Heavy SSTO - unable to take off?

[March Unto Torment]

Hey everyone,

Got a small problem here - if anyone could help, it'd be greatly appreciated!

So, I'm trying to build a heavyweight SSTO to get large volumes of cargo to orbit. It's loosely based off the Valkyrie shuttle from Avatar - a large, thickly-swept flying-delta design. I've taken up the Mk. IV spaceplane parts mod and B9 Procedural Wings to assemble it, and its engines are four 2.5m RAPIER-style engines. The whole thing, fully-fuelled, weighs in at about 368 tonnes, give or take.

However, it just refuses to get off the ground, no matter what I do. Thrust demonstrably isn't the problem - the thing hits over 160m/s before the end of the runway - but after the end of the runway, it slowly descends and can't gain altitude fast enough.

I added some VTOL engines from the same mod underneath, but that made practically no difference.

Thoughts -

1. It's a delta flying wing - is my centre of lift too far back?

2. I'm using B9 Procedural Wings, which was built for 1.0.2 - has the aerodynamic model changes significantly since then? (That said, the wings work fine for most other things.)

If anyone would like, I can throw up a craft file. Otherwise, I'd really appreciate anyone with solutions for how to get this abomination airborne - I'm really fond of its design, and I'd love to see it fly!

EDIT: Okay, so the answers were, for posterity -

1. Move CoM and CoL closer together.

2. Make sure landing gear are right behind CoM.

3. MOAR THRUST.

4. Switch modes when you're about to run out of air, not when you've got none.

Edited September 11, 2015 by March Unto Torment

[GDJ]

368 tonnes is a whack of mass to try to lift up. I've built planes that weigh in at 80 tonnes and they are a bit cantankerous to take off and land.

Might be your Centre of Lift. Shoot us a pic of the SSTO in the spaceport build screen with the CL, CG, and CT spheres in it.

[March Unto Torment]

I've gotten heavy stuff up before - max is probably around 120? But that wasn't SSTO, admittedly.

And I have no idea how to land this thing, no. I plan on dealing with that issue when I come to it (Although it's got five parachutes on the back, so that's a start.)

Anyhow, I'll throw up some images with those when I get home tonight. Thanks for the help!

[GDJ]

I've nudged 120 with cargo, but that's with modded Airliner Wings (80% upscaled with corrected lift, drag, fuel capacity) on a 3.5m body, but it was a Jetliner class plane. Think of a 777 with Twelve J-33 Weasley engines powering it.

[March Unto Torment]

Well, again, I'm using Mk. IV parts by Nertea and Procedural Wings, which is probably helping out some. Plus, 2.5m engines make all the difference in the world.

[March Unto Torment]

Alright, here's the images -

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[Captain H@dock]

Your center of lift is incredibly behind the center of gravity. This will make pitching up very difficult when in the air, and almost impossible when on the ground.

In addition, the rearest set of landing gear is so far behind the CoG that you would need an even bigger pitch authority to rotate while on the ground. I'm assuming you put it there to prevent tail strike, but this will prevent you from rotating all together.

Try on the grass area if 20km of runway is enough for you to take off. If so, you can focus on fixing the balance first (bring CoG and CoL closer), then the gear location second (a bit behind the CoG). Note that you can have an anti-tail-strike gear on the tail if you want, but it should not touch the ground when sitting on the runway: see concorde for instance:

[March Unto Torment]

How would you advise fixing the CoG issue? I can probably move the CoG backwards by taking some fuel out of the forward tanks, but I'm not sure how good an idea that is. Thoughts?

[Alshain]

How would you advise fixing the CoG issue? I can probably move the CoG backwards by taking some fuel out of the forward tanks, but I'm not sure how good an idea that is. Thoughts?

You have two choices.

1. Add or shift more mass to the rear of the plane.

2. Add or shift more lift to the front of the plane.

You are using some rather huge wings in a Delta wing design, you could add more wings to the front as canards. That may help a little, but your CoL is so far back I think it is going to need more than that.

[Brainlord Mesomorph]

How would you advise fixing the CoG issue? I can probably move the CoG backwards by taking some fuel out of the forward tanks, but I'm not sure how good an idea that is. Thoughts?

I wasted several hours trying to fix CoM issues by defueling aft tanks to move the CoM forward. The benefit goes away in flight. Realize that empty tanks (of the same kind) all weigh the same. So, as you burn up the forward fuel. the CoM shifts back to where it was originally.

If your plane can't fly full, it can't fly empty.

I fixed my problem by adding forward fuselage. Maybe you should add forward wings, canards?

(looking again at your plane)how about adding tail?

Edited September 8, 2015 by Brainlord Mesomorph

[March Unto Torment]

She flies!

Unfortunately, my computer's being finnicky tonight so I didn't do a full test run to orbit and back, but I'm registering 18,000 atmospheric dV, which is more than enough to get to orbit, and some very solid acceleration. She was already good for atmospheric flight when I pulled up.

Replacing the forward tank with a cargo bay both moved by CoM some distance backwards. Taking off was a little hectic - for reasons I don't fully understand, the thing refuses to go in a straight line on the runway - but it did manage to get airborne and she handles just fine in flight. Taking off the fuel load also cut my mass down by some 100 tonnes, surprisingly, so she's much more nimble now.

Oh, and on the subject of the CoM moving forward again in flight, that's not necessarily the case; as should be apparent, almost all of the plane's dry weight is in the rear, courtesy of its delta design and the absurd size of its engines. Empty CoM is only slightly forward of where full CoM is.

Anyhow, thanks for the help, everyone! Although I don't suppose anyone would happen to know why it keeps trying to do a roll while it's still on the runway? I feel like it might be a Procedural Wings problem - I've had this issue with them in the past.

[Vanamonde]

Wings in KSP produce zero lift at zero angle of attack. You can help your plane get off the runway if you adjust the landing gear so that it sits nose-high.

[Corw]

Replacing the forward tank with a cargo bay both moved by CoM some distance backwards. Taking off was a little hectic - for reasons I don't fully understand, the thing refuses to go in a straight line on the runway - but it did manage to get airborne and she handles just fine in flight.

My first guess would be that your wheels are not centred properly.

[Val]

She flies!

http://i.imgur.com/Qa5pGne.png

Unfortunately, my computer's being finnicky tonight so I didn't do a full test run to orbit and back, but I'm registering 18,000 atmospheric dV, which is more than enough to get to orbit, and some very solid acceleration. She was already good for atmospheric flight when I pulled up.

Replacing the forward tank with a cargo bay both moved by CoM some distance backwards. Taking off was a little hectic - for reasons I don't fully understand, the thing refuses to go in a straight line on the runway - but it did manage to get airborne and she handles just fine in flight. Taking off the fuel load also cut my mass down by some 100 tonnes, surprisingly, so she's much more nimble now.

Oh, and on the subject of the CoM moving forward again in flight, that's not necessarily the case; as should be apparent, almost all of the plane's dry weight is in the rear, courtesy of its delta design and the absurd size of its engines. Empty CoM is only slightly forward of where full CoM is.

Anyhow, thanks for the help, everyone! Although I don't suppose anyone would happen to know why it keeps trying to do a roll while it's still on the runway? I feel like it might be a Procedural Wings problem - I've had this issue with them in the past.

The plane looks great. Gratz on getting her airborne.

My first guess would be that your wheels are not centred properly.

Even if the wheels are perfectly aligned and strutted, they can still cause the plane to veer of the runway if there's too much weight on the nose wheel (or tail wheel for tail-draggers).

Too much weight on the nose wheel happens when the main landing gear is not close enough to CoM or if aerodynamic forces press the nose down.

[March Unto Torment]

I am seeing the wheels look like they're 'buckling' under the force of the craft. Might that be my problem?

[Merandix]

Yup, that's a problem. Think of your plane as a see-saw, and your main landing gear as the pivot. You want to have the CoM a LITTLE in front of your pivot, otherwise your nose will lift off the ground, but you will basically want to push the tail down with the control surfaces, so the nose will raise. With very long planes, there's a risk of tailstrikes, so you have to be careful. Don't pull up too hard

Don't forget to test with full and empty cargo holds though... Kind of annoying if your plane wants to do wheelies on landing XD

When your wheels are buckling though, this usually means one side is providing more friction than the other, which then leads to your plane veering off course.

[Val]

I am seeing the wheels look like they're 'buckling' under the force of the craft. Might that be my problem?

Wheel buckling is always the symptom. So you can't say definitely what is causing it. Could be any of the issue described in the above posts. Misalignment, CoM not above main gear, aerodynamics.

When your wheels are buckling though, this usually means one side is providing more friction than the other, which then leads to your plane veering off course.

I've never heard of friction being an issue. Can you explain that?

Yup, that's a problem. Think of your plane as a see-saw, and your main landing gear as the pivot. You want to have the CoM a LITTLE in front of your pivot, otherwise your nose will lift off the ground, but you will basically want to push the tail down with the control surfaces, so the nose will raise. With very long planes, there's a risk of tailstrikes, so you have to be careful. Don't pull up too hard

Don't forget to test with full and empty cargo holds though... Kind of annoying if your plane wants to do wheelies on landing XD

...

This is very sound advice.

Main gear needs to be right below and very slightly behind CoM. If part the wheel bay is not directly under CoM, then it's probably too far back.

[March Unto Torment]

It's definitely not misalignment; I'm using snap-to to ensure that everything's fine there. The wheel position itself seems reasonably okay - it's maybe a couple of metres back from CoM. If anyone wants, I can provide pics.

I'm wondering if it's a weight issue? I've got two pairs of heavy landing gear right beside each other, but then again, the craft does weigh about 300 tonnes on takeoff or something like that... perhaps the landing gear aren't designed to take that amount of weight?

[Val]

I don't think weight is the problem. My entry into the Stock Payload Fraction Challenge weighed ~308 t and had no wobble at all with just one pair of large landing gear. No struts and attached directly to fuselage.

My advice is to move the main landing gear closer to CoM.

Edited September 9, 2015 by Val
Link

[Merandix]

[...]

I've never heard of friction being an issue. Can you explain that?

[...]

Friction is the wrong word... I'm having non-native English troubles here

It's because only the bottom part of the wheels are actually acting like wheels (this is why the other wheels don't work upside down in KSP). If you put the wheels at an angle, especially DIFFERING angles (for example, when buckling) they will roll with different efficiencies. Also, if you tilt a wheel in real life, it wants to steer, that may play in as well.

It feels as if one wheel rolls less well than the other, which can be perceived as a sort of 'friction'. Anyway, end result is that your main gear will impart a steering force on your plane, causing it to veer off centreline.

I hope that clarified my failure of word-choice a bit

Also: 'a couple of metres' from CoM is actually pretty far. I think it's about a metre at MOST.

[March Unto Torment]

Well, thank you all! Adjusting the landing gear closer to the CoM fixed up my problem. She takes off just fine now.

One final question, I promise - for some reason, I hit apoapsis at about 5,000m and 320m/s, at which point it starts to descend again. Does anyone know why that is? I swear it should be able to climb higher than that. Does anyone know what my problem could be? Should I add on another pair of 2.5m RAPIERs for extra thrust? (I do know where I could fit those, by the way.)

[Val]

If you have enough power to take off, you should have enough. Rapiers really don't start working until you're going faster than 400 m/s. Don't climb above 300 m until you're going faster than that. See also the Chibi Skylon link in my signature.

[March Unto Torment]

Thanks for the advice! That worked great.

Anyhow, I re-engineered her to use some KAX liquid fuel tanks (sine the old Hypersonic Mk. 3 ones were a little low-res for my taste) and that worked great. A whole bunch of extra dV now, too, although at the expense of some extra weight. While I was at it, I added an extra pair of Mk. 2 RAPIERs (I think that the mod calls them BROADSWORDs), which now means that I have ample thrust. She still struggles to break the sound barrier, but once it's up past 425m/s, it pretty much doesn't stop accelerating. I actually had to throttle down to a third to prevent it from burning up from the sheer power of its own engines.

However, there is a problem.

So, how did this happen, you ask?

Well, I'm not quite sure, to be honest. I hit 20,000m or so (I'm not sure of the exact altitude - I was fiddling around with map view at the time) when all of a sudden, it violently pitched down. What followed was an exhilarating series of explosions and debris, some very well-timed use of engines and a solid landing with recovery parachutes (on the bright side, they work great!).

Any idea what would cause it to pitch down like that? It was really sudden - the whole thing must've gone from 10 degrees positive to 35 degrees negative in two seconds or so.

[GDJ]

^^ That might be a game issue and not related to your plane design. I've had planes blow up suddenly when I go from normal view to map view, especially when I was doing the Velocity Circumnavigation challenge. It's something about thermal temperature of your plane's parts and the map view. The hotter the plane gets, the map view seems to exaggerate this. It may also be related to the memory leak that the thermal gauges work off of.

As for the violent pitching down, I'm not sure either. The last time this happened to me is when I was (again) doing the Velocity Circumnavigation challenge and I was running the angle of attack too close to zero and I had to drop altitude, then the wings snapped off. I think the wings went into negative lift suddenly from positive, and I'm sure it was my plane's balance that was the cause.

Edited September 10, 2015 by GDJ

[March Unto Torment]

Wasn't an overheating problem - my heat was dropping rapidly as I passed the 17,000m mark, and everything looked fine when I switched out of map view (the same could not be said following a re-entry into low atmosphere at close to 2,000m/s) - miracle enough of it survived for a coordinated landing.

I'm wondering if my engines ran out of thrust? Last check, at about 19,000m, I was losing thrust rapidly - down to about 500kN per engine (relative to the previously-insane 4,000+kN thrust I was getting out of them).

So I'm thinking that I more or less ran out of thrust at 20,000m due to lack of air and the craft violently pitched down as a result. In which case, I should probably have switched over to rocket mode on the RAPIERs a few seconds earlier than I did.

TL;DR - Would a rapid loss of thrust account for sudden pitch change?