How to increase take-off speed....

[HoY]

I've been out of the modding game for a year now so I'm not sure if something has changed, but the stock game doesn't have an altitude or even air pressure throttle curve, it's a velocity based thrust curve, and an air pressure based ISP curve. I remember last summer someone had released a mod that handled those curves more realistically, but again, the limits of the stock game are working against you from all angles. You'll need to compromise somewhere for stock users, and keep it authentic for near/far users, and change it up when squad releases their new atmosphere in an upcoming patch.

[CaptainKipard]

I've been out of the modding game for a year now so

The open mod competition is on again, why don't you join?

[HoY]

The open mod competition is on again, why don't you join?

I don't have time for any real dedication now wife and 2 year old daughter just moved in with me finally and we have another one on the way. All I can hope to do is possibly expand the jet pack mod a little more in my free time.

[duckofthemoon]

Cpt. Kipard it looks like to make the craft function in stock you're gonna have to make something unrealistic. The best option looks like the thrust curve, and you don't have many other choices. Go for full realism in FAR. You have to make compromises in KSP.

[CaptainKipard]

Cpt. Kipard it looks like to make the craft function in stock you're gonna have to make something unrealistic. The best option looks like the thrust curve, and you don't have many other choices. Go for full realism in FAR. You have to make compromises in KSP.

[Gryphorim]

Out of curiosity, are you taking off under Air-breathing or Closed-Cycle mode? IIRC, Skylon is planned to use closed-cycle power to take off, the switch to air-breathing for the bulk of it's in-atmo run-up to mach 5-6.

[Pondafarr]

Generally, if your craft can't take off by 100m/s, you don't have enough lift. I understand that this is a specificly engineered craft, but perhaps if you increased your lift parameters slightly, you can make it work.

[CaptainKipard]

Out of curiosity, are you taking off under Air-breathing or Closed-Cycle mode? IIRC, Skylon is planned to use closed-cycle power to take off, the switch to air-breathing for the bulk of it's in-atmo run-up to mach 5-6.

Could you find me the source for that?

Generally, if your craft can't take off by 100m/s, you don't have enough lift. I understand that this is a specificly engineered craft, but perhaps if you increased your lift parameters slightly, you can make it work.

Yap

[sirkut]

Could you find me the source for that?

Yap

It's in the wiki if you consider that a reliable source.

http://en.m.wikipedia.org/wiki/SABRE_(rocket_engine)

[YNM]

Move the rear wheel forward, nearer to CoM (still behind it if you want to be stable). Make larger ground clearence. Often my light aircrafts can't takeoff on the runway just because of that; but be aware of tailstrikes !

[tetryds]

Hey, another thing you could do is increase the angle of the engines, making them point a bit more upwards will require less lift from the wings, you lose topspeed at low altitudes but at high altitudes the loss will be negligible.

[CaptainKipard]

It's in the wiki if you consider that a reliable source.

http://en.m.wikipedia.org/wiki/SABRE_(rocket_engine)

I do, but that's not what it says. It seems you didn't read it, or the article contradicts itself. I didn't read all of it, i just searched for "take".

Due to the static thrust capability of the hybrid rocket engines, the vehicle can take off under air breathing mode, much like conventional turbojets.

Move the rear wheel forward, nearer to CoM (still behind it if you want to be stable). Make larger ground clearence. Often my light aircrafts can't takeoff on the runway just because of that; but be aware of tailstrikes !

This has already been discussed earlier in the thread.

Hey, another thing you could do is increase the angle of the engines, making them point a bit more upwards will require less lift from the wings, you lose topspeed at low altitudes but at high altitudes the loss will be negligible.

Can't do it. It would take the thrust vector too far from the CoM. I already had to lower it a little because the plane flipped with heavy cargo in closed cycle mode.

[lo-fi]

Considering the changes you don't want to make, I think you have three options:

1) Move the CoM and CoL a little closer to the main gear, provided this can be done without messing something else up.

2) Increase the engine thrust for stock. I'm aware you're not keen on doing that, but you're painted into a corner with excessive drag. Given that you seem happy with the way it flies once in the air, I'd maybe set some clever tangents to give it a little extra juice in the takeoff envelope.

3) Change the lift a little. TL;DR, but I think you mentioned you based the lift off stock parts? If so, there will be body > wing < Nacelle aero interactions that a plain old delta wing of equivalent size won't cover. Also, do the body and nacelles have a lift component added?

Doesn't sound like you're far off at all, so a little subtle bodging between the three of those ought to get it sorted.

[CaptainKipard]

2) Increase the engine thrust for stock. I'm aware you're not keen on doing that, but you're painted into a corner with excessive drag. Given that you seem happy with the way it flies once in the air, I'd maybe set some clever tangents to give it a little extra juice in the takeoff envelope.

Maybe. The main reason I didn't want to increase the thrust is that I already did it by about 25% iirc. I say "about" because the official numbers aren't consistent in various sources.

3) Change the lift a little. TL;DR, but I think you mentioned you based the lift off stock parts? If so, there will be body > wing < Nacelle aero interactions that a plain old delta wing of equivalent size won't cover. Also, do the body and nacelles have a lift component added?

Those are interesting points. I did have a little lift coefficient (only 0.5, compared to the 12 of each wing) on the three largest fuselage sections, but I removed them for this round of tests, Thinking about it now it seems like the cargo bay segment could actually provide some significant lift realistically. I'd very much welcome some input from an engineer if one reads this. The OP of my Skylon Dev thread (in my signature) has some orthographic renders of the whole plane.

The nacelles themselves don't have any lift and I don't see why they would.

[lo-fi]

Nacelles will provide some lift, especially with that curved shape and a very hot, high velocity jet of gas coming out the back (which KSP doesn't model). I'd expect it to drive a fairly large flow over the whole nacelle body, and there will be some wing interaction too. I'm not an aero expert by any stretch of the imagination, but I have enough experience messing about in motorsport and with RC stuff to know that at high speed, even a house brick at the right angle creates lift. By that token, at high speed the body is capable of creating significant lift too.

All that being said, you can throw as many real stats and attributes at it as you like, and you're still stuck with once compromise that you can't change in the stock game: flying in treacle with a ridiculously simple aero model. It may just be a case of subtle tweaks here and there until it's playable and behaves within the bounds of what is expected in KSP; anything else probably belongs in the realm of a simulator. The day we have home computers powerful enough to run real-time CFD will be a good one!

My 2p worth, anyway. With my 'average player' hat on, I'd say you've made a fantastic job of recreating it from limited material, and if it flies roughly how I expect in KSP I'd be very happy. I hope you find a solution you're happy with too!

[Orange_Ignition]

lo-fi is right, anything with enough thrust behind it can produce lift. a school bus can take off if it has enough thrust or the wind speed is high enough:) it just won't fly very well...

really the way the engine nacelles are curved downward will produce some lift. the air moving under them needs to go somewhere and if the craft is moving faster than the speed of the surrounding air, the air will literally have to push upon the nacelles, which would create lift at certain angles. because they are rounded it won't create too much/minimal drag. even the under body of the skylon will produce some lift. Think of a javelin being thrown at a target; the guys who can throw it farther angle it slightly up so as to use the body of the javelin for that extra lift. It has no wings but can be thrown a large distance by hand. Now strap a rocket to the javelin and you've got... well a missile

Edited November 27, 2014 by Orange_Ignition

[White Owl]

Any shape can produce lift if it meets air at the right angle. Anything at all. Not as much as a carefully optimized wing, that's true, but it's just simple laws of motion at work.

When that airplane moves through the air, it pushes air out of the way. The air pushes back; that's your equal and opposite reaction. If the airframe is angled slightly then it pushes air more in one direction than another, so the air pushes back more in the opposite direction; that's called lift.

Consider: many airplanes can fly and maintain altitude with the vehicle in a 90 degree roll. Wings and elevators turned completely sideways to the ground and gravity, so not providing any practical lift at all. Popular stunt at airshows. This is possible because in that attitude the side of the fuselage meets the air at a slight angle, and that produces enough lift to support the vehicle's weight.

I'm not suggesting the Skylon should be able to do airshow stunts... but it's perfectly realistic to give various airframe components a small lift factor, just to make it work in stock air.

[Hodo]

Any shape can produce lift if it meets air at the right angle. Anything at all. Not as much as a carefully optimized wing, that's true, but it's just simple laws of motion at work.

When that airplane moves through the air, it pushes air out of the way. The air pushes back; that's your equal and opposite reaction. If the airframe is angled slightly then it pushes air more in one direction than another, so the air pushes back more in the opposite direction; that's called lift.

Consider: many airplanes can fly and maintain altitude with the vehicle in a 90 degree roll. Wings and elevators turned completely sideways to the ground and gravity, so not providing any practical lift at all. Popular stunt at airshows. This is possible because in that attitude the side of the fuselage meets the air at a slight angle, and that produces enough lift to support the vehicle's weight.

I'm not suggesting the Skylon should be able to do airshow stunts... but it's perfectly realistic to give various airframe components a small lift factor, just to make it work in stock air.

Try to do this with an Airbus, and you will be on the 6 o'clock news.

[Iskierka]

Maybe. The main reason I didn't want to increase the thrust is that I already did it by about 25% iirc. I say "about" because the official numbers aren't consistent in various sources.

This is probably because of the variance in its thrust in airbreathing mode - its TWR at static sealevel and transition point are about 9-10, but it peaks at about 14 at M=2.5 or so. I'd suggest using start and end velocitycurve values of 0.67, midpoint of 1, and setting the airbreathing thrust as 50% higher than quoted on wiki. (Unless the velocitycurve can go above 1? In which case it might be easier to handle the numbers using 1, 1.5, 1, and directly quoted value)

Those are interesting points. I did have a little lift coefficient (only 0.5, compared to the 12 of each wing) on the three largest fuselage sections, but I removed them for this round of tests, Thinking about it now it seems like the cargo bay segment could actually provide some significant lift realistically. I'd very much welcome some input from an engineer if one reads this. The OP of my Skylon Dev thread (in my signature) has some orthographic renders of the whole plane.

The nacelles themselves don't have any lift and I don't see why they would.

I'm going to confirm that the body does contribute lift - in the latest D1 specifications, the rear fuselage is even slightly flattened into a lateral ovoid, to promote its lift at high mach and improve trim characteristics. Probably not more than say 15%, but enough that it should be considered.

Finally, the runways are almost certainly too short by a large margin. Bearing in mind that a good part is for emergency braking, the runways specified for the actual vehicle are 5.5 km long, whereas I believe the longest KerbinSide runway is a mere 1.8 or so.

EDIT: Useful note, the vehicle's own CoM is positioned at around 35% from the front of the cargo bay, if you may need help positioning it. May also be useful for users to know that its quoted payload is only if the payload CoM is in a similar place. The user's manual, section 3.5 shows acceptable positionings for a good reference.

Edited November 28, 2014 by Iskierka

[CaptainKipard]

I didn't realise they updated the manual. I had an old version. There's a lot of new useful info in the latest one, thanks.