Radial mounted parts on atmospheric flight performance

[Levelord]

As of 1.0.2 KSP has had a revised drag model which greatly reduces the effectiveness of SSTOs with radial mounted tanks or engines. This thread serves as a rough experimental observation to illustrate how SSTOs are now needed to be adapted for 1.0.2.

As of testing various crafts, the ascent profile is a 30 degree climb up to 8000m where the pitch is lowered to 15 degrees. During this time as speed picks up, the sound barrier is (usually) broken and at around 20,000m the pitch is raised to 20 degrees, intakes closed and the RAPIER engines switched to rocket mode. All crafts used RAPIER engines and the desired apoapsis is 70,000m.

(1) The limiting factor for pre-1.0 SSTOs was the soupy drag, which requires all crafts to clear the soupy atmosphere as soon as possible to gain speed. However, engine thrust was constant and overscaled in high atmosphere.

(2) During 1.0 the limiting factor was overheating.

(3) 1.0.2 the limiting factor is now back to drag. Thrust is limited to speed and dependent on breaking the sound barrier, after which above 20,000m or so the engines die out.

Beginning with a one engine craft, we have one with radial mounted tanks and intakes. It makes it to orbit, but had to take a longer time to break the sound barrier.

Rearranging the parts around to make a single tubular shape (Minus 1 intake and 2 Type A nosecones. These are considered negligible.) We also switch out the NCS adapter for a FL-T200 tank in order to fit the 80 liquid fuel for an inline configuration. Liquid fuel was adjusted for 80 units to compensate.

Surprisingly at 5000m it was already breaking the sound barrier. The ascent profile needed to be adjusted higher in order to prevent overheating.

This craft too makes it to orbit, the ascent profile was a lot easier to manage although the craft aesthetics leaves a bit to be desired.

This is a 3 engine 23.934 tonne craft. This is to illustrate a brute force approach to breaking the sound barrier.

Here at 5000m it has broken the barrier. The ascent profile remains unchanged from the default pitch off stated in the above text. Note the drag values.

It makes it to orbit.

A second test with only 2 engines reveals that the craft is no longer able to break the sound barrier or reach orbit (even though it is 2 tonnes lighter due to 1 less RAPIER).

This is a craft of similar weight to the 3 engine variant but at 22 tonnes (due to 1 less RAPIER). All fuselage and intakes are place on attachment nodes and are inline.

Ironically called the 'Shortbus', it miraculously breaks the sound barrier easily on 2 engines.

And later reaches orbit with a full FL-T800 fuel tank.

Preliminary impressions:

1.0.2 makes drag a very powerful force that craft designs need to be designed around. Getting into orbit with reasonable amounts of fuel depends on leaving the draggy atmosphere as soon as possible. In order to leave the atmosphere ASAP, you will need to break the sound barrier ASAP. To break the sound barrier ASAP, you will need to reduce drag at all costs.

Currently inline placement to attachment nodes reduces this significantly. Attachment nodes are a requirement for the game to recognize an inline attachment to avoid drag. The game however does not recognize clipping and does not recognize inline placement of parts using the offset tools. This means that a streamlined looking craft will perform draggier than a not-so streamlined looking craft using attachment nodes.

For a minimalist like me, there are 2 methods of getting payloads into orbit. If radial attachments of engines/nacelles/fuselage is unavoidable in a design, a brute force approach is required to overcome excessive drag, regardless of the use of aerodynamic nosecones or intakes.

If a craft is an inline sausage, the number of engines can be reduced significantly to 2 RAPIERs. This also saves on the dead weight of having 1 extra engine in orbit.

I will perform more tests and will continue reporting in this thread. Other reports are welcome.

[SpaceChipmunk]

By SSTO you are assuming Space Plane variant? After all I see no problems using radially mounted parts on SSTO Rockets

[Levelord]

By SSTO you are assuming Space Plane variant? After all I see no problems using radially mounted parts on SSTO Rockets

This is mainly for SSTO spaceplanes as they often straddle around a TwR of 1. Rocket SSTOs are just another way of getting cargo into orbit, but using a larger craft and with a lot more fuel.

[Starhawk]

Drag is everything for spaceplanes now. Radial mounted stuff will cost you a lot of delta-v.

Of course, drag matters with rockets too, but you spend much less time in the thick stuff so it's less important than for air-breathing spaceplanes.

Levelord, in your 3 engine spoiler when you drop to two engines try replacing that centre intake with something pointy and see what result you get. Intakes are very draggy and I've found one shock cone or ram intake per engine seems to work fine.

Happy landings!

[Jouni]

Intakes have about as unaerodynamic shapes as possible. They're basically giant maws that try to suck in as much air as possible, hoping that the engines can do something useful with the air. If the engines don't need that much air, then all the intakes do is generate as much drag as possible.

Before 1.0, it was possible to close the excess intakes to reduce drag. The functionality is still there, but I haven't checked whether it actually affects drag.

[martinborgen]

If you right click the intake in flight, you can see generated drag. It does decrease when closed.

[Terwin]

Also, you might notice that the flat bit at the back of your plane where you dropped the 3rd engine also seems to have a lot of drag, I think this was mentioned in the helpful observations about 1.0 thread: you can get a lot of drag at the back of your vessel if it does not have a cone there as well(engines work too, but not as well, on the other hand, cones don't generally give thrust)

[Starhawk]

Also, you might notice that the flat bit at the back of your plane where you dropped the 3rd engine also seems to have a lot of drag, I think this was mentioned in the helpful observations about 1.0 thread: you can get a lot of drag at the back of your vessel if it does not have a cone there as well(engines work too, but not as well, on the other hand, cones don't generally give thrust)

Very good point. I forgot to mention that. You definitely want a pointy back end for anything that doesn't end with an engine.

Happy landings!

[magnemoe]

Intakes have about as unaerodynamic shapes as possible. They're basically giant maws that try to suck in as much air as possible, hoping that the engines can do something useful with the air. If the engines don't need that much air, then all the intakes do is generate as much drag as possible.

Before 1.0, it was possible to close the excess intakes to reduce drag. The functionality is still there, but I haven't checked whether it actually affects drag.

It should and will probably have an effect, still if closed they will should still have plenty of drag, exception might be the 0.90 shock cone intake. This would be mostly relevant for larger planes with plenty of intakes.

[moronwrocket]

Closing intakes currently has no effect on drag

http://forum.kerbalspaceprogram.com/threads/122595-Some-rough-test-results-with-different-nosecones

(although maybe because he wasn't using IntakeAir, the drag didn't change?)