[WIP/Stock] Skylon prototype SSTO

[Kerburettor]

Hi all,

I made this

 

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Description

This is a Skylon-like replica. I'm not saying "Skylon replica" because it doesn't have the same mass, size and ability to act as a cargo plane. It is merely an attempt at creating something that looks like the Skylon albeit in a twice-as-small version and with downscaled thrust.

I have eyeballed the angle of the velocity vector - or trajectory - compared to the horizon (which is inappropriately named "Trajectory-Prograde Angle (TPA)" in the vid - it's a mistake as prograde doesn't make much sense during atmospheric flight - but I still don't know how it is supposed to be named) and estimated it to be approximately 2 degrees at most during the initial ascent phase of a real-life Skylon prototype¹. This is ridiculously low and a KSP plane would have run out of fuel before reaching the Karman line if following the same ascent profile, but I guess the real SABRE engine has much more specific impulse than the RAPIER, which can't be helped. I rather pitch my airplane steeply at first to force a 25° angle and leave the densest part of the atmosphere as soon as possible before lowering the angle slightly to build up speed. I can only accelerate so much and reach a maximum of M_∞ ~ 3.5 before the engines can't provide enough thrust anymore, then coast at 28~29 km at which point I engage the closed-cycle mode of my RAPIER modules. The oxydizer mass is tuned to be burned along with LH2 and not leave much residual after circularization, or just enough for the reentry burn if all goes well.

In order to improve roll stability, I had to give the wings a dihedral angle. But this was unsufficient as the center of mass is just too high and can't be lowered further due to a lack of room inside the airframe. I had to clip control surfaces inside the middle portion to move the center of pressure up. These control surfaces generate quite a bit of lift, but also significant drag during flight which is detrimental to performance in the ascent phase. I am looking forward to getting rid of them once I manage to lower the CoM in later versions.

As stated in the video description, this is a first draft and I will improve this preliminary version upon unlocking the last remaining tiers of the tech tree. The fuselage should be wider and taller. It should also include a cargo bay to fulfill the main mission of the aircraft which is crew and payload transportation. With more volume, it can hopefully hold much more propellant and despite a lower T/W ratio (current is 1.09), it would still be greater than the forecasted thrust to weight ratio of its real-life counterpart : with a projected nominal take-off mass of 345 tons and a maximum gross air-breathing thrust of 2700 kN² and assuming that these numbers don't change much during the air-breathing phase (which is a valid assumption as mass only decreases by at most 6.25% at the end of the A-B phase), the Skylon will at most have a T/W ratio of 0.8, which explains the very shallow ascent profile.

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If anyone is interested, I will release a craft file in a near future once I correct a few things and tweak some stuff.

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Replica specifications

Part count: 135

Height: 5.0 m

Width: 11.8 m

Length: 41.8 m

Mass: 78.438 tons

 

Skylon specifications

Part count: N\A

Height: N\A

Width (wingspan): 25.4 m

Length: 83.3 m

Nominal take-off mass: 345 tons

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References:

¹ "Skylon users' manual" rev. 2.1 by Reaction Engines

² "Skylon users' manual" rev. 1 by Reaction Engines

Mehta et al. (2015). Skylon Aerodynamics and SABRE Plumes. 20th AIAA International Space Planes and Hypersonic Systems and Technologies Conference

Edited December 29, 2019 by Kerburettor

[Legolover]

I like it! How long did it take to make?

[Kerburettor]

All things considered, it took me more than a week.

The construction process is governed by a lot of trial and error and I wasn't really sure what I was trying to achieve at first. I knew that I wanted to build a replica of the SABRE engine, being inspired by Kronus' model which is pretty spot on in my opinion. However, his model suffers from an unhealthy amount of part clipping and I've already experienced fiery explosions from sticking a rocket engine on the mounting point of a RAPIER engine or on a grandparent part and moving the rocket around doesn't solve the conundrum. If I manage to fuse two RAPIER engines together without invoking the Kraken or logically overheating something, be sure that this will be found in the newest version of the Skylon replica.

As things went on, I thought the engine began to look the part. I decided to go for it and build the Skylon.

 

Edit: I've got to correct myself a bit. I did manage to stick rockets behind the RAPIERs. You've got to place them, translate them backwards until they get inside the RAPIERs and then move them outwards so they don't aggregate near the center where heat fluxes are tremendous. But the only ones that fit are the Terrier and the Reliant. The Terrier is by far the more economical engine from a fuel consumption point of view, but it isn't necessarily the most efficient considering its low thrust in vacuum. The Skylon is a heavy beast and 8 times 60 kN is not gonna cut it, plus the fact that you have to carry their dead weight around until they're being used. It's not worth the hassle.

Edited December 29, 2019 by Kerburettor