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SSTO questions.


cat5niper
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I am looking for both general design advice and have some specific questions about very large SSTO design.

 

Firstly are there any resources available for how aerodynamic drag is calculated within kerbal space program, or calculations for lift profile/forces of wing types/areas etc?

Is there a way to reduce drag of attached wing pieces at high speeds like IRL such as sweeping wings or telescoping wings or is this not modelled in kerbal?

Do any wing pieces provide better a better lift/drag ratio?

Does anyone have any previous designs I can look at for stable flying planes of mass higher than 300 tons? 

 

 

 

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For stock KSP, aerodynamic lift has a very simple model. There is no way to reduce drag. For the first few degrees of attack angle, drag increases very slowly, up to about 5 degrees, where you get maximum lift. You get rapidly increasing drag, but slowly increasing lift up to 10 degrees. From 10 to 30 degrees, you still get increasing lift, but drag increases extremely fast. At 30 degrees, your wing stalls and lift falls to zero rapidly over the next 10 degrees of AoA. The amount of wing that you need depends completely on your ascent profile, which varies tremendously between players.

All wings are identical for lift/drag. The basic swept wing, however, weighs twice as much as any other wing per unit area.

The BigS and FatS wings can additionally store fuel. The FatS wings have very low heat tolerance, however, and tend to explode when used in a spaceplane unless you are extremely careful.

Sorry I can't help with supersized planes. My biggest spaceplanes tend to be around 40 tonnes.

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11 hours ago, bewing said:

For stock KSP, aerodynamic lift has a very simple model. There is no way to reduce drag. For the first few degrees of attack angle, drag increases very slowly, up to about 5 degrees, where you get maximum lift. You get rapidly increasing drag, but slowly increasing lift up to 10 degrees. From 10 to 30 degrees, you still get increasing lift, but drag increases extremely fast. At 30 degrees, your wing stalls and lift falls to zero rapidly over the next 10 degrees of AoA. The amount of wing that you need depends completely on your ascent profile, which varies tremendously between players.

Do you know where I can find the exact equation for lift force - speed? To optimise the design I need to calculate the minimum amount of wing area required and ideally the drag to calculate the engines - drag needed. 

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The "exact" equation relating lift and velocity (if that's what you're looking for) is:

L = CL x S x 0.5 x ρ x V2

where

  • L is Lift (force)
  • CL is Coefficient of Lift (derived from the angle of attack of the lifting surface)
  • S is the Surface area of the lifting surface
  • ρ (rho) is air density
  • V is Velocity
Edited by Wobbly Av8r
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On 10/29/2020 at 3:40 AM, cat5niper said:

Does anyone have any previous designs I can look at for stable flying planes of mass higher than 300 tons? 

Bradley Whistance has some videos where he makes excessively large spaceplanes which I think you will find helpful.

This one in particular had a mass in excess of 18,000 tons (!):

Spoiler

Skip to around 1 minute into the video to see the relevant section.

 

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On 10/28/2020 at 10:40 PM, cat5niper said:

I am looking for both general design advice and have some specific questions about very large SSTO design.

 

Firstly are there any resources available for how aerodynamic drag is calculated within kerbal space program, or calculations for lift profile/forces of wing types/areas etc?

Is there a way to reduce drag of attached wing pieces at high speeds like IRL such as sweeping wings or telescoping wings or is this not modelled in kerbal?

Do any wing pieces provide better a better lift/drag ratio?

Does anyone have any previous designs I can look at for stable flying planes of mass higher than 300 tons? 

 

 

 

I don't even think Stratenblitz and Matt Lowne go over 250 tonnes for their designs. Why do you need a craft of this size? Nine times of ten its easier albeit more costly to bring up huge parts of a space station to LKO or HKO.  No difference in wing pieces. @bewing is correct, the BigS and FatS do carry fuel. I stay with Mk2 or Mk3 SSTO's for their flexibity and capability for various cargoes and missions. When I made Jumbo-5, my space station in HKO, I used a 50T craft that was unstable when returning but got the job done. I use a Mk2 for crew rotations and small satilites, as the cargobays are just big enough, and I can run my stations with a crew of 4. As much as I love SSTO's they aren't really worth-it until midgame. Because in late game  you can just put a massive rocket underneath the payload and send it on its way. In early-game, you don't have fuselage parts. So, in short, SSTO's for me at least, are very niche, and most reasons to use them can be done better with a massive rocket. 

6 minutes ago, EimajOzear said:

Bradley Whistance has some videos where he makes excessively large spaceplanes which I think you will find helpful.

 

Yeah, I love Bradley with SSTOs and challenges. He makes it sound easy.

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On 10/28/2020 at 8:40 PM, cat5niper said:

I am looking for both general design advice and have some specific questions about very large SSTO design.

Firstly are there any resources available for how aerodynamic drag is calculated within kerbal space program, or calculations for lift profile/forces of wing types/areas etc?

Here's a basic introduction to the drag model. This was made a few versions ago, so the data is presented in a little different way now, but all the concepts are still true. As for differences between wings, there are none when it comes to creating lift. The only thing the game cares about is the surface area, the angle of attack and equivalent airspeed. The "basic fin" has the best area to mass ratio, but it has the heat tolerance of flash paper so is completely unusable in SSTOs. By far the best wings to use for SSTOs are the Big-S wings, as they have the same mass to area ratio that most of the wings have but can also store fuel for free, the wing strake being able to store more per mass than the delta wing.

On 10/28/2020 at 8:40 PM, cat5niper said:

Is there a way to reduce drag of attached wing pieces at high speeds like IRL such as sweeping wings or telescoping wings or is this not modelled in kerbal?

  No, KSP does not model wing sweep or telescoping. It's 100% or 0%. The only way to reduce the drag on a radial part is to put it in a fairing/payload bay.

On 10/28/2020 at 8:40 PM, cat5niper said:

Do any wing pieces provide better a better lift/drag ratio?

  I have done a few experiments in the past and there was no statistically significant difference in any of the wings' lift/drag. Area, AoA and EAS is all that goes in to the lift/drag calculation.

On 10/28/2020 at 8:40 PM, cat5niper said:

Does anyone have any previous designs I can look at for stable flying planes of mass higher than 300 tons? 

Here's a tutorial where I explain and show how to build and fly large SSTOs. The one in the video is 180t and carries 90t to orbit but the design is very scalable and I provide all the math you need to scale it up.

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As for wings, you'll be sizing them mainly in function of your desired take-off/landing speed. Once you get up to speed you'll want the minimum area possible so you don't get restricted by friction drag. Try also using the least dry mass possible

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