple the lander stage and make sure to enable the fuel and oxidizer in the small Oscar-B tank. (They're disabled at launch to make extra sure that the transfer stage won't drain them.)

The lander stage has just barely enough delta-v to make it from about a 10 km Mun orbit down to the surface and back again. A suicide burn is not required, but there's not much margin for needless hovering on the way down, so try to be fairly quick about it. KER's "time to impact" indicator is quite useful here.

If you land on a slope, make sure the lander won't roll away when you leave the seat. The reaction wheel has plenty of control authority, so you can just tilt the lander onto a stable angle before disembarking.

With the high TWR and the overpowered magic reaction wheel, takeoff is best accomplished just by pointing the craft about 20 to 30 degrees above horizontal (depending on local terrain) and briefly toggling the throttle on and off.

Once off the ground, turn all the way horizontal and burn until your apoapsis (and hopefully your inclination, too) matches the orbit where you left the extra fuel tank and heat shield. (Note that there's no probe core on the tank, so the game considers it debris. If you can't find it on the map, check that you don't have debris hidden.)

The rendezvous is again pretty routine, although the lack of RCS (or even just any kind of probe control on the return tank) makes the actual docking a bit tedious. An added complication is that the docking ports are off-center, and you do want to line them up more or less correctly for re-entry. Adding a probe core, a reaction wheel and maybe even some RCS to the return stage might not be a bad way to use up its ample delta-v margin. Still, it can be done without.

Also note that, when the docking ports do engage, there's a fair chance that the pilot will get hit on the head by the fuel tank and knocked off the seat. If that happens, just fly back to the craft and board again. 

The Kerbin return burn is again routine, and should take a bit under 300 m/s of delta-v, resulting in an orbit like this:

If you wanted, you could burn off any spare fuel you had in a retrograde-radial burn just outside Kerbin's atmosphere to try and lower your apoapsis and thus your re-entry speed. However, this is not really necessary. What is necessary is maintaining the right attitude during re-entry so that the pilot stays safely behind the heat shield. Unfortunately, with this design, simply locking the craft to retrograde (or prograde, depending on which docking port you're controlling from) isn't enough; you need to actually tilt the craft a few degrees for proper heat shield coverage.

Once the craft is safely subsonic, jump off and deploy your parachute. (Did I mention you need KSP 1.4+ for this? Because you kinda do. In KSP 1.3, with no EVA parachutes, this re-entry technique is hazardous at best. )

 

Here's the craft file, if you want to give it a try yourself. Or you may prefer this improved (but untested) version with a probe core and RCS on the return stage. It's probably not the most kid-friendly craft to fly, but hey, it's pretty cool and kind of a fun challenge. 

Edited August 1, 2018 by vyznev

[Hotel26]

Well, what expert (and prophetic) advice you've all given.  Thanks also for the baseline.  I've had some fun trying to implement it.

Including on the last run, with just a Spark and a T100, I was a bit wasteful on fuel picking the landing site (avoiding a crater), so it didn't quite make it back into orbit.  Cathat Kerman had to bale out and use her jet pack to finish the job.  I didn't bother to try to finish the r/v with the jetpack, but never underestimate the dV of a very determined Kerbonaut!

I think I like the idea of a Mk1 lander can with a Mk2 can remaining in lunar orbit and returning for reentry.  That checks all the boxes and matches a suggestion above about Apollo?

I'll report back here when I'm done and also how well my 12 year-old enjoys it.