The SABRE precooled engine

[Kerburettor]

Hi,

This is an engine I created with stock and robotic (Breaking Ground DLC) parts. Like its real-life counterpart, it has a low-atmosphere air-breathing mode and a high-altitude rocket mode.
These can be activated by pressing two separate keys to toggle the KAL 2000 part. The controller synchronizes air-intake opening/closing with the engine mode switch.

Here is a video that goes through the main functionalities of the engine:

 

 

Performance

It generates 890 kN of thrust at sea level in Air-Breathing mode, up to > 2500 kN in flight and more than 1200 kN of thrust in rocket mode at sea level. However, due to the presence of 24 unshielded radial intakes for the ramjets, the engine also contributes its share of drag.

 

[Gordon Fecyk]

Try to turn the spring dampers on the landing gear from "Auto" to 1. That might help with the bouncy spawn-in.

[Kerburettor]

Thanks for the suggestion mate. What I usually do with the front landing gear is set the spring strength to a small number (typically .2 or .3) and increase damping. This allows the front to sink down and absorb most of the pitching moment. The distance between the tail tip and the rear landing gear is important and there exists a true risk of tailstrike if I don't pay attention to these parameters. But I was maybe wrong in my approach because I never properly characterized the landing gears on this particular system and just went full on guesstimation mode.

 

What bothers me the most is why wings tend to spontaneously snap at high velocities in the lower atmosphere. I don't have FAR installed, and I am aware that the aerodynamic model has probably been considerably improved in v1.8 but nothing from the dynamic pressure readings in the video would hint at too many Gs being the cause for spontaneous disassembly. Sometimes, I reach accelerations of over 2 Gs in the lower atmosphere at relatively high speeds (cf. Descent manoeuver in the transonic regime) and the wings stay put.

[JadeOfMaar]

4 hours ago, Kerburettor said:

but nothing from the dynamic pressure readings in the video would hint at too many Gs being the cause for spontaneous disassembly.

The joint strength of surface attached parts tends to be very low. For craft like a Skylon analogue, you'll easily meet the problem of your engines breaking off due to their weight (or their thrust) overpowering the joint strength of the wing structure between the engine and the center fuselage. The wing joints are further overpowered either by high drag due to AoA in low atmosphere adding stress onto them, or overpowered by the runaway thrust of a RAPIER cluster in mid-to-low altitudes.

This kind of thing won't appear in the flight log. There are two ways to help with your structural integrity problem:

1. Make your nacelles radially attached to the center, use Shift+Offset to offset them as far as you need, then strut them to the wings.
2. MOAR STRUTS.

[Kerburettor]

6 hours ago, JadeOfMaar said:

The joint strength of surface attached parts tends to be very low. For craft like a Skylon analogue, you'll easily meet the problem of your engines breaking off due to their weight (or their thrust) overpowering the joint strength of the wing structure between the engine and the center fuselage. The wing joints are further overpowered either by high drag due to AoA in low atmosphere adding stress onto them, or overpowered by the runaway thrust of a RAPIER cluster in mid-to-low altitudes.

This kind of thing won't appear in the flight log. There are two ways to help with your structural integrity problem:

1. Make your nacelles radially attached to the center, use Shift+Offset to offset them as far as you need, then strut them to the wings.
2. MOAR STRUTS.

It makes perfect sense! I will try this. 

Only option 1 seems feasible though, as adding struts is not possible anymore: somehow I can't attach struts between the wing and the cargo bay or between the wing and the root part of the nacelle which is the precooler. This problem has been occurring quite frequently on my bigger models but I can't put my finger on the reason why this is happening.

[Kerburettor]

Alright, your suggestions were actually pretty spot on!

I attached the nacelles to the fuselage using the mirror mode and offset them, then I removed all the struts and autostrutted the components of the engine instead. Turns out, the struts I put inside the engine (to hold the different parts into place) get "open" when the engine is attached to the Skylon. They generate a lot of drag and induce that nasty-looking Mach effect. I also made the mistake of autostrutting the engine components to their grand-parent part,which in my case were the mirror parts attached on the other side of the Skylon. This means the engines weren't attached to anything except the wing tip which is a very fragile joint. I decided to strut them to the heaviest part of the airplane which is that massive orange payload in the cargo bay. All of a sudden, the Skylon could tip its nose up right after take-off and reach high accelerations (~ 5 Gs) without a sweat and most of all, in one piece.

I also decided to remove the control surfaces I artificially clipped inside the fuselage in order to move the center of pressure upwards. I felt like the dihedral angle did not do much in ensuring roll stability and I thought I had to do that. It felt like cheating and not only does it not feel realistic, this "solution" also adds quite the amount of drag. All in all, having open struts in my aircraft and clipped control surfaces inclined at a high angle did not help at all. Thankfully, removing these clipped surfaces didn't seem to negatively affect roll stability, at least to an extent that would make my plane crash. In order to counterbalance the CoL moving downwards, I moved all the tanks and the engine nacelles down as well. I did observe a very slight tendency to nose down during roll though, as the CoL is significantly lower than the CoM. But the plane remains flyable and I dare say it flies much better than ever before.

I can now consistently reach orbit with a comfortable amount of Dv left (somewhere in between 300 m/s to 900 m/s depending on whether I fly without paying much attention to the controls or if I turn off the rocket mode early on and follow an optimal flight profile). Even better, as the Skylon doesn't need to have oxidizer leftovers in the tanks upon re-entry, I can decrease the amount of oxidizer and get less Dv once in orbit but more liquid fuel for atmospheric flight in the eventuality of a suboptimal re-entry profile.

I'll probably release the engine and the craft file when I manage to land it consistently. I haven't tried any landing this evening because I was already busy with ensuring structural integrity against aerodynamic loads, but I guess the bird can withstand the accelerations of touch down now that the wings and nacelles are strutted properly.

Oh yes, I also had to remove the Whiplash engines because despite the very beautiful blue color and small increase in Isp they provide, they were occluded by the wing tip and became a dead-weight when implemented on the Skylon. But now that the engines are attached to the central fuselage, I think I can safely place them back on.

Let's also note that I have added three sets of big-S wings. Wings don't generate as much drag as fuselage parts so I figured that I could get way more lift without a strong impact on my net thrust (thrust - drag). Now I get approximately as much lift as drag and my net thrust often reaches values up to 2500 kN. Plus now the Skylon looks more like the real model.

Edited January 29, 2020 by Kerburettor

[Kerburettor]

Sorry for the serial-posting but I've got to give some updates:

 

I've managed to land a Skylon which contains two Mk3 passenger modules. The set of modifications - compared to the previous version - can be found in the last post:

The engine nacelles aren't attached to the wing anymore but to the fuselage thanks to a suggestion by JadeOfMaar, which tricks the game into thinking that the engines cause virtually no stress on the wings. After removing open struts, draggy control surfaces that I clipped inside the fuselage and by adding plenty of big-ass wings, the spaceplane pretty much flies itself home with relative ease, with a lift to drag ratio of around 2.5 during landing.

These are not the numbers you'd find on a commercial aircraft but considering the previous version used to generate more drag than lift, I'm happy with the behavior of this machine. It doesn't need too much speed to maintain level flight.

 

More successful landings to come, hopefully!

 

 

Do note that this nacelle design doesn't leave much room for error. If one doesn't manage to land successfully during their first attempt, loading a quicksave prompts the game to reconsider its former choices. Because the RAPIER modules are enveloped inside an open fairing (nozzle), the game now somehow makes the fairings heat up. They subsequently explode which can jeopardize your second attempt, especially if you are in the transonic regime and undergo asymetric explosions. A workaround is to set the throttle at full thrust to blow up both fairings on each side of the aircraft at the same time. The RAPIERs will be unimpeded and you'll be able to land safely. Unfortunately, esthetics often conflicts with practicality.