A few space plane questions.

[Wingnutt]

Ive become pretty comfy with rockets and have unlocked enough aerospace to get the RAPIER engines so Ive been tinkering with a space plane, Im trying to start simple, just something to ferry scientists back and forth to my orbiting lab at 125km.

I have a few questions about some quirks I don't understand.

1: Velcro Runway.

For some reason, and ive tried it with multiple craft... the plane will not lift off of the runway until it runs off the end. Not in the sense that it only develops enough lift by the end, more like it is STUCK until the end.. A plane that happily climbs at 100ms.. but even at close to 200ms it will not lift off the runway, then when it reaches the end it leaps off the runway and can climb at 45 degrees for several thousand meters.

This is very frustrating, ive added more lift and more thrust, more and more and more because I thought My plane just wasnt.. plane enough.. the I realized it will just glued to the runway for some reason.

2: Engine performance profiles.

I get that the different engines perform better under certain parameters, currently Im trying to climb as high as possible using the aerospike engines then switching to the Rapiers to transition into space.

Ive read numerous times that I should be getting to almost 40K on air breathing engines before I need to switch to liqud/o2 but I cannot get above 25k without a flameout.. Im using the Ram-air intakes, as I have read those are the defacto best.

In this case there is almost too much info out there, and lots of contradicting advice, I think this may be due to the mass of changes in 1.0. I have read that adding more intakes helps, and I have read that it does nothing. I have read to use basic jet engines to get to the edge of space, and I have read to never use them.. etc etc

Is there a CURRENT chart/graph or something that shows the optimal performance windows for the current engine set?

3: Nose bobbing when adjusting pitch:

This is far and away the most annoying. No matter what I try, different elevator configs, canards, no canards.. the over all control of the aircraft is absolute garbage. when I try to point the nose higher or lower it bobs horribly up in down it does this at any speed and any altitude, and does it even if I switch to precise control (CAPSLOCK). Yaw control is not as bad. I've tried changing the center of lift closer to the center of mass, and further, neither seems to make an appreciable difference.

Thanks again for all the help.

[Belphegor]

for question 1, try placing your rear landing gears closer to the center of mass. your plane is creating leverage to point the nose up but at the same time it's pushing the rear wheels into the runway because they are too far back

sorry i can't help with 2 i haven't tried my hand at spaceplanes yet.. and 3 is a good question i'd like to know this as well

[mhoram]

1: How far behind the Center of Mass are your rear-wheels? Try putting them behind but close to the CoM. A good explanation of that effect can be seen here (quite far down in the wheels-section):

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

[DancesWithSquirrels]

1: It's all about attitude. Your plane will need a positive angle of attack on the wings, a slightly upward attitude for them, in order to develop lift. Is your plane set up so that when resting on the runway the nose is slightly lower than or equal in height to the tail? Then it will instead develop _negative_ lift and glue itself down. You won't be able to rotate to a positive angle of attack and develop positive lift until the runway falls out from under you.

You should have a neutral or slightly positive angle of attack when resting on the runway. Also, your rear gear should be fairly close to your CoG (as seen in the SPH), with control surfaces located _behind_ them so that they have a lever arm to lift the nose - if they are too far back you won't have enough control force to rotate up. Alternatively, canards up front can lift the nose as well. I try to place my rear gear forward as close to the CoG as I can without the plane slamming back on it's tail when placed on the runway.

2: Under 1.x air-breathers will fall off around 20-25km. 40km jets worked in 0.90 and earlier with intake spamming, but KSP has greatly modified performance in the direction of reality, and that's no longer possible. See some of the threads on 1.x SSTOs for discussion on this.

3: The SAS isn't the greatest for aircraft, and I can't really say for certain without looking at your craft, but I would suspect a case of overcontrol - too many control surfaces. Perhaps fewer elevators/canards?

You might try putting in a bit of pitch trim (Alt-S or Alt-W, Alt-X to go back to neutral) and see if that helps.

[Wingnutt]

OK, haven't tried it yet but I bet you guys nailed #1. Makes perfect sense when I think about it.. I'll shift the rear gear forward a bit and I bet that will fix it.

- - - Updated - - -

..I never tried it without having SAS on.. Never crossed my mind, of that fixes it I'm going to feel so dumb.

[FlipNascar]

snip..

2: Engine performance profiles.

I get that the different engines perform better under certain parameters, currently Im trying to climb as high as possible using the aerospike engines then switching to the Rapiers to transition into space.

Ive read numerous times that I should be getting to almost 40K on air breathing engines before I need to switch to liqud/o2 but I cannot get above 25k without a flameout.. Im using the Ram-air intakes, as I have read those are the defacto best.

In this case there is almost too much info out there, and lots of contradicting advice, I think this may be due to the mass of changes in 1.0. I have read that adding more intakes helps, and I have read that it does nothing. I have read to use basic jet engines to get to the edge of space, and I have read to never use them.. etc etc

Is there a CURRENT chart/graph or something that shows the optimal performance windows for the current engine set?

As others have said, that's long gone.

However, one can roughly assume that for each 13 tons of take off weight 1 Rapier will suffice. However the caveat is that while on paper you may have enough thrust, when push comes to shove you may not be able to breakthrough the high drag zone around the transonic speed boundary. A single RAPIER and all it's associated bits and bobs (ie fuel and enough intakes) is probably going to run you around 9 tons of mass. (Just cobbled together a rapier using mk 2 tanks it's 9.3 tons!) I typically assume 650 units LF and 550 units OX per engine for a LKO ascent.

My formula for it is to divide my desired payload by 3. I assume that I can lift 3 tons per RAPIER, and that typically leaves you on the safer side.

It would perhaps be worthwhile taking a look at the K-Prize thread in the Challenges subforum, and there's a thread in the Spacecraft exchange showcasing spaceplanes.

snip..3: Nose bobbing when adjusting pitch:

This is far and away the most annoying. No matter what I try, different elevator configs, canards, no canards.. the over all control of the aircraft is absolute garbage. when I try to point the nose higher or lower it bobs horribly up in down it does this at any speed and any altitude, and does it even if I switch to precise control (CAPSLOCK). Yaw control is not as bad. I've tried changing the center of lift closer to the center of mass, and further, neither seems to make an appreciable difference.

Thanks again for all the help.

If you're using the keyboard you may well be resigned to this somewhat. However, you may have too many control surfaces. Ideally you want your spaceplane to feel sluggish. To just have enough control authority to get by... The tricky thing here is that it's hard to tell if it's design or operator error.

Good luck and holler if you want some more help.

[A_name]

1. You need to place your landing gear closer to the center of mass, right behind it. Think of the plane taking off as a lever. If the fulcrum is too far back, you don’t have enough leverage to push down on the back and raise the nose. If this doesn’t cut it, consider adding some control surfaces at the front (but be careful that it doesn’t mess up your center of lift, you might need to move your main wings further back if you do this). Definitely forget about adding more wing and engine. For a successful SSTO plane you want to keep weight and drag as low as possible. Edit: Also make sure you have elevators on the tail and are not relying on the elevons on your main wing. The further back your elevators are the better they'll work.

2. You probably read the 40K number in an outdated tutorial. With the current aero, your jets will definitely flame out at around 25K if not earlier, no matter how many air intakes. Also, the shock cone intakes are actually better. Fly a test rig with two identical crafts only changing the intake and you’ll see for yourself. One intake per jet engine is plenty; don’t spam the intakes, that’ll only give you unneeded drag.

Also, not sure if it’s a typo on your part but it says here you’re using aerospikes for the initial ascent? You want to use jets or RAPIERS in air breathing mode for that. Yes, it is important to understand the performance profile of the engine you’re choosing. I think the max thrust paramaters appear in the SPH if you right click on the part in the parts menu. Make sure you design your ascent profile to make the most of that sweet spot. For SSTO’s generally the rapiers are better because they have highr thrust at higher speeds, which is where you want it if you’re trying to get to orbit, with the added benefit that you don’t need a separate rocket engine, which is heavy. The only reason in my experience to use turbojets is if you can’t break the sound barrier; since RAPIERs need higher speed to deliver their max thrust, you can sometimes find they won’t give enough thrust at low speeds to push Mach 1.

On an ideal flight profile, I usually switch to rockets when I’m going ~1400 m/s and slightly above 20km. I would say your speed is more important than your altitude at this point. Focus on building up speed. Oh and try n ot to burn up! If you’re burning up, you need to ascend a bit steeper to spend less time in the thicker air.

3. This is a known issue with no know solution thus far. You can mitigate it by turning off SAS and using trim, or with the Pilot Assistant mod.

Edit: Another useful trick I just remembered is to help the SAS by not allowing it to over correct. Example: You want to pitch up, so you press 'S'. As soon as you release the button, the SAS will try to cancel out your control input by pitching back down, only to pitch back up when it realizes it overdid the pitch-down, rinse and repeat. This causes a feedback loop that results in bobbing. You will see this happening if you pay attention to the arrows at the bottom left. What you need to do is give an additional quick tap on the 'S' button immediately after you release it the first time, so you don't give the SAS a chance to pitch down so much. Alternatively, pitch up to your desired attitude and then immediately after releasing the 'S', quickly tap the 'F' to temporary disengage the SAS for an instant. When it comes back on, it'll try to hold the current attitude.

Edited August 25, 2015 by A_name

[Nich]

You can also click on my Pro SSTO badge for a challenge with lots of current builds for 1.0.4

[RexKramer]

3. Nose Bobbing- My guess is the plane is either too stable or not stable enough.

A less stable plane will have the COL very slightly behind the COM, but very close to it. This will make an aircraft which is very maneuverable (pitch axis), but somewhat less stable. This is probably not your problem.

The further away (aft) of the COM the COL is, the more stable the airplane is. However, too much stability is not necessarily a good thing, and can even produce pitch-down moments which are difficult to overcome. An aircraft which is overly pitch stable has a tendency to weathervane into the wind. Deviations from your present pitch attitude will cause the pitch to try to return the pitch to where it was, sometimes with enough force to cause the nose to bob back and forth between nose above flight path and below flight path.

Try adjusting the COM to COL markers. I would suspect your COL is too far behind your COM, so try moving the COL forward, closer to the COM, and see how that affects handling. Personally, most of my aircraft have COL almost right on top of the COM.

Supersonic or hypersonic flight amplifies handling issues, so the problem is likely to get worse the faster you go. Thankfully, KSP is not 100% accurate regarding how supersonic flight affects aerodynamics. IRL, as you transition from transonic to supersonic flight, the effective COL moves aft, creating additional control quirks.

Again, try changing the relationship between COL and COM. Be careful to ensure that as you burn fuel the COM does not move aft of the COL, but try to get the COL closer to the COM.

[Warzouz]

3. Nose Bobbing- My guess is the plane is either too stable or not stable enough.

...

An aircraft which is overly pitch stable has a tendency to weathervane into the wind. Deviations from your present pitch attitude will cause the pitch to try to return the pitch to where it was, sometimes with enough force to cause the nose to bob back and forth between nose above flight path and below flight path.

...

Or sometime you stupidly forget to remove the "prograde" option and don't understand why you plane continues to dive to the ground. As I did more than once...

[A_name]

Or sometime you stupidly forget to remove the "prograde" option and don't understand why you plane continues to dive to the ground. As I did more than once...

Hahaha this happens too often to me as well

- - - Updated - - -

3. Nose Bobbing- My guess is the plane is either too stable or not stable enough.

A less stable plane will have the COL very slightly behind the COM, but very close to it. This will make an aircraft which is very maneuverable (pitch axis), but somewhat less stable. This is probably not your problem.

The further away (aft) of the COM the COL is, the more stable the airplane is. However, too much stability is not necessarily a good thing, and can even produce pitch-down moments which are difficult to overcome. An aircraft which is overly pitch stable has a tendency to weathervane into the wind. Deviations from your present pitch attitude will cause the pitch to try to return the pitch to where it was, sometimes with enough force to cause the nose to bob back and forth between nose above flight path and below flight path.

Try adjusting the COM to COL markers. I would suspect your COL is too far behind your COM, so try moving the COL forward, closer to the COM, and see how that affects handling. Personally, most of my aircraft have COL almost right on top of the COM.

Supersonic or hypersonic flight amplifies handling issues, so the problem is likely to get worse the faster you go. Thankfully, KSP is not 100% accurate regarding how supersonic flight affects aerodynamics. IRL, as you transition from transonic to supersonic flight, the effective COL moves aft, creating additional control quirks.

Again, try changing the relationship between COL and COM. Be careful to ensure that as you burn fuel the COM does not move aft of the COL, but try to get the COL closer to the COM.

These are very good points, but you also want to make sure your COL stays behind your COM even when the fuel is empty. Sometimes it's hard to do so without having to resort to pushing back your COL, so you're forced to fly a plane that's too stable in the initial phases of the flight. A solution to this is to have more drag on the top half of the plane, or the COT slightly under the COM. Both of these will give your plane a natural tendency to pull it's nose up, partially overcoming its inherent stability in your favor. Be careful when having the COT not in line with the COM though. It can be a real headache when in space if you don't have enough SAS or RCS to hold your nose straight during orbital maneuvers.

[Wingnutt]

HMM, yes this makes sense.

I'll move the COL forward a bit and leave SAS off

[TheDarkMechanic]

3: Nose bobbing when adjusting pitch:

This is far and away the most annoying. No matter what I try, different elevator configs, canards, no canards.. the over all control of the aircraft is absolute garbage. when I try to point the nose higher or lower it bobs horribly up in down it does this at any speed and any altitude, and does it even if I switch to precise control (CAPSLOCK). Yaw control is not as bad. I've tried changing the center of lift closer to the center of mass, and further, neither seems to make an appreciable difference.

I had this same problem pretty badly. Someone recommended that I disable most of the control surfaces and now my super cheap plane screams into orbit. I think something about letting SAS access to all your planes capabilities is over compensating for minor (normal) trajectory issues during flight. the only active control surfaces I have are canards for pitch and ailerons for roll. I keep even the rudder disabled during normal flight and just manually activate it if I need during reentry. Seriously, disable like everything and give it another go.

[KerikBalm]

2: Engine performance profiles.

I get that the different engines perform better under certain parameters, currently Im trying to climb as high as possible using the aerospike engines then switching to the Rapiers to transition into space.

Ive read numerous times that I should be getting to almost 40K on air breathing engines before I need to switch to liqud/o2 but I cannot get above 25k without a flameout.. Im using the Ram-air intakes, as I have read those are the defacto best.

I assume you mean that you climb on rapiers nad switch to aerospikes...

Rapiers, having a closed cycle, mean that you don't need aerospikes, but the added aerospike Isp can help. But each aerospike means more mass and drag, and a lost node that you could put a rapier on... so my space planes use the closed cycle of the rapiers for when high thrust is needed, and just have a few higher efficiency engines for the circularization (though I prefer LV-Ns for this, so I can carry excess LF, and not worry about not having enough oxidizer, or not having LF to fly to KSC in case I don't tiem the deorbit right)

The 40k info is outdated.

The engines have a hard limit on their maximum altitude/the minimum air density. No amount of intake spam will counter that.

I seem to recall turboramjets ending at about 23-25km, and rapiers ending at about 26-28km in 1.04, but I'm not sure on the exact numbers.

25km switch to closed cycle is fine and expected.

[xtoro]

Everything was pretty well covered except for your DCoM(Dry Center of Mass) which is where your CoM is when your tanks are empty. There's a couple of mods such as RCS Build Aid which will show you where this is. It's best to design your plane so that your CoM and DCoM are as close together as possible so that your CoL doesn't drift too far from your CoM as your fuel drains. Easiest way is to use the large wings with fuel in them, or place your main wings alongside your fuel tanks. For cargo planes, I place fuel tanks on the sides of the payload and CoM, with wings attached to the tanks.

Also, use the rotation widget to tilt your wings upwards just a smidge 5*-ish. This reduces the drag that the body of the plane is creating when you climb because the body of the plane is at less of an angle so less of it is "hitting the wind". Doing this also brings your CoL forward so you don't need nose wings/canards and you can keep your main wings further back.

[Geschosskopf]

A trick I've found to reduce (and often eliminate) nose-bobbing (and on all axes, not just pitch), it so lock engine gimbal. Engine gimbal is way overcontrol for the often very minor tweaks you need during a spaceplane's ascent.

[RexKramer]

Also, use the rotation widget to tilt your wings upwards just a smidge 5*-ish. This reduces the drag that the body of the plane is creating when you climb because the body of the plane is at less of an angle so less of it is "hitting the wind".

Well, that also reduces the lift generated by the fuselage. You could do the same thing to the wings, angling them down so less of them are 'hitting the wind", but that defeats the purpose of the wings..

[A_name]

Also, use the rotation widget to tilt your wings upwards just a smidge 5*-ish. This reduces the drag that the body of the plane is creating when you climb because the body of the plane is at less of an angle so less of it is "hitting the wind". Doing this also brings your CoL forward so you don't need nose wings/canards and you can keep your main wings further back.

This is great advice.

[Sharpy]

so my space planes use the closed cycle of the rapiers for when high thrust is needed,

The closed-cycle Rapier thrust has nothing on its thrust on the right altitude at the right speed.

Rapier closed-cycle vacuum thrust is 180kN and nearly 900kN in air breathing mode at mach 3.

[ajburges]

Okay, I'll play devil's advocate.

I actually prefer (nearly) unstable craft for landing. My craft have enough control authority to maintain stable flight with the smaller moment of inertia and the decreased stability makes maintaining/changing attitude much easier.

Note: unstable craft are a nightmare for reentry. Drag plays a larger role, so you need to be very careful with attitude between 20 and 36 km or you could go into a flat spin and blow your trajectory.

[KerikBalm]

The closed-cycle Rapier thrust has nothing on its thrust on the right altitude at the right speed.

Rapier closed-cycle vacuum thrust is 180kN and nearly 900kN in air breathing mode at mach 3.

What I meant was during orbital insertion, after airbreathing mode is gone.

Ie... you're at 30 km, trying to raise your apoapsis above 70km... do you just fire an aerospike, and have your rapiers sitting there doing nothing?

No, you fire your rapiers in closed cycle mode.

You could add many more aerospikes, but now you've got more dry weight, more drag.

I don't bother with aerospikes on my spaceplanes, I use rapiers when I need high thrust (raising apoapsis out of the atmosphere), and maybe a LV-N for a lower thrust circularization+ raising orbit.

I haven't found it worth it to add any other LFO using engine to a craft when its rapier propelled in the atmosphere

[xtoro]

Well, that also reduces the lift generated by the fuselage. You could do the same thing to the wings, angling them down so less of them are 'hitting the wind", but that defeats the purpose of the wings..

But using the fuselage for lift also defeats the purpose of wings. The logic behind it is while the wings generate lift, they're thin and generate less drag when compared to a large fuselage which would add very little lift, making the loss in drag not worth it.

Fly a MK3 plane and press F12 to see aero effects and look at the red drag line caused by the fuselage. Now tilt the wings and look again, the red line is shorter, meaning less drag, less wasted dV and more lift from the increase in speed.

[GoSlash27]

#1: It's really just a matter of moving the landing gear as close to the CoM as you can and moving your pitch control as far from the CoM as you can. Just maximize your leverage to get it to rotate. If you have a design that can't be rotated, then you can "cheat" it by building in some angle of attack. Just rotate the wings up slightly and it'll take off while still level.

#2: Spaceplanes all follow the same basic profile regardless of size, but it must be adjusted for the thrust to drag, engine type, and t/w of the closed cycle engine.

First step is to get it supersonic at around 10km altitude. Jets have an easier time doing this than RAPIERS. If you have high thrust- to- drag, you may blow through Mach 1 in a steep climb. If you have a very low t/d, you may need to do it in level flight or even a dive. But in any case, it should happen around 10 km altitude.

Next, get to your acceleration altitude and build speed in a gentle climb. For jets, this is about 16 km. For RAPIERS, it's about 18km. Get the nose down and let it build speed in a shallow climb. velocity is cheap here, so get all you can before you're over 20 km.

Once you're over 20 km and the air breathing is no longer adding velocity, you switch to rockets. If you have powerful and inefficient rockets you want to climb steeply to get over 43km and out of the drag as rapidly as possible. If you have weak and efficient engines, it's important to limit your climb rate so that your apoapsis remains at least 45 seconds ahead. Otherwise, you will fall back ballistically before you've had enough time to establish orbit.

These are the important checkpoints. Some SSTO designs can burn right through them in a single climb and others stairstep very slowly, but they all hit these marks.

#3: Several people have hit on this, and I just want to really stress it: The stock SAS/ control surface system sucks. It's too powerful and too laggy and *that's* why your spaceplane bobs around like an epileptic duck. You need to balance the plane so that it needs very little control authority to keep it flying straight, and then disable as many control surfaces as you can get by with. A lot of my small designs have no active controls whatsoever because the SAS/ reaction wheel is adequate to keep them in line. For most light/ medium spaceplanes, a single pair of control surfaces for pitch and roll is all you need.

It's counterintuitive, but it's all those control surfaces that make it uncontrollable.

Good luck!

-Slashy

Edited August 25, 2015 by GoSlash27

[Wingnutt]

Ok, problem #1 is fixed,

I still cant get anywhere close to space, let alone orbit, its an absolute joke how bad it is. I tried the really stupid sounding idea of putting shock cone intakes on the backs of the rapiers because "its bad to have an open node" that resulted in a plane that would not move.

The controls are such garbage that no matter what I do the nose bobbing is present, furthermore, despite the plane flying perfectly straigh with no signs of issues at 20K @1000ms, turning OFF SAS sends it into an uncontrollable spin instantly, it cannot be flown without at high alt and speed.

Im not closer to getting to space than I am walking on water.

[xtoro]

Walking on water is just as easy as getting a working plane, you just need patience. (wait till winter and walk on the ice)