MK3 space plane (NOT SSTO), need some advice

[SalehRam]

I have managed to mess around and build this MK3 crew transporter space plane, this is NOT an SSTO (or you can consider it an SSTO if you ignore the 3 stages )

It can transport 16 kerbals + the 4 crew so total 20, I was able to get to orbit fast with it, but for some reason, I did not like the final fuel status when I was in orbit...

The way I fly it:

Take off, pitch up to 40-45 and keep climbing until ~22,000m.

When at 22,000-24,000m dump 2 Whips, activate the Swivels, close intakes, and shutdown the other 2 Whips.

Keep burning until I get my AP, then at AP, burn to get orbit.

So I'm looking for some advice if someone with better experience with SSTOs/Spaceplanes who can take a look on it and tell me about anything not right in it...

I have attached the craft file below:

https://drive.google.com/file/d/0B4vxG7-rvmD2a2xTVGRJbHVPMVk/view?usp=sharing

Screenshots:

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Appreciate your help as usual

Edited July 18, 2015 by SalehRam

[Zephram Kerman]

I'm too lazy to mess with the .craft file. Could you post an image instead? (Or even cooler, one of those Sketchfab thingies.)

But, just based on your description, I'm pretty sure you can get more out of those whiplashesesh by nosing down some around 10,000m. I usually start out at 30°-45° like you said, but then temporarily level off to only 10-15° or so. The reason for this is the turboramjet really hits its stride here; at mach 2-3 you'll get several Gs of acceleration. If you time it right, you'll run out of intakeair just as the skin is getting too hot. Then gently pitch up to 45° again to finish the climb out on rocket power. Doing it this way gets you hundreds of m/s of extra speed, which means more fuel left over.

[SalehRam]

I always tend to forget the screenshots :S

I could not manage to do the Sketchfab, so I uploaded normal screenshots, and edited the original post with the screenshots.

[11of10]

I'm not sure you really need that much turborams, I think you could get away with 2-3 only.

[InterCity]

It's not power what matters, it's your maximal speed. You want a TWR between 0.60 - 0.80 for your jets and 1.20 for your rockets. This thing made it to space with 2 whiplashes and 2 LV-T45s or what is their name nowadays (the gimbaling 1,25m engines):

The magic was really pitching down whenever the plane stopped accelerating. Be careful, though, for it might get VERY hot. Oh, and in the new aero, streamlining your plane helps you get higher speeds. Shot swept/delta wings will increase your delta-v budget dramatically, albeit they are not the best in terms of controllability - don't forget your SAS.

Edited July 17, 2015 by InterCity

[LordFjord]

As others already mentioned, it's most likely your ascend path.

Pitching up to 40-45 might get you up there quickly, but your speed probably will be lower -> more rocket fuel needed to get into orbit.

Try a lower pitch, lets say 10-20.

You will need to watch the temperature though. You have a quite many engines and the high TWR might be too much so you end up in a fireball.

There is a fine balance between your speed, temperature and pitch, ideally you can get to beyond 1200 m/s and not burn up while leaving the atmosphere, from there its merely another ~1200 dV to orbit.

You also might want to try to replace a few turbojets with rapiers, those perform better at high altitude and higher speeds.

Good luck!

[SalehRam]

I've been trying to do the ascend on an angle lower than 40, but I end up failing to reach space at a height about 18,000m or around it...my TWR was around 0.85, so I thought about giving more power to climb fast... I will reduce the engines and work on getting at an angle between 10-30 and see what will happen...

Also, what about the plane design? should I add or remove something?

[Wanderfound]

Since you're going for a staged design anyway, you could take it further...

http://s1378.photobucket.com/user/craigmotbey/Kerbal/Beta/Kerbodyne%20Showroom/Sabotage/story

[LordFjord]

I've been trying to do the ascend on an angle lower than 40, but I end up failing to reach space at a height about 18,000m or around it...my TWR was around 0.85, so I thought about giving more power to climb fast... I will reduce the engines and work on getting at an angle between 10-30 and see what will happen...

Also, what about the plane design? should I add or remove something?

Without breaking your design (which is fine and totally be able to go to space), you can do this:

I don't think that you need that many struts to the wings for example - the struts do have a weight (even if its just a little) and they do create drag.

Same with the fuel lines. Airbreather engines draw fuel evenly from all liquid fuel tanks, so you only need to take fuel flow into account for LFO engines and nukes.

Are the wings filled with LF?

You might want to switch the swivels to rapiers (unless you insist on them as a design feature/challenge).

If you do a testflight and barely get to orbit, observe your LF and Oxy fuel levels, then adjust it that you don't bring too much excess of one or another.

I prefer to have a bit excess LF to be able to fly back to KSC if I fail to hit it on reentry.

How fast/high is it going when you switch to rockets?

[Nich]

To much lift, too much thrust, to much rudder. Go to a smaller rudder, add a small mk3 tank of fuel, and fix you assent profile and you should be quite happy. Should be doing 1400ish on air

[SalehRam]

I'm not sure you really need that much turborams, I think you could get away with 2-3 only.

I changed the engines to 4 rapiers only, and my speed would start to drop while I am ascending at 20-10 degrees to a point the plane will start falling backwards.

Intakes are all working so I don't think this is the issue...

Also to mention it, this same problem that forced me to have many engines on the plane, as I need to worry about my maximum speed, and I was unable to get that high speed no matter what I did except this design I put...

[LordFjord]

The rapiers have a certain minimum speed that they need to "kick in". Aim for something beyond the sound barrier, lets say 380-400 m/s, from there the rapiers should carry it. You usually still need some turbojets to be able to get there, so try maybe a 50%-50% turbojet/rapier setup.

[SalehRam]

Without breaking your design (which is fine and totally be able to go to space), you can do this:

I don't think that you need that many struts to the wings for example - the struts do have a weight (even if its just a little) and they do create drag.

Same with the fuel lines. Airbreather engines draw fuel evenly from all liquid fuel tanks, so you only need to take fuel flow into account for LFO engines and nukes.

Are the wings filled with LF?

You might want to switch the swivels to rapiers (unless you insist on them as a design feature/challenge).

If you do a testflight and barely get to orbit, observe your LF and Oxy fuel levels, then adjust it that you don't bring too much excess of one or another.

I prefer to have a bit excess LF to be able to fly back to KSC if I fail to hit it on reentry.

How fast/high is it going when you switch to rockets?

Struts are to hold the wings, because on high speed they will just crash and bring the whole plane down, but I'll mess around to reduce them

For the fuel lines, I needed that for the rocket stage, as I still want fuel to flow without my interaction to all engines...

For Swivels, it is not a challenge or anything, I just found them the best fit for their gimbal ability and good thrust in space/

Wings have LF, which is not adding that much weight though.

When I do a test flight, and finish my orbit burn, I end up between 300-150 fuel left in tanks (or what equals to 300-150 ms/dV), this won't be enough to make my rendezvous maneuvers with the station that I am targeting (sitting on 100-120k orbit)

When I am ascending, I tend to switch to rocket mode at 22,000m high, where I will be no less than 1,000m/s (sometimes I can do it on 12,000m/s).

---

In general I think the issue is in my ascending profile and the number of engines, but I could not get anything other than this to get my MK3 into orbit. However, I did a different design before with an MK2 staged spaceplane (again staged for engines drop only), and it was much better, but that was for cargo delivery, and I needed something for Kerbals transport, so here comes MK3...

I tried to follow the same design rules for this plane same as the MK2 one, but still I'm not fully satisfied about this.

---

I will do the 50-50 setup between Whips/RAPIERs and see what might happen...

[SalehRam]

Ok, so

I ended up replacing the Swivels with RAPIERs, I also put a stack decoupler before all the remaining Whips.

I take off and pitch between 15-25, then once I reach speed between 500-600m/s, I activate an action group, which will drop 4 Whips, and activate 4 RAPIERs, then I pitch up to 45degrees.

By the time I reach 22,000m, I get my AP already around 50,000m, and my speed should be more than 1,000 m/s already, I activate another action group which will dump all the Whips, close intakes, and switch RAPIERs mode.

When I finish executing my orbit burn, I end up with an OK-ish amount of fuel left for my rendezvous with the station and refuel on it.

Here are screenshots of fuel status and the new craft file if anyone wants to check it

Craft file: https://drive.google.com/file/d/0B4vxG7-rvmD2cVd6b19Jc2VDWWM/view?usp=sharing

Screenshots:

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