aphenine

Meh, I just tried with two variants, one 1400t and another 1100t and both blew up the runway. Yey for B9's HX. Incidentally, the second one was fine, it just wouldn't move anywhere.

*sigh* I get where you are coming from. I even agree with it a little. But not entirely. I started playing KSP because I wanted to know whether it was possible to create cheap vehicles that would allow people to access orbit. At the moment, it's blindingly obvious that rockets are not the way forwards if we want to do that. I want to know because, since I was young, I've always been a huge fan of sci-fi. I've devoured it and lived in the various future worlds other people have created and its been fun. But, I've always wanted to know things like how likely it is and this is one of the fun ways I have of exploring that. RSS is fun, and a chasing of realism is fun, but I find it limiting. I want to know what the next generation of technologies might do and whether they make any sense. To do that, I want to have some realistic models that would let me play with and figure out how they would work. I don't just want to make something up. Why scramjets? Well, I started playing KSP and I could design, using the standard KSP and B9 parts, spaceplanes that would fly up into orbit so successfully that it's not a design challenge any more. The designs I have are really simple and, because of the incredible fuel efficiency and the loveliness of Kerbin, they work amazingly. I want to know whether a similar approach will work on the Earth, what I would need for it to work or whether the scaling factors preclude that. There aren't really any designs that would allow that except for hybrid ramjet/scramjet engines paired with SABRE engines and I've been wanting to explore that. I've created some curve-based scramjet engines and I want to go further. As for the fact that most of the research is classified, this is true, and it is annoying. However, there's still a lot of stuff out there. The paper I have compares its model to real life results in a wind tunnel. That's just one paper I have access to outside a University paywall, not the other stuff that's so tantalisingly out of my reach. There are probably others, and you all seem to have forgotten HySHOT and a couple of other programs that are out there doing stuff, plus the fact that Lockheed Martin wants to build the successor to the SR-71 Blackbird using a ScramJet engine design (it will be called the SR-72), with the engines provided by Rocketdyne, if they secure funding from Congress (which is doubtful, but could happen, and if not now, at some point). I don't understand why you're happy to have modelling for SABREs (which have a similar could happen approach) and not for scramjets. As for my approach, I have no idea whether its the right one. I think, though, I would like to give it a go, since I think only testing it would see if that's so. I'm asking if anyone has any specialised knowledge that might be useful and might help me, or any useful experience, assuming you want to give it. For example, you all seem to be bright people, so I thought one of you might have done maths numeric work (I'm afraid my numerical analysis work has tended towards analysing parametrisation meshes in n-dimensional spaces), so you might have experience about whether what I'm doing is stupid or not (I was surprised about the number of papers and methods surrounding that one field alone, let alone something as well travelled as the fields of ODEs and PDEs). Or about the aeronautics, which I know nothing about. Still, you've been light on that information and instead told me your opinions about whether what I'm doing is worthwhile in the first place. Of course its worthwhile to me... Grrr....

Well, I think I am a stupid idiot, because I just realised how hard all this is and how much I don't know. I've worked out that the NASA EngineSIM definitely uses subsonic flow inside the engine itself, because the first equations for the intake works out the stagnation pressure and temperature, both of which assumes the air slows down almost to zero. I was kinda hoping there would be some hints as to dealing with supersonic flow from the equations given with regards to the intakes, or there would be some hints as to handle velocity inside the engine. But no. Also, the paper I was looking at regarding scramjet fuel flow, I've just realised how they solved things. They used a full set of ODE solvers written in Fortran90 on a set of differential equations I'm still looking at and not understanding. No amount of physical intuition is helping. The whole thing is scary and terrifying and I couldn't think of a way forwards. For one thing, using any kind of ODE solver scares me, since I don't really understand the complexities of all of that, nor do I know how to get one working in C#. However, it also strikes me as severe fail in trying to get something simple to work on limited processors in real-time during the game. However, having said that, I was lying in the bath after Zumba (why are baths such good places to think?) and I realised that I might be able to do something with solving a scramjet engine if I modelled the interior as a set of discrete volumes. If I assumed that temperature, pressure, velocity, density and volume were all constant over such a volume, and I made the volume small enough to work through, I could maybe achieve something. It also kinda matches the method used in EngineSIM, which merely models the temperature and pressure through discrete parts of the engine (except supersonic flow appears to need way more variables of state). It might also help because different effects occur at different parts of the engine and I might be able to accurately model certain bits with less complex code. That would require me understanding what the hell was happening in the engine and writing good equations (it seems I might need something like ODEs in terms of points along the engine length) to fully model the engine. But wait, where have I seen ODEs in terms of points along the engine engine length? Yes, in that paper I said scared me and I didn't want to solve. So maybe I could use those equations (or bits of them) and work them out point by point, or something, using the method outlined above? Anyone with numerical analysis experience know why I should not do this (speak now or forever hold your peace)? I know it would be a cludge, but it might give us something slightly more accurate than the default KSP model and right now something has got to be better than that, right? Anyway, if you followed that, any thoughts? (And if you didn't, well, writing it out helps me think, so thanks for just being there).

Thanks of considering it, anyway. I can understand that you must be insanely busy given the number of mods you do! If I'm desperate, I'll make a new part for now. None of the PP parts are attached to Tweakables. In fact, the craft has Tweakables attached to PP parts only. The core chassis from the root part to those components are all PP.

Thank you camlost. I have a feeling it's going to be really useful, but I'm not going to jinx it by saying it will be. However, I've already identified a few equations I know are in the code, and the indices are suddenly very explicable. Thank you.

One of the things I'd really like to do is to write some code that would take into account scramjets. At the moment, the NASA EngineSim code doesn't do scramjets and it would be nice if it did. I don't know if it's practically possible for me to do this, but I've already had a bit of fun trying to look into it and, well, scramjets are both more complicated and less complicated than normal engines. They're more complicated than normal engines because they have fuel mixing effects and supersonic shock boundaries. However, they're simpler in that the air-flow further down the engine doesn't influence air-flow up the engine and I've already found a paper that uses a one-dimensional model to model the fuel mixing efficiency. I know this is really hard, and I'm aware it's unlikely I'll succeed and I'm OK with that. But I'd like to try and basically, I was wondering, does anyone on here actually understand how the NASA EngineSim code works and would they be willing to field stupid questions from me?

I've been using the awesome mod at the moment to build spaceplanes with good ballistic efficiency. However, I keep running into a problem where my designs tend to generate more heat than the default 1100 limit in RSS. As a result, my designs tend to melt and die when crossing the Mach 4-6 border. I know that there are other materials which can be used that have better thermal limits. These are in NASA's EngineSIM and I think they get denser and more expensive as they go. Off the top of my head, titanium has a thermal limit at 2500, Stainless Steel at 3300 and active cooling (like the Reactive Engines pre-cooler) at 4200. Anyway, I was wondering if it would be possible to include a right click menu parameter, which would change the material and hence the heat, weight and cost properties much like you can do with tank types? It would certainly be more elegant than creating clones of the current tanks with different heat and weight levels, which I think I'm going to do in my quest to Build One That Works. P.S. I don't mean to be pushy, but does anyone have any ideas about the bug I posted before? Is it a bug? Has it been seen before? Would anyone like more info, or the craft designs?

I'm not sure if this is a PP bug and it strikes me that this might have happened before, so you may know the answer to this, but if you could help, that would be wonderful. Anyway, here goes. I built a Skylon-inspired spaceplane for testing in RSS, both in a clean build install (exactly like the instructions say) and a previous one where I was more liberal in interpreting the instructions. In both of them, this bug happened: As you can see, the front part disappears inside one of the other parts, leaving no way for me to get it out. It's happened on the end as well, which you can just see. The attachement node on the front of the other part that the first goes into also disappears, so I think it gets moved or something. I don't know what exactly causes it, but some things that are correlating include when the spaceplane gets over a certain length in the SPH, or it might be to do with crashes, since it only pops up once the games crashed a few times.

I have a question regarding TweakScale and compatibility with the B9 Aerospace parts mod. I've had a look at the TweakScale config for B9 and it looks complete and professionally done. However, the scale tweaking stops at any of the chassis types. I'm thinking this is deliberate and I was wondering, is there any reason for this? Did someone try letting TweakScale work on them and did it work?

Yey, I just went and visited Berlin for four days! I wish my German was as good as your English. At best I could say please and thank you. Berlin is... strange. Very traumatic, with all of its recent history. I liked it though. The Reichstag dome was amazing. There is a release of the Undine, because, y'know, Voculus liked it. I decided that I wanted a cool company name for all the things I produced and in honour of the SABRE engine manufacturer, I picked Research Aerospace. Everything I make will have the form RA-XYZ, where X is the chassis number, Y is the series number and Z is the version number. It seems like a logical way to do it and, to be honest, my previous scheme wasn't working well. I've stripped ECLSS stuff out of these and also taken off MechJeb, as not everyone likes it. I think I'll create a release thread, maybe. Save spamming this forum if I do more releases. Everything will also have a name, because that's one of the most fun bits, IMHO. Link: https://www.dropbox.com/s/mbt1vmucoezgvix/RA-302%20Undine%20%28Release%29.craft (also located above) Also put up the Skylifter and the Spacelifter, because I think it would be nice for people to see some of my earlier stuff, so they can see that building a huge fully capable SSTO takes a lot of trial and error. These are preceded by AI in the version number, but using a similar code as the above. I might also post the ones that didn't work, maybe, or at least, screenshots, to give new designers a bit more confidence. These have whatever mods are used to make them fly. Probably no ECLSS, but no guarantees. Feel free to spot the design mistakes. Links are located above. Finally, I also have posted the X-693. I think I will post a few more experimental designs because maybe I'm being too shy and silly about posting them. I'm sorry about that. However, the X in front of the version number means experimental. I take no responsibility in anything the craft does. I thought the X-693 might be fun because, well, it's 500 tons plus. Link: https://www.dropbox.com/s/8930blpy0nt6m5x/X-693%20%28Public%29.craft

Yey, well done!

Thank you Voculus, I'm glad you liked the Undine. And thank you HeXeCtiC. Um... I didn't know that anyone would want to have the craft files. I can put up the historical ones straight away (although they embarrass me). But would you mind waiting for the others? Some of them I want to release when they're ready. Some need things like ECLSS components stripped out of them. Is there any one in particular you wanted to try?

I have a lot of weird designs I'm playing around with. None of them are really polished enough to post and go "oh, look at my shiny craft" with the exception of the Entente Heavy SpaceLifter. But some of them are a little exciting and I want to show them off. Plus there are things I've built and played around with and learnt from. So, if no one minds, I feel like posting some of the designs I'm working on and some of the ones that got me to where I am. Feel free to comment. Full Album Contains descriptions and all the images plus extra ones of the designs in flight. SkyLifter (Mk4 Chassis) Craft File This is the first spaceplane I built that ever flew all the way into orbit. I was building it at about the same time as the SpaceLifter (next one). I was having trouble getting anything up into orbit and I was playing with HL chassis spaceplanes that somehow kept losing control or not going into orbit. One of the things I learnt with this design is that less can be more. Because the Mk4 chassis doesn't have any engine mounts at the rear, I had to use engine mounts on the wings. So I used lots of small SABRE engines mounted above and below the wing with a Eurofighter-style delta wing-plan. This worked better than mounting fewer SABRE M engines nearer the rear. Once I learnt this lesson, I went back and redesigned the SpaceLifter. I did transport cargo to orbit successfully with this, but the awkward loading bay at the rear caused an explosion in the Munar module (the Command module was fine) when somehow it interpenetrated with one of the sections. I think it was a bug. A reload later and the module blew up in the cargo hold. The SkyLifter didn't notice, though. At the time, I remember how it handled the weight beautifully and was a joy to fly. I've since revisited it and it's not nearly as good as I remember: getting it off the runway requires several attempts and it still has bad low altitude handling. I've never landed it successfully, as the explosion did take out the battery packs and it was a remote-guided test flight. Once I switched off the engines, it lost power and did the whole re-entry unguided. It was still flying when it spiralled gently into the ocean many minutes later, so a good design. But it would have been nice to land it. SpaceLifter (HL Chassis) Craft File Bizarrely, every time I flew this in a mission, the rear SABRE M blew up partway through the flight. Not only did this not affect the flight, it flew better, which again taught me a lot more about spaceplanes and how whacking more engines doesn't always generate more lift. I used this lifter to construct my prototype station and to work out how to attach cargo inside spaceplanes. Dart (S2 Personnel) My first personnel transport. In a test flight, I flew it up and docked it with my space station. I was really pleased with this design, but when I came back to it, it did flips of death on the runway every time it tried to take off. By this stage, I was confident enough of what I was doing that I went back and redesigned it to create the next spaceplane. Undine and Undine Plus (S2 Personnel) Craft File (Released Version) The Undine (don't ask me what led to the name, I don't really know, although I think I was playing lots of water-based people in League of Legends) was pretty much a redesign of the Dart and, because I vaguely know what I'm doing now and can look at the FAR derivatives and vaguely understand what they're telling me, it pretty much flew first time, after some landing gear adjustments. Hodo took me to task over the Entente's landing gear and, well, he had a point. This has beautiful landing gear layout. So there The only thing I didn't get right was yaw stability under sideslip. This I fixed by canting the wing-tips a little. This is the best craft that I've ever made in terms of handling and stability. Of course, the Undine is wonderful but its lack any range to actually get into any orbit greater than 80km. So I went and added an extra S2 fuel tank. By now I was starting to be very aware that changing one small thing can lead to an almost completely different design, which makes redesigning from scratch imperative sometimes. The Undine Plus was an example of this. It handles slightly worse than the Undine in nearly every single way and is prone to transsonic instability. It's still a good craft and it gets to 366km where my space-station is, so I'm happy, but I have the feeling that if I redesigned it completely, it would have been an impovement. Blended Wing Proto 2 This one I'm really proud of, although it hasn't got a name yet. I'm thinking of taking this to a release level and replacing the SpaceLifter as my small Heavy Lifter. It's completely experimental and came from being inspired by the NASA X-48B design and seeing that the B9 inspired procedural wing had an option to change the thickness. The idea of a blended wing is to mix the wing and the body so the body produces lift. In spaceplanes, which are pretty heavy, and in which the weight is concentrated in the middle, this can be exceptionally useful and can take a lot of strain off the wing-body junction as well as maximise lift. The awesomeness of this design is that it has really good handling characteristics at all altitude levels, from what I can tell. It has none of the problems of the Entente in that regard. The blended wing design also allows for easier attachment of landing gear in a manner that works sensibly, so this thing is the most stable thing I have ever put down a runway and takes off the best. But, despite all of this, it still has a 40m/s greater take-off speed than the Entente. Weird. X-693 Super-heavy Tanker Craft File (Non-released experimental version, so you can play, because who doesn't want to launch 500+ tons?) Because it had to be done, I descided to try to build the biggest, heaviest reusable spaceplane to take the most fuel into space that I could. Behold the X-693 Super-heavy tanker. The picture maybe doesn't do it justice, but those are the 3.75m large tanks from KW Rocketry. The thing weighs 518tons on take-off and holds a staggering 40,000ish amounts of fuel, of which 3/4 seem to get into LKO. I haven't actually got a use for such a beast yet, as my space station doesnt have enough tanking capacity and, in fact, probably never will. Drat. But, still, if I want to create a fuel dump at some point, it would be worth having. I'm also interested how it'll handle in DREC once I install that. The ballistic coeeficient should be small, so I think it should handle it, but... *fingers crossed* X-683a Piggyback Lifter Designed to deliver piggyback loads into orbit, particularly in this variant, four 3.75m fuel tanks. Do not pull up sharply (the wings will rip off), do not put too much power (the engines will blow up) and so on and so forth. Not really sold on this design at all, but the high wings give some nice stability and maybe I could redesign it to work. With care, I have gotten into orbit. Atmospheric Lander (Mk4 VTOL) I'm still not sold on this design. It's designed to be a jack-of-all-trades lander. Designed to fly (a little), land on airless moons (a little) and carry lots of equipment for EVA (a lot), it's the engineering cludge of doom. Maybe I can tweak it but at the moment I feel it's probably not going anywhere, unless I can move the rear engines onto the tailspike. Anyway, that concludes my tour of the random stuff I'm doing. If you read through that, hope you liked it and thank you.

I think you've been pretty clear and I like the idea. I was wondering if it was possible to create fly up boosters for spaceplanes and you've certainly shown that it's possible. Nice. Of course, I want to do this with B9 and really big spaceplanes

Oh, wow, I recognise that craft. I saw it before on the forums and thought wow. I think I've seen some of your earlier designs too, back when I started my first HL chassis cargo lifter, because I think these are new, right? I think I had come up with something similar at the time, although I hadn't quite worked out a lot of things yet (it was my first craft with FAR and I was till getting over the fact that my lovely AN-225 style wings had peeled straight off at Mach 1), and it really helped to see a design that worked from someone else that was similar but more polished. It inspired me a lot! If you don't mind me asking, what did you use to create the black fuselage on the 2.5M nacelles? I've been using the 2.5m KWRocketry tanks in a similar nacelle-driven design and they don't look quite right, but those look awesome and I've been dying to know since I've seen it. Oh, wait, you'll have posted the .craft files. My bad. I'll go find them, I think I know where. Sorry. But thank you for commenting on my craft. That's awesome. About the landing wheels. I know what you're trying to say. I know that you're right. I'm just trying to say that, despite that problem, it still lifts at 160m/s really well, even with the pivot effect. I have had other craft with the same wheel layout design and they do have the problem you describe, but not this one. Also, I created a similar craft but using a Blended Wing design. Because it was so easy to mount lots of landing gear, I could put them in straight lines clustered around the CoM as you said without buckling. Despite this, it still lifted at 200m/s. It was the most stable craft I ever sent down a runway, and took off the best, but still.... 40m/s faster take-off speed unloaded. I do not understand it at all...

Yeah, but did it flip around an axis defined by the line running between the single (front?) landing gear to one of the double (rear?) landing gear? If it did, then that's the dead giveaway. You might not even have time to notice the landing gear going, or the weight distribution could have caused that (or just some residual velocity). Some of my designs have gone that way If it didn't flip on that axis, then it's something else.

Thank you AmsterMan, and thank you Voculus Also thank you Hodo. The landing gear design isn't optimal, no. Unfortunately, it's the only way to spread the load enough. I wish there was some mod that created landing wheels in different sizes and shapes, but I haven't found one yet. However, if I went for a more traditional layout, this would create a lot of load on the runway. I know that KSP doesn't care about it, but I remember reading how the Skylon, a lighter craft with a traditional 3-gear system, has to have a reinforced runway to take off from with a 40 ton load. The Entente should not have this problem and should land and take-off on a commercial runway, which is good and important to me. I did experiment briefly, in another design, with playing with the steering on such a arrangement, getting the rear wheels to have their steering reversed and having the middle wheels respond to controls less than the front ones, much like lorries are designed to do and it was viable, although I don't know if it was practical. Running off the runway strangely doesn't seem to be a problem and it is one of the better craft I've designed for travelling down and getting off the runway. I didn't think it would either and I don't know why. I think it might be that the nose canards. They have a permanent 5 degree angle of attack. So this might be enough to pull the nose up and then activate the delta-wing effect. But I would not swear to this. Hmmm... I'd noticed the load moved my CoM forwards and that messed with the CoM/CoL arrangement, making the craft less responsive to pitch control. I had just written it off, but you might be right. *goes off and thinks* Thank you. *Is guilty* That's my one major worry about the reasonableness of this craft and you noticed that. The game doesn't seem to mind at the moment. I've never used nuclear engines. Why would I use them here? As you say, TWR doesn't have to be high in space and the craft works fine as it is, plus putting extra weight on there that I can't use is wasteful. So what's the benefit? Yey! Awesome! Any pictures?

Um, so, ever since I got KSP I've been mainly interested in building spaceplanes. They're supposed to be the path of the future and, if we're ever to get out there into space, supposedly we'll need them as the stage between rockets and the space elevator, or at least, that appears to be the consensus in sci-fi. Anyway, I've been playing around in KSP trying to build different versions of spaceplanes based on existing designs, to see how they would work and what's necessary to get a good spaceplane design. FAR has certainly made my life difficult, if instructive and I have decided that aerodynamics is officially evil. I've been looking around the forums drawing a little bit of inspiration from other people's designs and I thought that I'd post one that I haven't seen around the forums, which isn't to say it doesn't exist. Entente Inspiration: Concorde: https://en.wikipedia.org/wiki/Concorde The name comes from the squabble over Concorde's name. The English changed the name to the English spelling, Concord, over a perceived slight, but the science minister changed it back to the French, with the nationalist uproar dying down when he stated that the suffixed 'e' represented "Excellence, England, Europe and Entente (Cordiale)." Stats: Cargo Bay: 32m length Max Rated Cargo Weight (to LKO): 95 tons (probably could do 100, but someone braver than me can try that, 95 was struggle enough) Craft Weight: 276 tons History: I was trying to make a fuel tanker based on some successful delta-winged designs I'd made with a 16m cargo bay. Unfortunately, because fuel is very dense, the spaceplane wasn't long enough and the thing had not enough wing area, leading it to fall out of the sky at 10,000m, as well as not having enough mounting space for engines. Initially, I tried to compensate by adding in structural sections in between the fuel tanks, giving me a longer design than I was used to, so I could have bigger wings and more lift. So I went looking for inspiration and realised Concorde had the same long slender body, so I could use the same wing plan as an inspiration for my design. Unfortunately, I couldn't build the right wing plan using the traditional parts from B9, but then I discovered Procedural Wings. The tanker was too heavy to ever fly, but I realised if I replaced the tanks with cargo space, I'd have an awesome cargo-lifter. Craft: https://www.dropbox.com/s/pjkunifucj4b0k2/HL%20Entente%20V1_1%28Cargo%20Lifter%29.craft Mods required: B9 Spaceplane, Procedural Wings, MechJeb Slight warning: It's a bit big, so it might stress older machines. Mine's about average and it struggles a bit when it's fully loaded. Characteristics: The nicest thing about this vehicle is the massive cargo capacity and the very stable flight and low drag at very high velocity and altitude. Unfortunately, this is belied by the thing's absolutely dismal low-altitude handling. It bucks and it drops like crazy. Apparently, Concorde had the same problem and they fixed it by using a phenomenon with delta wings, which involves a vortex happening at high angle of attack. This is why Concorde has the really insanely high front landing gear. This effect does seem to be modelled in the game. This spaceplane takes off at about 40m/s less than a similar but much more stable design I tried, and will hold well if held at an angle of attack through the whole lower atmosphere. Otherwise, though, I have to use flaps to land the thing along with full engines, as the drag seems to make the thing more stable. In space, it's not very manoeuvrable and expends a lot of monopropellant. It has enough to rendezvous with a station if you want it and then come down. However, it's not really meant to be manoeuvrable and it's easy enough to push its cargo into orbit and then let a tug push the cargo where it is supposed to be. I haven't checked what orbits it can achieve, but it gets to 360,000km pretty comfortably with fuel to spare. Flight Profile: Take off: Take off speed is about 160m/s empty and it only goes up significantly if you really fill the cargo bay. It has a tendency to tail strike, so it's worth being careful on lift-off. If it's lightly loaded or empty, the engines are grossly over-powered, so be very careful of hitting high dynamic pressure. This happens almost immediately after take off, so the engines need to be throttled back, often to 30% if empty. 20-30 degrees lift is good for the first 10,000m, then reducing to 10 for the rest. Engines need to switch below 26,000 from air-breathing to rocket, although in practise it's rare to get the thing above 22,000m on air-breathing with any load, as the engines loose too much power and start to overheat. Space: The big rocket engines cannot handle full rocket thrust for any length of time. Manually limiting them after take-off is a must whenever I've done any MechJeb manoeuvres. 66.5 works well. Landing: The awesome high-altitude characteristics and it's sharp ballistic profile make re-entry a joy in this thing. The steeper the better, and use the flaps to bleed off velocity. Then pull up when you've had enough plummeting through the atmosphere like a dropped stone. Unfortunately, this is balanced by it's absolutely dismal low-altitude handling. I could never get the whole thing to fly well at low altitude and the only way I can land the thing is by using the rear wing flaps (key 5), landing gear and the engines on full power. The increased drag seems to increase the stability of the flight and allows a successful coast over the runway. Adding the full flaps and bleeding off the power allows it to be brought in. I have landed it in one piece once using the keyboard, otherwise I tend to take the bottom engines out. I'm sure that could be fixed with a few extra wheels, but meh. Keys: 1 - Switch Air-breathing/Rocket mode on the SABRE engines. 2 - Toggle cargo bay 3 - Toggle all flaps 4 - Toggle front flaps 5 - Toggle read flaps 6 - Toggle solar panels 7 - Toggle cargo bay lights

I can't tell you for certain, but there are two suggestions. I know on take-off, craft tend to flip if they're front heavy or if the front landing gear buckles. The front landing gear might not be strong enough when you touch down and the forces can cause it to buckle to the side, spinning the front out and sending the craft into a flip. Or, the fuel you're burning might have come from different tanks, leading to a completely different weight distribution on landing that might not be stable.