Second stage burns up after RTLS. Solution?

[Motokid600]

I am so close to a perfect RTLS capable rocket. Infact Ive nailed it with my first stage. The second is the issue. By time I get back around to the upper stage its already falling back down into the atmosphere. I'm using MechJeb and BDArmory with an edited 750km load range. This way I don't have to do it the "stock" way which is a ridiculously steep trajectory which puts the first stage at like 100km to give you the time to manually boost the second stage to orbit.

No with that 750km load range and MJ I am able to model real life more appropriately with a much more favorable launch trajectory and a 50km staging. And it works. My god it works great. After the boostback MJ lands the first stage square on the runway. But MechJeb is having trouble with the second stage. I don't know whats happening. Either MJ is shutting down the engine not long after I switch away ( despite the massive load range ) or its hitting the target Ap of 150km ( which sucks I wish I could have this much lower ). I need that high Ap to give me the time to land the booster and switch back. But I also need that high Ap so that MJ with corrective steering can fly to orbit in one burn. Or ateast get close. 750km should be plenty enough. Theres no way I'm hitting that mark and the vessels unloading. But something is happening where MJ cannot fly the upper stage down range far enough no matter what target Ap I set. I return to the craft, the engines immediately throttle up where I left them, but I'm at 40km and falling.

Its ashame because were it not for this one quirk... this setup is go. Three times in a row this booster lands perfectly, but the upper stage is holding it back. So what can I do? I understand this is a unique approach here, but I'm just curious if by chance anyone has any ideas of what I can do. I'm so close to just moving into RSS and doing this there. Because theres no way MJ will hit my target Ap before I land and should just keep motoring on, but.. I don't know.

Bear with me here...I cant just chuck the second stage to some super high Ap. I need this to be efficient. The upper stage needs 1700 Dv left in orbit for a Duna transfer and injection. So in order for the upper stage to not only boost 75% of the way to orbit and reach low Duna orbit it needs something like 3,4 - 3,700 Dv. A large upper stage for a 3.75m rocket that had to fly back home. Its easy to make, but that makes RTLS a nightmare. Because the lower stage needs 1,250 to return. So the heavier the upper stage is the more it cuts into the Dv I can put out with the first stage. Which in turn cuts into the Dv the upper stage needs to fly to orbit. So to help this I can fly a steeper trajectory ( which also helps with ultimate issue here ) But that then cuts into the Dv needed to rtls.

What does this have to do with the aforementioned issue? Funds is a factor and I cant just throw an overpowered second stage on an overpowered lower stage to 300km.

How the heck does SpaceX do it?! At this point its just like holy crap I can take off the landing legs, the thrusters, the rcs, the air brakes and put 30 tons to LDO.

Edited January 4, 2016 by Motokid600

[Snark]

Don't have an answer for you because this sounds like primarily a MechJeb issue and I never use MechJeb.  One suggestion, though-- when you use a jargon term that's not in common KSP parlance, like RTLS, it's not a bad idea to hyperlink it or something so that people know what you're talking about.

For example, if googling it doesn't turn up an obvious answer for what you're talking about, you're likely to confuse people. 

 

[Temstar]

Do you have to do it in two stage reusable SpaceX style or would you accept SSTO reusable? Delta-V requirement to LKO is so low compare to LEO and the sort of issue you are running into are so annoy that in KSP it's actually easier to just make your launch vehicle SSTO and recover that rather than splitting it up into stages.

[Motokid600]

1 hour ago, Snark said:

Don't have an answer for you because this sounds like primarily a MechJeb issue and I never use MechJeb.  One suggestion, though-- when you use a jargon term that's not in common KSP parlance, like RTLS, it's not a bad idea to hyperlink it or something so that people know what you're talking about.

For example, if googling it doesn't turn up an obvious answer for what you're talking about, you're likely to confuse people. 

 

Sorry, I didn't figure I had too.

1 hour ago, Temstar said:

Do you have to do it in two stage reusable SpaceX style or would you accept SSTO reusable? Delta-V requirement to LKO is so low compare to LEO and the sort of issue you are running into are so annoy that in KSP it's actually easier to just make your launch vehicle SSTO and recover that rather than splitting it up into stages.

There no way I could get a giant rocket stage to survive reentry and planes have no where near the payload capacity. Cost is also and factor and well.. Space X is all the rage. Its really ashame.. this works really good if not for the distance restrictions the game puts upon us. DBArmory is a way to circumvent that, but for some reason I suspect MechJen is shutting down not long after I switch crafts. And now I'm wondering is I could use a KoS script...

[Temstar]

Just now, Motokid600 said:

There no way I could get a giant rocket stage to survive reentry and planes have no where near the payload capacity.

You just have to combine the best features of both:



I like to flog the fully reusable rocket idea because I want to see other people's take on it.

[Motokid600]

Its not quite the solution I'm looking for... However not to say I wouldn't want to try that later down the line. Do they fly with FAR?

[Temstar]

2 minutes ago, Motokid600 said:

Its not quite the solution I'm looking for... However not to say I wouldn't want to try that later down the line. Do they fly with FAR?

I don't use FAR so I don't know, but I can't imagine why one couldn't design something similar with FAR since Energia II was suppose to use something like this concept.

Besides wings, people have also designed rockets that can lift 600 tons to orbit in one stage and still return in one piece to Kerbin with ballistic reentry.

Edited January 5, 2016 by Temstar