Laythe lift-off

[Shoddyfrog]

I am trying to design a rocket which can launch from Laythe surface to orbit. Easy enough but I cannot seem to get to orbit without the same Delta V as I need to reach orbit from Kerbin. That does not make sense, Laythe is smaller and has a thinner atmosphere so should take less Delta V right?
Wiki says you need 3400 for Kerbin, my rockets can reach orbit with that fine. But wiki says Laythe should only need 2900 delta V to reach orbit. As I expected, less due to smaller radius and thinner atmosphere. However I always have to give a Lander 3400 for it to return to orbit.
Any idea what I might be doing wrong? I need a different launch profile for Laythe too starting gravity turn later and pitching over less sharply. Similar to launching from Eve which suggests to me that Laythe has a thicker atmosphere than Kerbin, but re-entry heat is much less than Kerbin showing that it is thinner, so I am confused.
I can make something that works, so maybe I should just be satisfied with that. But why i need as much Delta V as I do to make it work is bugging me.

[RoofCat]

Kerbin has 2290m/s orbital speed at 72km, Laythe 1880m/s at 52km iirc. Which is 410m/s less. Also you would have less losses due to smaller gravity and less drag, so 500m/s less sounds realistic. Plus your engines should be tiny bit more efficient due to higher Isp in thinner atmosphere. But since you already use Laythe Isp in delta v formula, it shouldn't change the value found on internet and roughly estimated by me right above. That's delta v in Laythe numbers. Even if you would measure delta v for Laythe using Kerbin based Isp by mistake, it would decrease the delta v number for you rocket rather than increase due to worse efficiency on Kerbin.

1. I have no idea what's your TWR for Laythe (start hovering?)
2. I have no idea about your design drag (pics?)
3. Gravity turn should be quite similar to that of Kerbin, except a bit shallower due to 20km shallower atmosphere. Start like on Kerbin up to ~100m/s to avoid nosedive and then just dive tiny bit quicker once you are more stable.

You are doing something wrong. And without more details about the rocket I have no idea what.

[Shoddyfrog]

https://steamcommunity.com/sharedfiles/filedetails/?id=1656129798
Repacked top XL chute after landing to help aerodynamics.
Science bay and landing legs ditched immediately after take-off.
1st Stage-2429 Delta V (Vac) TWR 1.40 (Laythe sea level) Reliant engine
2nd Stage-930 Delta V (Vac) TWR 1.84 (Laythe sea level) 2xTerrier Engines
About 200 m/s short to reach the 70km orbit I am trying for, could probably get to 55km have not tried, design is still a work in progress.
Suggested gravity turn sounds right, but does not seem to work for me, am waiting till 6km altitude to start turn and only pitching over to about 45 deg. Using similar or shallower to profile I use to leave Kerbin uses way more fuel for some reason.

[Jupiter3927]

I used a plane with the Rapier engine when I went to Laythe.
https://steamcommunity.com/sharedfiles/filedetails/?id=548319140

It was able to get to orbit around Kerbin from the runway so I assumed it was good enough for Laythe.

[RoofCat]

TWR looks perfect. You could have less TWR on Terrier stage, but whatever. I see it is design related. Not sure how you get such high TWR with Terriers, can't notice decouplers. Well low fuel volume then I guess.

Though why would you use VAC delta v for launch stage? That's where you lose some 150m/s - rough guess. You actually don't have those, your math is just wrong.

If you start Terriers the same time you start Reliant - at sea level, you are wasting a lot of fuel at low Isp. Not just that, you might also have lower TWR than expected and thus more hovering. That's where you could lose another 150 or so m/s. You should wait for 10km or so on Laythe, not sure. Just check the Isp so it is close to 300sec the very least - rightclick on Terrier.

Large chute doesn't have great aerodynamics nor slowdown for its mass. I usually avoid it. It's like blunt nose cone or even worse. Also worse than 3 separate radial chutes. That could reduce delta v a bit further.

You have slim engine on bottom. Rear drag because of that ladder is a thing too. Large engines actually make that a better transition. Much less relevant than frontal drag in KSP, but still you might lose around 50m/s there - another guess.

Anyway, all drag estimates are inaccurate. Depends on gravity turn and speed a lot.

All those chutes are adding to drag as well. Drag chutes aren't really that great for Laythe. You could try to use 2 mechanical airbrakes to reach chute speeds, not sure (depends on landing mass) and one or two regular chutes extra for landing. The bad thing about drag chutes is they have very poor performance below certain velocity thus not really helping with safe landing. Only to reach speeds needed for main chutes. Which airbrakes can often do better due to higher speed tolerance. If your final touchdown is 1..2m/s too fast, use tiny engine assistance instead of extra 2..3 chutes. That fuel weights less if used at the right final moment.

Is that intended for repeated landings? You could place landing chutes on radial decouplers and drop during early takeoff (to blow them up) if it is one time lifter.

Gravity turn is not about altitude. It's a ballistic curve you balance using speed (thrust). It must be smooth incremental 90..80..70..60..50..40..30..20..10..0° not 45° at certain altitude.

If I had to guess I would rate that rocket with ~23t mass. You sure it has TWR 1.4 during launch? Well, except those tanks are 1/3 empty, it shouldn't.


Regarding drag one 2.5m profile without nose cone can easily kill 300m/s or even more compared to rocket with nose cone on Kerbin during regular launch. Yours shouldn't have that issue, but
1) I think your delta v math isn't accurate to start with (use sea level for Reliant for rough guess - the truth will be in the middle and make sure you don't start Terrier at sea level)
2) you still have few spots with drag that could be improved a bit. But I can't guess their impact close enough.

[Shoddyfrog]

21.8 tonnes at launch (nice guess), Mechjeb set to Laythe says 1.40 TWR.
The terriers don't fire until stage 2 by which time I am clear of the atmosphere, so there is only the 2x90 fuel in their own little tanks then and I have the Reliant on a decoupler so it is ditched when they fire that is how it is 1.84 TWR Still have the empty rockomax tanks of course, can't remember how much they weigh empty, bit of a weird design I know but I wanted a nice fat bottom stage for stability when landing and all the science is in the 2.5 meter service bay so the rockomax tanks seemed the logical choice.
So from what you say it does sound like drag is the issue, I did not realise that a 1.25m engine underneath a 2.5m fuel tank would cause more drag than a 2.5m engine. Also did not realise that parachutes make a noticeable difference. I will try your suggestion of airbrakes instead mounting those and what parachutes I do use on decouplers. Swap the XL parachute for a pointy nose cone oh and maybe smaller fins too...hmm back to the drawing board :)
Thanks for the tips, can't believe I have been playing this game for over 900 hours and still have so much to learn.

[RoofCat]

Well don't get me wrong, I couldn't find any serious issue with your design. But if I had to guess something you could eventually improve, that's about it.

I'm confident your tail doesn't kill 400m/s. 1/10 of that - maybe. As I said, tail drag is much weaker than frontal in KSP. Rather your Reliant stage has slightly less delta v than you mentioned due to start at sea level not vacuum. Then some chute drag probably.

Laythe has 7.85m/s² gravity. Single Reliant I would guess has around ~220kN ASL there. Rather not full vacuum 240kN yet. You have 21.8t. So 7.85*21.8=171.13. 220/171.13=> TWR=1.285
Not sure, but you also mentioned you have trouble getting it up. So that would be it - slightly low power and a bit more lost during hovering. 20 extra seconds spent in high G on Laythe = 20x7.85=157m/s lost. Some 150m/s probably missing from wrong delta v on Reliant. Some drag. Launch going up for longer thus having less orbital speed and more vertical - another loss. It's more like a few small issues coming together and math.

I would drop Reliant with the larger tank in one piece. The very least. Empty tank has 1/9 mass of full one. So that 9t tank still has 1t while empty. Large decoupler is 0.16t. Adds at start, but saves a lot for vacuum stage after. Since you don't need small decoupler for Reliant now, it's even less - 0.16-0.04=0.12t extra during launch. And each chute weights 0.1t. You could shredd another ~1t there if you don't have to return just placing them on radial decouplers in packs. But that's not about missing delta v. Rather just a few options to push closer to limits.