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mbound

SSTO spaceplane not gaining speed

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Hi All!

I've been trying a little more with SSTO spaceplanes recently. I've done some in the past, mostly small ones with stock parts, but in this heavily modded career game I wanted to try big, so I built this one using moslty OPT parts, which I wanted to use for a space camp mission (bring 15 tourists to orbit).

It actually has a pretty decent TWR, not a lot of DeltaV probably, but I haven't tuned it yet...mostly because I can't get it past 370 m/s or so. In the picture attached you can even see that I'm pitching down, but even pitching down, my speed was DECREASING...:/

So. I'm showing the aero overlay, by doing some googling, I saw people seem to suggest drag from clipping parts might be a thing. I can't honestly think of anything else, given the large wingspan and high TWR.

Is drag what I'm looking at? If so, how do I fix it?

Thanks in advance! :D

cDWVWsZ.png

 

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That's a lot of drag.

Are you sure the parts in your stacks are snapped to the attach node rather than inadvertently surface attached? If you surface attach the aero model will see every part's flat front. I'm asking because I see a lot of drag vectors there, and if your stack was built as a stack, I'd expect to see far fewer of them.

-- In this situation I would rebuild the craft from scratch, making sure each part in the stack snaps to the attach node. Use the alt key to make sure of this.

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There are certainly a lot of drag indicators very close together.

Part clipping itself does not create drag, however may give the illusion of an aerodynamic craft. In stock aerodynamics be aware: Part attachement not placement is used for drag calculation. So if you radially attach something it creates high drag at front and backside. Rotating or moving inside another part does not change this.

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18 minutes ago, mbound said:

It actually has a pretty decent TWR

What engines are you using, and what's the mass of your craft?

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1 hour ago, Snark said:

What engines are you using, and what's the mass of your craft?

I'm using 4x OPT-E High Altitude Turbo Ramjets (340 kN stationary thrust) and the craft's wet mass is about 188 tons 

Edited by mbound

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You are in "MK2 hell".

MK2 parts (while pretty) have enormous drag compared to their performance. They have two benefits on reentry (high drag and high temp resistance), but on ascent if you cannot brute force them through the sound barrier (at about 370 m/s) then you will never reach space in the first place.

So, redesign your plane and throw away every last MK2 part on that craft. Replace them all with equivalent MK1 parts. Fly your craft easily to space. Then wonder why it seemed so hard the first time.

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1 hour ago, bewing said:

You are in "MK2 hell".

MK2 parts (while pretty) have enormous drag compared to their performance. They have two benefits on reentry (high drag and high temp resistance), but on ascent if you cannot brute force them through the sound barrier (at about 370 m/s) then you will never reach space in the first place.

So, redesign your plane and throw away every last MK2 part on that craft. Replace them all with equivalent MK1 parts. Fly your craft easily to space. Then wonder why it seemed so hard the first time.

Got rid of pretty much all of the MK2 parts, apart from the engines, and replaced with Mk1. Still not enough, potentially even worse. I also removed a payload bay that I had carrying a satellite and replaced it with a fuel tank of equivalent mass. Still nothing.

I was able to go past 400 m/s only when I tried completely removing the side engine pods and tanks, removed the LF+OX linear aerospike that I was planning to use as the liquid fuel stage and put the 4x turbo ramjets in place of that. But at that point my TWR at liftoff was above 1, so I was basically flying a rocket.

Might it be an issue with OPT J parts? Or is it something more basic?

2 hours ago, CBase said:

There are certainly a lot of drag indicators very close together.

Part clipping itself does not create drag, however may give the illusion of an aerodynamic craft. In stock aerodynamics be aware: Part attachement not placement is used for drag calculation. So if you radially attach something it creates high drag at front and backside. Rotating or moving inside another part does not change this.

Yes I thought about that as well, but all of my side pods have aerodynamic cones or intakes, and I can see equivalently long (if not longer) drag lines from the center vs the sides. So not sure the drag is caused by the side engine pods anymore (if indeed it is drag).

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If you open the F12 menu, there should be an option under Pysiscs -> Aero to turn on aero information (including total drag from that part) in part action windows.  Turn that on and hunt down the offending parts.

Edited by Lt_Duckweed

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23 hours ago, Lt_Duckweed said:

If you open the F12 menu, there should be an option under Pysiscs -> Aero to turn on aero information (including total drag from that part) in part action windows.  Turn that on and hunt down the offending parts.

Thanks, I've tried that and looks the main body parts and the wings are those with by far the highest drag, with an ascent at around 10 degrees, they oscillate from 50 to 65 in terms of drag value. But they are also all lifting bodies... so, is this what's slowing me down?

Also, maybe worth mentioning that the wings are tweakscaled, but I suppose that shouldn't change much?

 

At ths point I'm clueless, other than just reducing the weight and thus increasing the TWR beyond 1.0...

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Been there. In your case, i'd throw away the wings and start over, they are killing the design. Sometimes, it just happens :(

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So, there have been some updates.

After numerous attempts at fixing the original design, I gave up and basically redesigned it from scratch, and just decided to go overboard with the TWR. I strapped 4x J-60D hybrid engines and I also had the linear aerospike because I really wanted to use it (I hadn't initially realized that the J-60s are actually dual mode, and they are also incredibly OP, but nevermind).

Got to orbit pretty fast and thought everything was golden, center of lift always behind center of mass, etc. Except...when landing, I realized that it was basically a brick. It wasn't flying, it was falling out of the sky...:/

SO I eventually realized that I had to first understand a bit more about aircratft and SSTO principles, and hence started from the basics. I have to say Bradley Whistance's SSTO spaceplane guide on youtube was extremely helpful!

Re-built it again from scratch and did a bunch of tests: looks like I cracked it now. Ended up just having 2x J-60Ds and nothing else, which gives me a TWR at liftoff of about 0.73, and eventually gets slightly over 1 during high atmospheric flight. And this time I was actually able to get it to orbit, with plenty of deltaV leftover, and fly it back to Solitude safely (I'm using After Kerbin).

I'm still tuning it, but so far it seems to perform. The wings are tweakscaled-up, because they were a bit too small.

Thanks a lot to everyone for the suggestions!

Below a picture of the new beast, currently in passenger + small probe configuration:

gH3IDYJ.png

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I'm glad you got things working.

Cargo bays often fail to shield the parts within from drag. That might be a big cause of the drag in your original design.

OPT is a fun parts pack, suitable for science-fiction style playthroughs. Those Screamjet engines are amazing for large shuttles.

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Also, as a few of my own threads atested, the rooting of parts affect drag calculation; namely, do not use PL Bay as root.

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OPT parts have horrible drag,  worse than mk2.   I think the authors of the mod calculated drag based on volume of the fuselage section, so as they have more internal volume than mk2 drag is scaled up accordingly.   This seems to be how squad calculated drag  on the stock parts -  mk2 parts have more volume than mk1,  so they gave them more drag.    Except that in real life,  that wing body blending reduces drag at transonic/supersonic speed (at the cost of higher weight),   and in real life,  the wing/body blending can be used to store a lot of extra fuel,  whereas in game,  mk2 parts contain no more fuel than mk1 of the same length.

There is a wonderful mod called Kerbal Wind Tunnel that will tell you if your design can break the sound barrier without even leaving the SPH.   It also shows top speed at each altitude, so you can work out the optimal altitude to attempt to get supersonic at.

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