Sideslip, how to fight it?

[cicatrix]

All right, all my spaceplane designs have this one in common. I'm using FAR and whenever I exceed M 3.5-4.0 my plane starts to veer sideways (I think it's called sideslip) and it's very difficult to control it. Usually it ends up with a major stall. Is there anything I am missiing or this is generally expected and there's nothing that can be done? I repeat - this does not concern any particular spaceplane - it's all of them. How do I fight it?

[Vectura]

A) Do you have decently sized tailfins?

Have you tried the mod that re-orders the engines and intakes to stop asymmetrical flameouts? https://kerbalstuff.com/mod/444/Intake%20Build%20Aid

[cicatrix]

A) Do you have decently sized tailfins?

Have you tried the mod that re-orders the engines and intakes to stop asymmetrical flameouts? https://kerbalstuff.com/mod/444/Intake%20Build%20Aid

And it happens BEFORE the flameout.

[Jimbimbibble]

The tail looks like it is not far enough behind the CoM. Your plane is pretty short compared to its width.

[Laie]

Don't know how much FAR has in common with stock, but...

In stock there's a brief phase of asymmetric thrust when the engines first underrun their air supply. It's nowhere as bad as the real flameout (which won't happen until much later), but it can cause real trouble nonetheless.

[Empiro]

I've found that having having the wingtips (but not necessarily the entire wing) tilt up at around 45 degrees can have a huge improvement on stability at high speeds. You might want to also use the simulations mode where you input a small sideslip and see if there's any major oscillations.

Sweeping the wings back instead of having them flat might reduce drag while improving stability.

[Xavven]

The tail looks like it is not far enough behind the CoM. Your plane is pretty short compared to its width.

This, I think. Your rudders don't have much control authority with such a small moment arm.

[cicatrix]

The tail looks like it is not far enough behind the CoM. Your plane is pretty short compared to its width.

This! Finally, I made it relatively easy, but aesthetically I'm not overjoyed:

[MarvinKitFox]

This! Finally, I made it relatively easy, but aesthetically I'm not overjoyed:

http://i.imgur.com/lEWQZuA.jpg

Don't worry, this is no uglier than your first design.

Nothing could be! ;-P

Your original design has 3 flaws. None of them lethal, but they add up.

1) the yaw control(rudder) is much too close to the center of Mass. Indeed, I'm thinking that when your fuel is about used up, it might even be ahead of Com?

2) Your wingspan vs length ratio is ideal for a low-speed glider, NOT for a hypersonic spaceplane. You want long and narrow.

3) Your center of drag is WAY too forward. Air intakes are very high drag items, and especially that intake on the nose causes problems.

the effect of these are:

1 - > your ability to control yaw is very much reduced

2 - > unneeded drag, under FAR

3 - > your plane's natural orientation is backasswards.

[cicatrix]

Don't worry, this is no uglier than your first design.

Nothing could be! ;-P

Your original design has 3 flaws. None of them lethal, but they add up.

1) the yaw control(rudder) is much too close to the center of Mass. Indeed, I'm thinking that when your fuel is about used up, it might even be ahead of Com?

2) Your wingspan vs length ratio is ideal for a low-speed glider, NOT for a hypersonic spaceplane. You want long and narrow.

3) Your center of drag is WAY too forward. Air intakes are very high drag items, and especially that intake on the nose causes problems.

the effect of these are:

1 - > your ability to control yaw is very much reduced

2 - > unneeded drag, under FAR

3 - > your plane's natural orientation is backasswards.

I'm aware of the intakes being highly draggy. The thing I do not understand is how to put them farther behind. I would place them even behind the nozzles (and I think that this might actually work in KSP), but my inner drive for being nice to physics objects when I do this. Yes, this is a problem - all my planes when freaking out turn backwards (retrograde) and I cannot figure out a design good enough to prevent that.

[eddiew]

By and large when designing with FAR, try to take your cue from planes that fly on earth; most of them have a long fuselage, the main wing and engine weight in the middle, and a secondary wing and vertical fin at the far back end. Yours... doesn't

Your plane is kinda short; as you've discovered, this means you don't have much leverage around the CoM for your control surfaces to exert themselves on. Tbh, I'm surprised you can get it to lift without canards or a tail plane... You might do well to just make it longer, even if you use empty fuel tanks to do it. I'd also suggest moving the nacelles forward, as this will shift the CoM away from the back end, so the tailfin will naturally fall further behind it.

With the CoM moved (ideally to the centre of the cargo bay), you can shrink and move the main wing forward, which then gives you space for a tail plane. That should help stabilise the flight, as the CoL will be above the CoM, meaning the plane will want to stay the right way up. A little upsweep on the wing tips will help keep roll under control.

Overall, I've had much more success with longer MK3 planes than small ones. This is a brute, but it flies pretty solidly, and doesn't have a lot more wing area than yours I think:

Good luck with the refinement - it's a long process, but when you finally get a good stable lifter, it becomes the pride of your fleet

[cicatrix]

Another question then - I have a tendency to overload my spaceplane with fuel. While I can calculate exactly how much I have to take to get to orbit with rockets I tend to take extra 'just in case' for space planes and thus making things worse because of increased weight. dV calculator from KER or MJ are virtually useless so, more frequently than not, I simply take a wild guess.

[Jovus]

Both your 'not long enough' and your 'too much fuel' problems will be solved by switching to the Mk 2 instead of Mk 3 fuselage. Unless you have some cargo bay I didn't see?

[Laie]

Another question then - I have a tendency to overload my spaceplane with fuel.

The only answer I have is experience. After the first flight, remove the amount you didn't need and try again. If you've been badly overprovisioning, you may need to repeat this two or three times. Don't know about you, but my flyers all follow the same design principles and have a similar flight profile, so the amounts I learned from one plane are good starting values for new designs.

For what it's worth (probably not much), my new designs start out with 120units for jets that run through the entire flight, 60-90 units for those that don't, plus 200m/s worth of rocket fuel. I guess that yours will differ.

[eddiew]

Oh I have exactly the same problem The only way I can really calculate the right amount of fuel for a spaceplane is to fly it a few times.

That said, you can make a decent guess. A good flight plan for a spaceplane is to hit mach-5 on air breathing engines, as high as you can possibly get. The best I've seen using FAR is Wanderfound's Kerbodyne Dolphin which can be stretched all the way to 37km on air. Most of mine are lucky to break 30km, but this is acceptable with enough spare fuel. Needing rockets before 25km generally means a failed design that won't break the atmosphere.

Point is, that you know that mach 5 is about 1500m/s. You also know that orbital speed in LKO is ~2200m/s. Without air drag, if you kicked to rockets at mach 5, you'd need 700 delta-v. You can assume that getting to mach 5 will take 'a bit' of fuel. Not a huge amount in general. Take maybe 10% of the oxidiser out of the biggest tank, put your rapiers in rocket mode in the SPH, and see what delta-v KER is showing you. Less than 2000 would make me suspicious that it won't get to orbit and be able to de-orbit safely.

What's throwing you off is that the weight, lift, drag, and TWR of your plane are all variables that are specific to the design, as is the altitude at which you'll be switching to rocket mode. Lift and drag are much higher than in conventional rocketry, which is why the whole exercise is much harder. This is where 'fly it and see' comes in

My suggestion would be to overkill: i.e. design and test using the maximum load you ever want to lift. Over the last week, I've been gradually refining a

carrying an orange tank + 2.5m monoprop container to orbit each time. On the runway with this load, it has ~0.77 TWR, developing to about 0.9 by 25km, which is a comfortable if not lavish amount.

This drone's actual purpose is to carry MKS base modules to LKO, which are much lighter; but the CoM is in the middle of the cargo bay, and I know that if I can lift 35T of fuel depot, then anything lighter can be attached in the right place and the plane will still fly. Sometimes it goes up with 1.0-1.2TWR and has a ton of fuel spare, and sometimes it goes up close to baseline performance and comes back almost dry. For all loads it even has enough spare fuel to top up the nerva-powered spacetug that'll be taking the payload beyond LKO, which is actually preventing the need to ever launch the fuel depot that I built it to lift in the first place!

One big gotcha; make sure it flies stable, and re-enters the atmosphere safely, when completely empty of fuel and cargo! You need to be sure that the CoM when empty doesn't suddenly shoot back behind the CoL, which is very easy to do if you've been designing with a heavy load in the cargo bay. Which is why you want the CoM as close to centre of the bay as possible.

While the design process can be a pain, in the end, what you get is a 'one size fits all lifter' with a known good capacity, and when you start putting it into service you put an end to designing a new set of rocket stages for every mission. Bung the payload in the bay, and launch with confidence!

[cicatrix]

Both your 'not long enough' and your 'too much fuel' problems will be solved by switching to the Mk 2 instead of Mk 3 fuselage. Unless you have some cargo bay I didn't see?

That whole fuselage is a cargo bay. Fuel tanks are only in the nacelles and the rear adapter.

Edited February 24, 2015 by cicatrix

[Jovus]

If you don't want to lengthen your fuselage - because you need that diameter of cargo bay and don't want to add even more fuel - there are a couple of things you can do to help with your yaw problem. First, sweep those wings to at least 15 degrees. You're doing yourself no favors by having them straight like that. (In fact, were it my plane I'd go with a low aspect-ratio delta, probably.) Second, give them a touch of dihedral - no more than 5 degrees should be necessary. This will help with the spiral mode instability I'm betting you're experiencing, where sideslip leads to roll leads to more sideslip. Finally, take those wing extensions you have and turn them into winglets on the very ends and backs by making them vertical and moving them as far back as possible. You might even want a larger wing for this, like the small deltas.

As an alternative, take off the nacelles and make up the extra fuel by extending the fuselage with Mk 3 parts. With swept wings (and possibly less wing; you have a lot), you don't need the extra 4 RAPIERs. Then you can have rudders further from your CoM.

Edited February 24, 2015 by Jovus

[capi3101]

Another question then - I have a tendency to overload my spaceplane with fuel. While I can calculate exactly how much I have to take to get to orbit with rockets I tend to take extra 'just in case' for space planes and thus making things worse because of increased weight. dV calculator from KER or MJ are virtually useless so, more frequently than not, I simply take a wild guess.

Well, I dunno about FAR; I do however know some magic formulas that work for stock aero, and that includes fuel. The general rule there for RAPIERs is 40 units of Liquid Fuel per tonne of mass at takeoff, and 25 units of Oxidizer per tonne of mass at takeoff. That number assumes the engine will switch over to rocket mode with the craft going 1750 m/s at whatever altitude you're at (and therein lies the difference between FAR and stock aero - I don't know how fast the RAPIERs will get you while you've still got air to work with).

Now, my design process for figuring out fuel load typically goes like this:

1) Determine the mass of the payload. This is anything besides engines, fuel tanks, wings and intakes. The command module counts as payload, your cargo bay mass counts, anything in the cargo bay counts.

2) Multiply that mass by four - assume that's going to be the takeoff mass of the plane. That's going to give you a "25% payload fraction", which is what a lousy spaceplane design will usually give you.

3) Determine the number of engines needed to get that into orbit. For stock aero, the rule for RAPIERs is 13 tonnes per engine at takeoff. Your 54 tonne plane would require five RAPIERs in stock. My understanding is that the nerfing the Turbojet and RAPIER received in FAR was done to prevent those engines from being so powerful they'd rip your plane apart, and that the more realistic atmospheric pressure profile with height compensates for it (takeway there - you might consider reducing the number of engines on your plane. Whatever floats your boat there).

4) Now - and this is key - that maximum mass per engine is what you assume for determining fuel. If you've got 8 RAPIERs, you assume each one will lift its maximum tonnage of 13 tonnes, so you assume the takeoff mass of the plane will be 13*8 = 104 tonnes. So you need 104*40 = 4160 units of Liquid Fuel and 104*25 = 2600 units of Oxidizer. If you've got 5 RAPIERs, you assume each one will lift its maximum tonnage of 13 tonnes, so you assume the takeoff mass of the plane will be 13*5 = 65 tonnes. So you need 65*40 = 2600 units of Liquid Fuel and 65*25 = 1625 units of Oxidizer.

There is no tank in the game that gives you a 1.6:1 LF/LOX ratio - those Mk2 Rocket Fuel Tanks certainly don't - so you usually have to wind up draining oxidizer from the tank. For the 65 tonne example, you could use a Mk3 Rocket Fuel Fuselage; it'd put you 350 units short on Liquid Fuel, but you can make up that difference easily enough with outboard fuel tanks and adapters (like those Mk2 Bicouplers you're already using.

Like I said, I use stock aero - though I'm thinking of making the switch and I've been asking these kinds of questions about how things work in FAR. So take this advice with a big grain of salt. It might work for you, it might not; it's worth a try anyway.

[eddiew]

Well, I dunno about FAR; I do however know some magic formulas that work for stock aero

...

Like I said, I use stock aero - though I'm thinking of making the switch and I've been asking these kinds of questions about how things work in FAR. So take this advice with a big grain of salt. It might work for you, it might not; it's worth a try anyway.

Alas there are no magic formulae for FAR, only the big complicated ones

But make the switch! Remember you can turn off disintegration so your planes don't fall apart, and just fly with the better aerodynamics if you want to. It is so much nicer to be able to take real-world solutions and see them work in-game

[-Velocity-]

Don't know how much FAR has in common with stock, but...

In stock there's a brief phase of asymmetric thrust when the engines first underrun their air supply. It's nowhere as bad as the real flameout (which won't happen until much later), but it can cause real trouble nonetheless.

Cicatrix- THIS ^^. If you're building large space planes, IMO you MUST get this mod to balance intake air. The thurst assymmetry described above killed ALL my large space planes until I got the mod. This problem happens WELL before flameout. The reason your spaceplane is yawing is most likely due to this problem; all you're probably doing by setting the tail back like that is providing enough aerodynamic force to offset the thrust imbalance. Instead of putting a bandaid over the problem, FIX the problem by getting the air intake balancing mod:

http://forum.kerbalspaceprogram.com/threads/104704-0-90-Intake-Build-Aid

[capi3101]

I'll second the suggestion to install Intake Build Aid. Running your engines full throttle at 38k without loss of control up to that point appeal to you? Then get the thing. It's not cheatsy at all, it just does a job that needs doing with the push of a single button, with the end result of you will go to space today. Awesome regardless of aero model.

Alas there are no magic formulae for FAR, only the big complicated ones

But make the switch! Remember you can turn off disintegration so your planes don't fall apart, and just fly with the better aerodynamics if you want to. It is so much nicer to be able to take real-world solutions and see them work in-game

I'm still not sure I hold to the notion that none of the general guidelines that apply in stock aero can't also be applied to FAR. I might have to install it just to either disillusion myself or come up with a few. Either way, a disintegrating plane is one sure-fire way of learning what doesn't work...