Some Spaceplane Related Issues

[Jade Falcon]

Hi guys I've been building various spaceplanes according to the numerous video tutorials and helpful comments on the forum but there are some things that occur that I can't explain.

1) Typically I follow the guide to ascending but now and again when I hit a certain point the aircraft seems to want to pitch down. I have enough intake air and my speed is over 1000m/s but it still seems to want to go nose down. I can keep the nose up but its kind of like a bouncing effect. To begin with I thought maybe it was due to having the landing gear too far forward but that wasn't it. Then I thought maybe its drag causing it or its the SAS but if I switch the SAS off my plane tends to be awkward to fly. Or its the plane bouncing on the atmosphere or something. However it doesn't always happen at high altitude either so I'm confused. I know it must be something I'm doing wrong but I don't know what.

2) Sometimes despite having the velocity and what I think is a high enough altitude my AP suddenly starts falling and it doesn't matter how I orientate my aircraft but I just end up heading towards the ground again. Is this due to Kerbin's gravity and me not having a high enough altitude? Same problem occurs for me with some rockets so I think it must be a personal design flaw.

3) Ideally how much oxidiser do I need for a spaceplane since I often find I run out too quickly. I can get into orbit but circularising often costs me all my oxidiser. Is there a specific ration of fuel/oxidiser I need to apply?

Any suggestions will be greatly appreciated. I'm only 100 hours into this so far so I'm sure I'm still making stupid mistakes.

Edited July 24, 2014 by Jade Falcon

[Hodo]

First question, do you have a picture of the craft?

Second question are you using FAR or NEAR?

[Jade Falcon]

@Hodo No I don't have a picture of the craft to hand and no I'm not using FAR or NEAR.

[Hodo]

@Hodo No I don't have a picture of the craft to hand and no I'm not using FAR or NEAR.

Ok. There are a few thoughts without a picture it is going to be hard.

It is most likely your CoM moving forward as your fuel tanks get empty. You may want to see what your wet and dry CoM positions are in relation to your CoL. Also check to see if your CoT is still in line with your CoM. If it is over it, it will push your nose down.

[Pecan]

Hodo is far better at this than me (look at his title!) but you should practice with 'fine' controls - toggle with caps-lock - and trim - alt-WASDQE - to balance the plane. When you're in 'fine' mode the yaw/roll/pitch indicators at the bottom-left of the screen display in blue and you have much better control of your planes - rockets are less sensitive and usually don't need it (although it can be good for docking). With trim you can remove the need to keep pressing the control keys and should be able to balance the plane so it flies without SAS or input. You'll need to adjust trim fairly often, as the speed, altitude, fuel-mass and distribution change, but it's a lot easier than 'bouncing'.

[Jade Falcon]

Ok. There are a few thoughts without a picture it is going to be hard.

It is most likely your CoM moving forward as your fuel tanks get empty. You may want to see what your wet and dry CoM positions are in relation to your CoL. Also check to see if your CoT is still in line with your CoM. If it is over it, it will push your nose down.

Yeah I guess I should think to check that cheers. I actually just tried to see if I had the same issues with the Aeris 4A and while for the most part the nose always kept its pitch up there was a point when the same issue happened. I think now what the problem is is that I'm letting the intake air run too low and despite the engines still working there isn't enough air running through to keep them at top performance.

I actually noticed this when I move the camera around and I could see my jet trail had actually dropped in altitude from where I previously was. Plus the Aries has the nose winglets and they probably help a lot and not all of my designs use those so maybe a rethink is in order.

[Jade Falcon]

Hodo is far better at this than me (look at his title!) but you should practice with 'fine' controls - toggle with caps-lock - and trim - alt-WASDQE - to balance the plane. When you're in 'fine' mode the yaw/roll/pitch indicators at the bottom-left of the screen display in blue and you have much better control of your planes - rockets are less sensitive and usually don't need it (although it can be good for docking). With trim you can remove the need to keep pressing the control keys and should be able to balance the plane so it flies without SAS or input. You'll need to adjust trim fairly often, as the speed, altitude, fuel-mass and distribution change, but it's a lot easier than 'bouncing'.

Thanks I do use the caps-lock and the alt-trim controls but I never noticed them display in blue at the bottom left so thats definately new to me. I do tend to use the fuel lines which I picked up from Cruzan's tutorials since I learnt about the balance problem from a Scott Manley video I think. As I said in my last post to Hodo I think the bouncing is the intake air problem, i.e enough air to keep the engines going but not enough to maintain the lift of the plane itself. I will need to run some more experiments to know for sure.

[Hodo]

Yeah I guess I should think to check that cheers. I actually just tried to see if I had the same issues with the Aeris 4A and while for the most part the nose always kept its pitch up there was a point when the same issue happened. I think now what the problem is is that I'm letting the intake air run too low and despite the engines still working there isn't enough air running through to keep them at top performance.

I actually noticed this when I move the camera around and I could see my jet trail had actually dropped in altitude from where I previously was. Plus the Aries has the nose winglets and they probably help a lot and not all of my designs use those so maybe a rethink is in order.

Canards, the nose winglets, are really great for keeping your nose pitch up, but can also stall out before the rest of the craft making them useless. So don't become overly reliant on them.

[Jade Falcon]

Canards, the nose winglets, are really great for keeping your nose pitch up, but can also stall out before the rest of the craft making them useless. So don't become overly reliant on them.

Yes I have noticed that in my tinkering. It seems to depend a lot on the aircraft, some designs they are really useful other designs not so much. In fact in some cases I have noticed adding them just ends up shifting the CoL in front of the CoM which is really bad. To be honest I really hate them on planes kind of destroys the aesthetic look for me which is another problem I have since I often feel the need to make something look nice but it seldom flies nice. lol

Regardless I just tried something new and adapted the Aeris to my purposes so I could understand why it works. I now believe my problems arose from not noticing enough how the fuel shifts in flight, pilot error in regard to not shifting my pitch enough with Alt-WASD and also I think trying to get through the atmosphere a little too severely I think I'm trying to power through it and not coasting along enough rising my velocity. Plus also letting my intact air get too low and not realising how its affecting my ascent.

Anyway I got this shameless Aeris ripoff into orbit, circularised and despite the low fuel readout at the bottom left I do have enough to burn retrograde and finally fly down to Kerbin. So many thanks to you both I will consider all this new information for future craft. Many thanks.

[Hodo]

No problem, glad to be of help. And remember one simple rule with SSTOs, patience.

[Jade Falcon]

No problem, glad to be of help. And remember one simple rule with SSTOs, patience.

True the quick and easy path leads to the Dark Side of SSTO design. I will endeavour to learn patience Master Hodo. Just as a pure experiment I thought I'd give NEAR mod a go, not sure if I'm ready for FAR yet but this plane I just built gets into orbit quicker with more velocity and plenty of fuel left and I'm now confused and wondering why that is because from where I'm sitting NEAR is way better than vanilla. Are vanilla aerodynamics really out of whack or something? Plus I had none of that weird bouncing either.

[Hodo]

NEAR and FAR both change the atmosphere density from that of nearly frozen split pea soup, to that of well.... air.

So aircraft, that are built with aerodynamics in mind, tend to perform better in NEAR and FAR then they do in stock KSP. Same for rockets, but rockets are a bit trickier because if you turn to fast with them they will spin out of control.

[Jade Falcon]

NEAR and FAR both change the atmosphere density from that of nearly frozen split pea soup, to that of well.... air.

I laughed out loud at this. Thanks for the explanation.

So aircraft, that are built with aerodynamics in mind, tend to perform better in NEAR and FAR then they do in stock KSP. Same for rockets, but rockets are a bit trickier because if you turn to fast with them they will spin out of control.

Ok cheers. As for rockets who cares I prefer planes anyway. The game is called Kerbal Spaceplane Program right?

[Pecan]

And also remember that SSTO doesn't mean spaceplane ^^. It is perfectly possible to SSTO with a rocket as well.

[Hodo]

And also remember that SSTO doesn't mean spaceplane ^^. It is perfectly possible to SSTO with a rocket as well.

So true.

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[Jade Falcon]

And also remember that SSTO doesn't mean spaceplane ^^. It is perfectly possible to SSTO with a rocket as well.

Yes I know. SSTO is single stage to orbit. Even a tin can with an engine on it that achieves orbit is an SSTO. Besides I've actually been enjoying your Exploring Campaign PDF recently and that has tons of interesting ideas. I merely meant that I personally prefer planes or spaceplanes. Its looking at the stars through the Mk I cockpit nothing beats it for me and yes I could bolt that onto a rocket if I wished but its not the same. Different strokes for different folks and all.

[Pecan]

No worries - Hodo started it in the post you quote at the top of this page. It's just one of my bugbears - "SSTO" doesn't equal Spaceplane. The other is "Asparagus" doesn't equal ugly.

[i'm glad you enjoy spaceplanes and wish you all happiness. I don't like spending all that time messing around in atmosphere, which is why I stress that point at the start of my tutorial ^^. I also stress that spaceplanes are good though, for those that like them :-) ]

[Hodo]

No worries - Hodo started it in the post you quote at the top of this page. It's just one of my bugbears - "SSTO" doesn't equal Spaceplane. The other is "Asparagus" doesn't equal ugly.

To me it doesn't matter if it is a traditional plane design SSTO or a tail sitter Buck Rogers rocket SSTO. The rule is the same, patience.

As for "Asparagus" launchers, we will agree to disagree on that one.

[Pecan]

As for "Asparagus" launchers, we will agree to disagree on that one.

Asparagus:

The only difference between parallel and asparagus is the fuel lines and you can't really see them. Therefore, if parallel isn't necessarily ugly neither is asparagus. In contrast, if parallel IS ugly, then asparagus isn't what's making the vehicle ugly. Either way, the ugliness does not derive from asparagus staging. Hence, it is wrong to say, as have many other people who haven't thought about it, "asparagus is ugly".

If you want to say "any rocket that isn't a single stack is ugly" then that's an entirely different matter - and still nothing to do with asparagus.

Edited July 25, 2014 by Pecan

[Laie]

2) Sometimes despite having the velocity and what I think is a high enough altitude my AP suddenly starts falling and it doesn't matter how I orientate my aircraft but I just end up heading towards the ground again. Is this due to Kerbin's gravity and me not having a high enough altitude? Same problem occurs for me with some rockets so I think it must be a personal design flaw.

That one, at least, is simple: you're going parabolic.

Your plane is pointing where you're going(1), which may at first coincide with where you want to be going, so you don't notice the problem just yet. But with increasing altitude and airspeed, your thrust decreases. Eventually you hit a peak and go downhill from there. By now it's obvious that something is wrong, but it's also too late to do anything about it.

(1) I'd like to call this "weathervaning" but am afraid that this word has a special meaning to actual pilots.

I've encountered this several times and haven't found a single, definite reason why it happens.

In the more obvious cases, it was a case of excessive drag in the rear, together with a wrong ascent profile. The combined intake/tank/nacelle parts are especially dangerous in that regard, having both a high weight and an intake's drag. Though even that usually is not a problem unless you're going too fast. Yes, that's right: one can be too fast. High-powered spaceplanes can reach near-orbital velocities at 30-33km -- no problem there, as at these altitudes "near-orbital velocity" amounts to perhaps 80-90% of terminal velocity. However, at ~20km that kind of thrust will allow you to go at 2-3 times you terminal velocity, creating insane drag forces.

Making the problem go away is a question of climbing to your desired altitude without going too fast; if you plane can't do that, add more engines or wings so you can still climb at less ridiculous airspeeds.

If you absolutely have to go at several times your terminal velocity, try to balance your center of drag and/or add more canards.

[Jade Falcon]

That one, at least, is simple: you're going parabolic.

Your plane is pointing where you're going(1), which may at first coincide with where you want to be going, so you don't notice the problem just yet. But with increasing altitude and airspeed, your thrust decreases. Eventually you hit a peak and go downhill from there. By now it's obvious that something is wrong, but it's also too late to do anything about it.

Yes that is exactly what happens and exactly how you describe it.

(1) I'd like to call this "weathervaning" but am afraid that this word has a special meaning to actual pilots.

Isn't actual weathervaning to do with crosswind affecting aircraft on the ground which tends to turn the nose into the wind? If so I see what you mean by this because the effect at altitude I experienced is a similar effect, not the same but certainly similar in the fact that the nose tends to pull down sometimes.

I've encountered this several times and haven't found a single, definite reason why it happens.

In the more obvious cases, it was a case of excessive drag in the rear, together with a wrong ascent profile. The combined intake/tank/nacelle parts are especially dangerous in that regard, having both a high weight and an intake's drag. Though even that usually is not a problem unless you're going too fast. Yes, that's right: one can be too fast. High-powered spaceplanes can reach near-orbital velocities at 30-33km -- no problem there, as at these altitudes "near-orbital velocity" amounts to perhaps 80-90% of terminal velocity. However, at ~20km that kind of thrust will allow you to go at 2-3 times you terminal velocity, creating insane drag forces.

Making the problem go away is a question of climbing to your desired altitude without going too fast; if you plane can't do that, add more engines or wings so you can still climb at less ridiculous airspeeds.

If you absolutely have to go at several times your terminal velocity, try to balance your center of drag and/or add more canards.

Yes after the first couple of replies from Hodo and Pecan I came to the conclusion my problems arose from bad balancing of tanks in the sense of not paying attention to the effect when drained as well as too steep an ascent. Too much speed is something I am certainly guilty of I will have to learn to start making my speed more gradual. Excessive drag in the rear which you mentioned may also be another of my design problems. Thanks for the reply some very useful info there.

[Laie]

Too much speed is something I am certainly guilty of I will have to learn to start making my speed more gradual. Excessive drag in the rear which you mentioned may also be another of my design problems.

Your drag can't be excessive as such, or you'd notice immediately after liftoff.

The core of the problem is that Ramjets (or Rapiers, for that matter) have their peak thrust at 1000m/s. On the runway, you have only half their thrust at your disposal. When going 1750m/s, their thrust is perhaps 66% of nominal (and possibly even less, when air-deprived). This means that any ramjet-powered vessel that can function as a spaceplane (or take off easily, for that matter) will be totally overpowered in-between.

There's a saying about a horse with no reins: "It gets you somewhere, quickly." Until you know your ascent profile, throttling down is much better than going somewhere.