Wing Patterns and Stability / Hypersonic Performance

[Raideur Ng]

Hate to drag up an old topic, but how does your craft even FLY, slashy? I've made one roughly the same, and weighs 1 ton less, four Rapiers, and it cannot lift itself, no matter what wing AoA, beyond 5km up. The TWR is simply not good enough, and yours would be even less than the 0.34 I'm getting. It simply makes no sense. The craft cannot carry itself.

Your large tail-fins are very heavy and it makes more sense to use small fins. Regardless of how many and their strength, the vehicle just cannot accelerate while keeping the AoA required to not fall out of the sky. What is this witchcraft?

[Val]

2 hours ago, Raideur Ng said:

... and it cannot lift itself, no matter what wing AoA, beyond 5km ...

What kind of Ascent profile are you using?

When I make craft with Takeoff Mass around 30 t / RAPIER I use something like this one:

1. Take-off using full runway length.
2. Build up speed to 450 m/s at sea level. (Below 300 m)
3. Pitch up gently to 10° above horizon, without losing speed.
4. At 7 km start pitching down, so you’re level around 9 km and build up speed.
5. At ~1100-1200 m/s pitch up slowly to reach 10° above horizon before 12.5 km.
6. At 22 km switch RAPIER Mode (Action Group 1)
7. Keep going at 10° above horizon.
8. When AP is above 45 km point the nose Prograde.
9. Throttle down when AP is a couple km above your desired altitude.
10. Circularize at AP.

Example craft:

• 1 RAPIER - 31 t - Triana
• 2 RAPIERs - 64 t - Chibi Skylon
• 4 RAPIERs - 115 t - High-Five (only 28.75 t / RAPIER)

The important part with a craft like this, is that it has low enough drag to complete step 2. That often requires a very specific tuning of AoI, so that the craft can fly at 0° pitch without losing or gaining altitude at around 350-400 m/s at sea level.

 

[Plusck]

5 hours ago, Raideur Ng said:

Hate to drag up an old topic, but how does your craft even FLY, slashy? I've made one roughly the same, and weighs 1 ton less, four Rapiers, and it cannot lift itself, no matter what wing AoA, beyond 5km up. The TWR is simply not good enough, and yours would be even less than the 0.34 I'm getting. It simply makes no sense. The craft cannot carry itself.

Your large tail-fins are very heavy and it makes more sense to use small fins. Regardless of how many and their strength, the vehicle just cannot accelerate while keeping the AoA required to not fall out of the sky. What is this witchcraft?

Intrigued, and wanting to muck about with planes for an hour or so, I decided to make a (nearly) strict copy of Slashy's craft. Didn't put any docking port or means of generating power on it, which was probably a bit daft, but it's in orbit now... Craft file here

My only real problem was the long tail cone at the front. On medium difficulty, it blew up at about 1300m/s and 15km-17km altitude each time. SAS off, 90% pitch trim for take off, gradually taking that down to 0 trim on going supersonic at under 1000m altitude. Just following a 6-8° incline from there got me to a 90km orbit with 40t LFO in the tanks - 34t useable (discounting locked tank and LF/Ox imbalance).

So it may be that your decision to separate the wings - and therefore the need to add more control surfaces - plus the shielded docking port, is just making the design too draggy to work. If anything, the tailplane should maybe be downsized since yaw control in KSP is apparently less important than IRL.

[Raideur Ng]

Val, the craft in question is roughly on par with yours, at 31~t per Rapier. The issue is it simply cannot accelerate in level flight to create enough lift to hold itself up. So it seems. I'm aware the accent will be very flat and gradual, and thus the shielded docking port for maximum heat resistance during such a shallow profile.

Plusck, I tried my craft with a tail cone, aero nose cone, etc. It's not a matter of drag at high speeds, it's a matter of thrust at low speeds. I don't have nor use MechJeb to fine tune my profile so any pitch command will bleed off massive speed to the point the vehicle isnt producing enough thrust to hold itself up. Concerning the wings, moving them at no effect.

The problem is such a poor thrust vehicle flies at nearly 5-8 degrees AoA just to stay level, and this prevents it from climbing or accelerating, This INCLUDES Plusck's vehicle, which I also flew on Val's profile, no less, the results were always the same, if I didnt hit the ocean immediately, it would stop accelerating at 3-5km due to nose AoA.

Not sure what black magic you're using to keep your craft level, but it simply will not fly like that for me. Also, pitching up with keyboard inputs 'softly' is effectively impossible without tuning down control surface power to the point they can barely control the vehicle.

 

No ideas besides more boosters engines. If you add an additional engine on the back instead of a cone, it flies quite nicely, like I would expect a well designed aircraft would. But why..

P.S. Tail cones are really terrible at dealing with heat, theres NO way you can get past 1300 without it exploding, and below that, you're not getting nearly what you want out of the Rapiers.

Edited October 3, 2016 by Raideur Ng

[Plusck]

49 minutes ago, Raideur Ng said:

Plusck, I tried my craft with a tail cone, aero nose cone, etc. It's not a matter of drag at high speeds, it's a matter of thrust at low speeds. I don't have nor use MechJeb to fine tune my profile so any pitch command will bleed off massive speed to the point the vehicle isnt producing enough thrust to hold itself up. Concerning the wings, moving them at no effect.

The problem is such a poor thrust vehicle flies at nearly 5-8 degrees AoA just to stay level, and this prevents it from climbing or accelerating, This INCLUDES Plusck's vehicle, which I also flew on Val's profile, no less, the results were always the same, if I didnt hit the ocean immediately, it would stop accelerating at 3-5km due to nose AoA.

Not sure what black magic you're using to keep your craft level, but it simply will not fly like that for me. Also, pitching up with keyboard inputs 'softly' is effectively impossible without tuning down control surface power to the point they can barely control the vehicle.

I'm not using MJ or anything.

Really, no SAS at all to start with. On the runway, leave SAS off and add pitch trim until it's nearly all the way up to the top of the slider. Take off - it should just run off the end of the runway and avoid the sea by several millimetres. Once it climbs to 100m or so, start very slowly reducing pitch trim. If it starts climbing, knock pitch trim down a couple of keypresses.You want it to be in virtually level flight until it gets well above mach 1, at low altitude. By that time, you should be finding that pitch trim is at or around zero, AoA is also about zero, and it wants to climb on its own. Then you can activate SAS and let it follow prograde up to a reasonable altitude. I actually kept switching between prograde and SAS-hold.

Incidentally, flying with pitch trim alone (i.e. hardly ever touch A or S without also pressing alt) is the only way I've managed to get KSP flying to feel "right"...

[Raideur Ng]

Well, I wasn't doing it quite that intensely. I prefer vehicles I can maneuver aggressively, especially during re-entry and landing and they will not spork me for doing so. I guess I'm not cut out for the ultra-efficiency SSTO club.

[Plusck]

Ah well, I have no idea if I'll be able to land the thing. I don't even know if I can deorbit it - It's sitting in orbit sans nose tailcone, probe core exposed at what is now the nose.

I've tried to be ultra-effiient before but with much much smaller craft (such as making orbit with droppable Junos and a Terrier). Big craft aren't really my thing. As I said earlier, the problem intrigued me and I reckoned it would be good to add this sort of design to my portfolio. I reckon I'll end up using some of the lessons to ferry large numbers of crew to and from orbit, and that's it.

Edited October 4, 2016 by Plusck

[Raideur Ng]

While ultra high efficiency orbital haulers is a brilliant thing, I cannot divest that amount of effort into each launch. I want a simple, no nonsense payload to orbit with virtually zero room for failure and lots of room for dick-ups. i suppose that;'s truly my greatest measurement of SSTOs over all other considerations. The margin for error, because we're all human... er KERBAL.

[Val]

40 minutes ago, Raideur Ng said:

While ultra high efficiency orbital haulers is a brilliant thing, I cannot divest that amount of effort into each launch. I want a simple, no nonsense payload to orbit with virtually zero room for failure and lots of room for dick-ups.

You're in luck then. If you want a high payload fraction then launching with ~18 t / RAPIER is the most efficient, according to the Stock Payload Fraction Challenge.

And craft with that kind of TWR are a pleasure to launch.

Example craft:

• 2 RAPIERs - 36 t Triana Mk2
• 16 RAPIERs - 290 t - S-108 Long Mk3

But they also break Mach 1 at sea level-ish. 

 

[GoSlash27]

I'm also intrigued as to why @Raideur Ng's design doesn't fly. Just looking at it, it *seems* like it should.
The original design was intentionally set up to run close to the minimum thrust/ mass so that I could demonstrate the design techniques required to make a successful SSTO without spamming engines. Perhaps there's some glitch that's creating excessive drag in his derivation?

Best,
-Slashy

Edited October 6, 2016 by GoSlash27

[GoSlash27]

Here's a copy of the original craft.

http://wikisend.com/download/621292/Mk3SP.craft

I checked it in 1.2 to make sure it still works. It does. Not only that, but with the revamped aero, it now hauls an additional 10 tonnes to orbit on the same profile; over 10t of pure payload per engine.

 I think the problem you're having is in piloting. Getting this design to fly is now much more taxing than it was when I first made it. When it first leaves the runway, it's on the backside of the power curve, already in a stall. Very high wing loading and very low t/w means that it's just *barely* flying. You have to get it out of that high- alpha condition without running out of altitude, so you have to *very carefully* trade altitude for airspeed. Gaining airspeed allows you to maintain level flight at lower alpha. Once you've got it down to 5° alpha in level flight, it's generating more thrust than drag, and you can begin the profile to orbit.

 After that, it will accelerate to the necessary speed to get you to orbit. The trick to getting it up there without overheating is to allow it to climb into thinner air without overrunning 20km altitude. Don't try to get all of your speed at sea level. Just get 380 m/sec and climb out at 5°. Once you get a temp warning around 1,380 m/sec and 20 km, switch to closed cycle and pull up to get into thinner air. It'll do the rest on its own.

Best,
-Slashy