Trying to get this to the mun

[Ketatrypt]

been working a while trying to revise this design to get to the mun/minmus with a 7.5t cargo.

And I am really close..

 

Currently, with that 7.5t of cargo, I can get to LKO with about 1200 dV remaining. which, is close, but I would be more comfortable with 1800 dV.

And there is 2 issues I see right away which I am not sure of.

With the whiplashes, I can only seem to get up to ~1100 m/s easily. (about mach 3.6) this happens at around 18km, I level off a bit from 15km to pick up speed, and then start climbing again, and turn on the rocket engine at about ~20km, which leads to the next issue - getting up thru the atmosphere quickly. To keep max speed, when I switch on the rocket engine, I am only climbing at about 120m/s, which I feel is really slow, and I am not really sure how to go about the problem. I could gain speed lower in the atmosphere, pull up steeper, which would get me thru the atmosphere quicker, but, with low TWR I would need to ignite the rocket engine at a lower altitude to keep a high speed. So I am in a pickle, not sure where to get this last 400m/s dV... 

 

Any ideas?

Edited February 15, 2016 by Ketatrypt

[Temstar]

Why do you want to spaceplane to the Mun?

[Ketatrypt]

5 minutes ago, Temstar said:

Why do you want to spaceplane to the Mun?

 

To deliver/retrieve cargo.. I could use a space tug but I am already so close with this, I figure why not. mostly just a lesson on efficiency, as I am hoping that I can learn to make my ascent more efficient. 

[Evanitis]

My impression is that you pitch up too quickly and too much once you need to switch to rocket mode. ~1100 m/s on 18km sounds all right. It could be higher, but I often can't reach any higher on Whiplashes. It might just originates from the plane's TWR. Rapiers feel better in this regard, though I don't think it will be needed for your craft.

The ascent profile stays important in rocket mode too. If you have a high TWR at that point, it's good to pitch up to escape the remaining atmo, but it doesn't worth it if your TWR is lower than ~1. In this case, the speed you lose by raising your AoA thus accelerating slower and presenting that huge wide profile to the airflow is a lot more than you save by leaving the atmo faster. Nuke planes leave to orbit by a 10-15° AoA. I'd aim 20-25 maximum 30° in rocket mode with your plane. I wouldn't shoot for more than 40°, even with a very high TWR plane.

Keep in mind, you don't have to fully reach an AP of 70km on the first go. It's no problem if you only manage to get it to ~60km, but you can raise PE to ~71km (so it becomes the new AP), it won't decrease much. Than it just takes a tiny burn on the other side to circularize.

[Snark]

5 hours ago, Ketatrypt said:

 

To deliver/retrieve cargo.. I could use a space tug but I am already so close with this, I figure why not. mostly just a lesson on efficiency, as I am hoping that I can learn to make my ascent more efficient. 

If you're doing it just for the challenge, sure.

But in general, in KSP, a specialized spacecraft that's designed for one particular purpose will always beat out a spacecraft that tries to do multiple things at once.  Sending that whole spaceplane to the Mun is really wasteful, it's lugging along lots of extra mass that's needed for getting off Kerbin into orbit, but not for going to the Mun.

A little tug that's just equipped with an LV-N (or a Terrier, if the mass is low enough that LV-N's not worthwhile) will bring more cargo to the Mun, for less fuel, with a lot more safety margin.  You can refuel the tug from the spaceplane each time.

But of course, that's all moot if your reason for doing this is "engineering challenge for spaceplane" rather than just "get stuff to the Mun".