Laythe Spaceplane

[Jebediah Kerman Jr.]

I'm in orbit of Laythe with 6 Kerbals!

My craft is a spaceplane with a nuclear tug. The tug will insert the spaceplane into a suborbital trajectory and then detach.

The spaceplane is just a Minmus SSTO. I used the tug to bring it to Laythe.

It has quite a bit of fuel left over when returning from Minmus, enough to render any aerobrakes unnecessary. The question is, will it be able to get from Laythe's surface to Kerbin?

I don't want to do any unnecessary gravity assists, either.

Can anyone help me with the DV? I am too incompetent to work it out myself. 

[Pecan]

You have a spaceplane for reaching orbit from the surface of a moon without an atmosphere? ... Riiiight.  180m/s dV for SSTO from Minmus then.

Laythe launch and Kerbin transfer requires approx 4,360m/s, with capture burn.  Looks like you're a long way short.

[Jebediah Kerman Jr.]

Oh no, the SSTO was originally designed to get from surface of Kerbin, to Minmus, and back. I didn't mean SSTO from Minmus surface to orbit, I meant all the way from Kerbin to Minmus. Sorry for the confusion.

Wait, did you say around 4400 m/s from surface of Laythe all the way to Kerbin?

[bewing]

Yeah, if you have enough to get from KSC to Minmus, then you have enough to get from Laythe back to KSC if your craft is fully fueled. 5km/s of dV in round numbers each way, assuming a decent transfer window at Laythe.

 

[Pecan]

9 hours ago, Jebediah Kerman Jr. said:

Oh no, the SSTO was originally designed to get from surface of Kerbin, to Minmus, and back. I didn't mean SSTO from Minmus surface to orbit, I meant all the way from Kerbin to Minmus. Sorry for the confusion.

Wait, did you say around 4400 m/s from surface of Laythe all the way to Kerbin?

Oops, my figure is from orbit and uses old figures.  Using the standard dV map ...

... just add all the numbers from where you are (Laythe surface) to Kerbin orbit - plus a couple of 100m/s depending on what you want/need for landing.

Laythe is top-right, in green.  It's 2,900 m/s to get to orbit there, plus another 1,070 + 930 m/s to escape Laythe to Jool's greater SOI.  Another 160 + 980 gets you into sun-orbit, with a final 950 capturing at Kerbin.  2900 + 1070 + 930 + 160 + 980 + 950 = 6,990m/s from Laythe surface to Kerbin orbit.  That's complicated, however, by the fact that you'll be using jets for the launch-to-orbit at Laythe, so that part at least won't use oxidiser.  Simpler to compare to vacuum-Minmus if you assume orbit-to-orbit instead and forget the launch (so 6,990 - 2900 = 4,090m/s).

Now, to find out what your vehicle is capable of we need the similar orbit-to-orbit figures for Kerbin/Minmus (bottom-right of the map) which is 930 + 160 = 1,090m/s + 180m/s if you're landing; 1,270m/s.  Since you can do a return trip simply double that, to show you have between 2,180m/s - 2,540m/s.  That's without plane-change (another possible 2 x 340m/s <= 3,220m/s) and "quite a bit of fuel left over when returning from Minmus" which, for the sake of generous round numbers I'll take to mean an upper-end of around 4,000m/s.

From that it's easy to see that the plane should easily do the launch-to-orbit, LF-only part (2,900m/s required for Laythe, 3,400m/s capable on Kerbin) but is going to be very tight on the space-transfer using LF+O: 4,090m/s required Laythe-orbit to Kerbin-orbit, 3,220m/s and "quite a bit of fuel left over when returning from Minmus" demonstrated.  If you've got ~800m/s there you should be good.

It all comes down to that last, left-over fuel whether you can do it comfortably.  Since you can aerobrake at Kerbin though chances are you can squeek through, even if your residual fuel isn't quite enough.

Edited December 3, 2018 by Pecan

[Jebediah Kerman Jr.]

1 hour ago, Pecan said:

Now, to find out what your vehicle is capable of we need the similar orbit-to-orbit figures for Kerbin/Minmus (bottom-right of the map) which is 930 + 160 = 1,090m/s + 180m/s if you're landing; 1,270m/s.  Since you can do a return trip simply double that, to show you have between 2,180m/s - 2,540m/s.  That's without plane-change (another possible 2 x 340m/s <= 3,220m/s) and "quite a bit of fuel left over when returning from Minmus" which, for the sake of generous round numbers I'll take to mean an upper-end of around 4,000m/s

So, 1270 m/s to Minmus surface, 1270 m/s back.

But wait, it doesn't take 1270 m/s to get back from Minmus, the 1270 figure includes a deceleration burn at Kerbin periapsis.

I am not the most efficient player of the game, so I'd say when I use the Minmus SSTO, I spend ~300 excess DV.

That gives me ~3500 m/s, plus the ~3500 m/s I use to get to Kerbin orbit.

My spaceplane/SSTO has 4 Rapiers and 1 Nerv.

I probably won't need much oxidiser to get to Laythe orbit. So ~3000 m/s needed to get to Laythe orbit, I probably have about 3150 m/s left in orbit of Laythe, taking into account the higher ISP of the Rapier on jet mode as compared to the Nerv.

What am I going to do? I do have some "emergency procedures"

1) Do a chain of gravity assists off of Jool's moons, but I am terrible at gravity assists.

2) Rendezvous with the tug in orbit, line up the Nerv and 1.25 meter decoupler, switch to the other vessel (the tug), and puuuuusssshhh the spaceplane out of Laythe's SOI to gain some extra D/V.

3) Don't land on Laythe at all. I could do the alternate mission instead, a Bop landing, but I could go to Bop any old time. Laythe is the favored destination. This is a worst-case scenario, I do not want to abort the mission.

Edited December 3, 2018 by Jebediah Kerman Jr.

[Plusck]

1 hour ago, Jebediah Kerman Jr. said:

What am I going to do? (...)

1) Do a chain of gravity assists off of Jool's moons, but I am terrible at gravity assists.

(...)

Definitely this one. But first:

I find reaching orbit in a plane on Laythe far, far easier than on Kerbin, much easier than the dV maps seem to suggest. The annoying thing on Laythe is how much you need to spend to get free of its SOI afterwards.

However, that annoyingly high escape velocity also implies a high Oberth benefit from your low Laythe orbit burn. As with Kerbin, burning hard out from low Laythe orbit will reap great dividends for escaping the Jool system. I can't remember the exact figures but it is much less than the dV map suggests. Land and re-orbit as efficiently as possible, and you might well still make it home without any gravity assist.

Next: gravity assists:

You only need one, which is Tylo. Passing close by Tylo (tangentially, passing in front) on entering the Jool system can get you (for free!) into orbit around Jool with Pe near to Laythe and Ap a bit higher than Tylo.
There is therefore no reason not to do the reverse: leave Laythe so that you catch Tylo when it is approaching the point where it would eclipse the sun, pass (very close) behind it, and you'll be thrown out of the Jool system retrograde. For minimal expenditure (perhaps a small burn in low (≈100km) Tylo orbit), you are guaranteed a Kerbin intercept.
Travel time from Jool to Kerbin is so long that you can leave at any time; minimal corrections to your path can easily change your Kerbin intercept time by months. The only possible concern is if you have poor heatshielding, since minor corrections can make a significant difference to the speed at which you hit the atmosphere on arrival.

[Pecan]

6 hours ago, Jebediah Kerman Jr. said:

My spaceplane/SSTO has 4 Rapiers and 1 Nerv.

You have a nuke engine?  From using a separate tug I assumed you hadn't.

In that case disable the rapiers once you reach orbit and you'll effectively double your dV because of how much more efficient the nuke is.  Plus, you can benefit from all that LF-only you have on Kerbin but won't need at Laythe, so that's at least another 500m/s :-)
Downside is long burn times but at least you get home.

Anyway, you seem to have the dV & map idea now so good luck, hope my figures at least helped you think about what's possible.

Edited December 3, 2018 by Pecan

[Jebediah Kerman Jr.]

Update: I had plenty of DV for my return to Kerbin, but I still had to abort the mission. I tried to land the spaceplane about 20 times. Every time I tried to land it, it exploded. It could not get under 50 m/s as it hit the ground. I am so furious right now.

Next time I'll use bigger wheels.

[bewing]

1 hour ago, Jebediah Kerman Jr. said:

It could not get under 50 m/s as it hit the ground.

Try landing it in the water, offshore. The water is flatter, and has a higher speed rating. Bring the thing down to 5 meters altitude, and try to fly exactly horizontal, and wait for it to splash down by itself.

 

[Jebediah Kerman Jr.]

I can't maintain level flight and keep it going at less than 70 m/s. It probably needs bigger wings.

[Guest]

12 hours ago, Jebediah Kerman Jr. said:

I can't maintain level flight and keep it going at less than 70 m/s. It probably needs bigger wings.

Possibly, or it could be that your CoM is too far forward which causes the nose to droop when you slow down, which then flies you splat into the ground. The solution to this is more control authority or moving the wings forward a bit. 

Planes generally fly better with wings though, and they don't weigh all that much, so if in doubt, yeah add more wings.

[Pecan]

Odd.  If it works on Kerbin, it should work on Laythe.
*Shrug*

[Jebediah Kerman Jr.]

My second Laythe landing attempt is underway. Larger wings, bigger landing gear, and CoM/CoL issues fixed. Crew:

Commander: Malin Kerman

"Pilot" (copilot): Linlee Kerman

Senior scientist: Burrey Kerman

Junior Scientist: Ribfrod Kerman

Head Engineer: Nataliana Kerman

Junior Engineer: ? (I forgot his name)

[Jebediah Kerman Jr.]

Oh yeah, I think it was a CoL issue. In my first attempt, the plane kept tumbling out of control in the upper atmosphere.