FAR/RP-0 Plane: Sequoia, aka the "turbojet-spam-that-somehow-miraculously-flies"

[SyzygyΣE]

Designing planes has always been my nemesis when it comes to KSP. I spend at least three times as long tweaking my planes than I do tweaking my rockets. I've just simply never really preferred planes over rockets, but for the sake of fulfilling contracts and allowing for better efficiency than launching non-recoverable rockets, I took on this challenge of designing a plane capable of surpassing Mach 1 early in my RP-0 career to earn some income.

The Sequoia features 51 parts amassing to a total of 32 tonnes. With 68 000m/s of dV, it is capable of a 14 hour flight. Its most striking characteristic is the addition of 11—yes, 11—turbojets, powering the vehicle up to a TWR of 0.76. I chose a large plane over a smaller jet because smaller planes tend to have greater oscillations, which I still don't yet know how to prevent.

Despite reading up on FAR guides and watching countless tutorials, when it gets to the actual design, I still have little idea how to proceed. But, at least this plane can fly, which is a rather significant milestone for me considering the dozens upon dozens of times I've crashed my SPH creations. 

If you have any suggestions as to how to improve on this design or designs for the future, please leave a message. I will appreciate any input.

Mods featured in slideshow:

• B9 Procedural Parts Modified
• Ferram Aerospace Research
• Procedural Parts
• RCS Build Aid
• Real Solar System
• Realism Overhaul mod collection
• Realistic Progression Zero
• Scatterer
• Texture Replacer

Edited July 13, 2016 by SyzygyΣE

[Van Disaster]

For starters, ditch half the engines - you don't need anything like that twr; not sure what thrust those give but won't 4 work? cute looking thing though. Kinda hard to tell without the FAR panels up but yor CoM might be a bit far aft.

Low TWR supersonic is mostly about drag reduction ( keep the cross section down and the craft long and thin ), low AoA which just keeps drag down in general, and patience to get to thin air.

Edited July 13, 2016 by Van Disaster

[SyzygyΣE]

10 minutes ago, Van Disaster said:

For starters, ditch half the engines - you don't need anything like that twr; not sure what thrust those give but won't 4 work? cute looking thing though. Kinda hard to tell without the FAR panels up but yor CoM might be a bit far aft.

Thanks for the reply. I'll see if I can get the FAR panels up for you later on. I don't really understand them so all I look for is the derivatives staying green. The TWR is to just surpass 350m/s for a contract/milestone. But that is one of my questions: how much TWR should there be on a plane? Does it vary depending on its purpose? My assumption is that the more TWR, the faster a plane can fly. Is that necessarily correct?

Edited July 13, 2016 by SyzygyΣE

[Van Disaster]

TWR is an odd figure for non-combat craft; basically in level flight thrust has to counter drag & wing area counters weight, TWR in essence determines climb rate ( and basic acceleration of course ). More wing area means more drag, so they're not completely independent. Less wing area for the same weight means higher AoA or flying in thicker air which is also more drag, so of course the less weight the better all round.

I built an endurance craft for a contest recently; its takeoff TWR is 0.17, but thanks to putting in a lot of work to keep it's drag down it's capable of M1.28.

[SyzygyΣE]

So is that why very fast planes like the X-15 have such small wings surfaces, to keep drag to a minimum? By weight, you don't mean the mass of the aircraft, do you? I think weight is something different but I'm really not sure. So, just to confirm, having more weight on the aircraft means the AoA required to maintain level flight increases, but by having a greater wing surface area, the AoA to maintain level flight can be reduced?

Edited July 13, 2016 by SyzygyΣE

[Van Disaster]

Weight is just mass * gravity - and while it's confused and misused often, in the case of lift weight *is* the right terminology and yes you had it right more or less: for the same airspeed in level flight, more wing area ( technically more lifting surface but let's not get hung up on body lift ) means lower AoA for the same weight. The term to describe it is "wing loading" ( weight per sq unit of wing ) which you might come across in aeronautical docs. Using mass & weight interchangeably might change once we start living on other bodies. Mass is a description of the composition of a thing, which by itself isn't really much use - it becomes useful when you're working out how the thing interacts with the universe.

The X-15 is a bit of a special case; it flew so fast that heat was a major concern in the design, and also by flying so fast each unit area of wing could produce an awful lot of lift ( so that kept the drag down ). Unfortunately that means the landing speed is quite ridiculous, but that's one of the compromises of aircraft design

There's a few types of drag, but the ones you need to think about at the moment are induced drag, and form drag. Induced drag comes from the process of generating lift, and form drag is drag due to the shape of the aircraft. Bigger wings will mean more induced drag as they generate more lift, and smaller wings will mean more form drag because a higher AoA means a larger cross-section if you look at the plane from the front. For supersonic craft there's also wave drag, which is a bit hard to describe simply ( it's minimised by keeping your craft cross-section as near to the same as possible, and minimising sudden changes in cross-section ) but at first don't worry too much about it.

So, initially, you need to have big enough wings that your AoA is low, and small enough wings that you don't have a lot of induced drag. Next thing to investigate after that would be shape, as there's more drag at the wingtips than the wing root, but don't overly worry about that when you're starting out.

[SyzygyΣE]

@Van Disaster

Ah I believe I get it now. That was a great explanation. I can probably keep note of it for future reference. 

I have the screenshots of the various FAR pages for Sequoia you asked for earlier. If you could have a check and see if something is out of the ordinary, that would be much appreciated. I excluded the lat/long simulations page because I'm really not sure what it's meant to do or how to use it properly. If you would like me to tweak the settings and attach some more screenshots, just inform me and I'll boot up the game to get them.

 

Edited July 13, 2016 by SyzygyΣE

[Van Disaster]

The first thing to take note of there, is AoA vs airspeed in the second panel; 118m/s is really quite fast for takeoff but you need 17 deg AoA to hold a steady altitude. That's not terrible if you were building something hypersonic like a spaceplane because the tradeoff would come with much reduced drag at altitude ( I don't consider outbound spaceplanes as aircraft, really ), but you're building something reasonably low speed. Try that again at say, 6km altitude and mach 1.15 ( that's 356m/s on Kerbin, might need to add a bit of altitude for Earth ) and see what the AoA says - and then try a few lower mach numbers because you still have to accelerate, and there may be a point where you have more drag than thrust & you'll never get any faster. 74m^2 reference area ( that's the lifting surface figure ) for a 32t supersonic aircraft is pretty reasonable, but given there's issues like wing shape to take into consideration ( delta wings of the same area as long straight wings will work differently at different speeds, for instance ) in the end you have to judge whether it's enough or too much at your target numbers. In your case, 350m/s.

All the other numbers look pretty reasonable; the yellow line in the first graph is quite important for stability, it's how much natural pitch force is in the plane ( which is dependent on where CoL is vs CoM at a given moment ). If it ever crosses the 0 line then the craft will naturally pitch up on it's own at that point, and it's not dynamically stable; not always bad, but something of an advanced topic you also want to be looking at all those with the gear pulled up ( hence the big Raise Gear button - don't forget to put it down again before launch! ).

I'd just try pulling most of the engines off; your takeoff run might take forever & it'll take a whle to climb, but you'll need less fuel( don't forget to remove fuel! ) and you'll have less dead weight also from having less engine, and you'll have less form drag too. Lastly, does it say 350m/s in level flight, or could you do it in a dive?

One other thing: it's been quite a while since my last RO install, do you have any 0.625m parts open? you can build a Juno powered SSTO plane in stock+FAR pretty easily, and that's obviously ( way past ) supersonic

Edited July 13, 2016 by Van Disaster

[SyzygyΣE]

9 minutes ago, Van Disaster said:

Try that again at say, 6km altitude and mach 1.15 ( that's 356m/s ) and see what the AoA says.

Unfortunately it can't seem to reach 350m/s while in level flight. I have to pull into a shallow dive and even that just barely scrapes it. The level flight top speed is about 310m/s. I'll definitely gather these for you but since it's getting late here, I'll do so in the morning.

I do wonder though, is there any rule as to what type of wing and what surface area would suit what purpose? So far when it comes to making wings I just plug in some random numbers and hope for the best. They usually all look like the ones on Sequoia, except either scaled up or scaled down. I'm generally not very comfortable with just adding random figures without knowing what they do—and unfortunately this applies to a lot of parameters when designing my planes.

[Van Disaster]

You can put the numbers into the second panel, you don't need to fly them

Wing shape is complicated & I don't know enough to properly summarise ( hopefully someone else can ); more slender wings have less induced drag ( and less CoL shift when you go supersonic ) but there's obvious structural issues - which we can ignore a lot more than reality - and as the cross-section is bigger, you'll get more drag that way. Sweep reduces the cross section without reducing length but you can get low-speed issues, taper as we've mentioned before is more efficient due to the roots being less draggy than the tips. The extreme of all of them is a delta wing, but you can't use flaps with a delta so landing speeds are high and at really high AoA - fortunately you have to have to have a *huge* AoA before a delta drops out of the sky. Ignoring hypersonics, generally some tapered, slightly swept wing of some sort works fine whether it's airliner-shaped or F-15 shaped, and yours seems ok. I just don't think you have enough of it for the craft mass!

[Empress Neptune]

4 hours ago, SyzygyΣE said:

Unfortunately it can't seem to reach 350m/s while in level flight. I have to pull into a shallow dive and even that just barely scrapes it. The level flight top speed is about 310m/s. I'll definitely gather these for you but since it's getting late here, I'll do so in the morning.

I do wonder though, is there any rule as to what type of wing and what surface area would suit what purpose? So far when it comes to making wings I just plug in some random numbers and hope for the best. They usually all look like the ones on Sequoia, except either scaled up or scaled down. I'm generally not very comfortable with just adding random figures without knowing what they do—and unfortunately this applies to a lot of parameters when designing my planes.

 

From the third image you posted showing FAR diagrams, you can see the effect of the many engines on your cross section. There is too much cross section in the middle of the plane. Your engines are giving you massive drag at high speed. You need better area ruling, this is a direct analogue to the classic F-102 Delta Dart problem:

 

https://en.wikipedia.org/wiki/Convair_F-102_Delta_Dagger#Design_and_development

 

The plane was not shaped properly and couldn't go supersonic. Ideally you want your green cross section curve to be smooth for supersonic flight. This should help you get past 310 m/s. Your nose also looks blunt. Could you try a thinner and pointier nose?

 

With wings you want enough area to hit your best L/D at cruise speed, and as much aspect ratio as possible for efficiency across the board. However when you go supersonic, you are forced to limit your aspect ratio because of area ruling and shockwaves. If your goal is Mach 1, you will want moderate sweep and a mid-low aspect ratio probably between 3 and 4. Your current wingspan may be too much for the length of your aircraft as it is now. The shape look good, perhaps try clipping off the wingtips bit by bit until performance improves. If you do this while removing away engines, you shouldn't see too many negative effects.

 

A final note, I've never used FAR, so I don't know how it works specifically. I'm generalizing based on real aircraft design.

[Van Disaster]

I would make the craft longer before removing wing, tbh - that is a pretty stubby craft, and structural fuselage is lightweight. I was also trying to avoid wave drag until he'd got the relationships between normal lift/drag sorted a bit

You can apply real design principles to FAR, although you're limited to one aerofoil so no supercritical wings or anything, and there's no spanwise airflow effects ( there are some wingtip effects, I forget how much ). Also no ground effect.

Edited July 13, 2016 by Van Disaster

[Empress Neptune]

9 minutes ago, Van Disaster said:

I would make the craft longer before removing wing, tbh - that is a pretty stubby craft, and structural fuselage is lightweight. I was also trying to avoid wave drag until he'd got the relationships between normal lift/drag sorted a bit

You can apply real design principles to FAR, although you're limited to one aerofoil so no supercritical wings or anything, and there's no spanwise airflow effects ( there are some wingtip effects, I forget how much ). Also no ground effect.

True, lengthening the plane might be the better way to go, especially if it doesn't add much weight. Maybe the weight and AoA of the plane without the excess engines should be posted to make it clearer which is the best route to go down. Sorry about jumping to wave drag, but I saw that the goal for the plane was supersonic flight, so I thought it would be helpful to know. The area graph will be really important for this project.

 

Thanks for the info on FAR. I want to try it eventually, but I don't feel like I've "finished" stock KSP yet.

[Van Disaster]

A couple more things for the OP:

* Add a degree or two of incidence to the wing ( ie, tilt it upwards slightly ): you'll get more induced drag because the AoA of the wing is higher ( also more lift, of course ) but the fuselage AoA will be lower at the same speed. Ideally you want near zero AoA on anything but lifting surfaces at your optimal speed, so it needs fiddling with. You get a little lift from every part at an angle to the airflow, lifting surface or not, but again you have to balance out if that tilt is worth the extra drag.

* Dammit, I forgot. But yes, it does sound like you're running into the sound barrier - considering your excessive thrust you might have to take note of wave drag a little sooner rather than later.

If you go read up on drag & it's sources you'll find I've simplified a considerable amount ( and mangled a bit, especially with induced drag & AoA ) but those were basic ideas to get you thinking about wing/lift tradeoff with weight.

[SyzygyΣE]

9 hours ago, Van Disaster said:

One other thing: it's been quite a while since my last RO install, do you have any 0.625m parts open? you can build a Juno powered SSTO plane in stock+FAR pretty easily, and that's obviously ( way past ) supersonic

The turbojets I'm using for the plane right now is 0.625m, I believe. I also have Tweakscale installed so I can modify some parameters.

Edited July 14, 2016 by SyzygyΣE

[Van Disaster]

In which case, can you build an unmanned craft at the stage you're at? if so, then put a long 0.625m tube on the front of a turbojet with a pointy nose, add an intake, add enough wings to get it in the air, and you've probably completed the contract. Also, now you've got a drone for research

That was an investigatory spaceplane, you can get an idea of the scale from the small landing gear.

Anyway, before jumping on that try refining your current craft.

[SyzygyΣE]

3 hours ago, Van Disaster said:

In which case, can you build an unmanned craft at the stage you're at? if so, then put a long 0.625m tube on the front of a turbojet with a pointy nose, add an intake, add enough wings to get it in the air, and you've probably completed the contract. Also, now you've got a drone for research

That's certainly a plausible idea, but since I'm playing with RemoteTech as well, if I stray too far from my launch site I may lose connection. Close proximity flights shouldn't be a problem though.

On another note, I have the FAR screenshot of 6km and Mach 1.15. I raised the gear because there isn't really any reason why I would have gears lowered at that speed.

Also, just another quick question, what does Mass-Strength Multiplier % in wings and control surfaces actually do? I always turn it down to 0.25 because I once saw it being done in a tutorial but apart from shifting mass—which it seems to do—what is it used for?

Edited July 14, 2016 by SyzygyΣE

[Van Disaster]

Mass-strength multiplier affects joint strength, I'd be wary of turning it down too far on anything remotely aerobatic, and control surfaces. Well, those are pretty nice numbers - I was expecting AoA to be around that but really needed to check it wasn't something ludicrous - just need to get the Cd down.

I installed AJE to see what the engines were like - you're using the J85, yes? I ( fairly literally ) threw a craft together to try them - it's aerodynamics are horrible, but it does have reasonably wave drag, and so with four engines it's hitting 390m/s at 7km-ish ( on Kerbin, so that's about 9-10 on Earth ). The wing shape is pretty awful ( the trailing edge especially ) but it was a 10 min test effort...

Wave drag - not going to explain what it is, you can look that up yourself, but it's minimized by keeping the rate of change of cross-section down the craft as low as possible. A long nosecone will not only change cross-section in small steps if you slice it up from the point backwards, how much each change differs from the previous one will also be small. Blunt shapes change very quickly, and those you need to avoid. Remember you're taking the entire cross-section of the craft as one, so if you stuck two engine pods parallel with each other they'll count as one big engine pod ( and as an even bigger fuselage ). The back edge of my wing is a bit of a nono, it needs to be swept a bit so the thickness of the wings don't disappear all in one slice ( you'll notice a clipped tank behind the wings, that's to help change the cross-section profile more gently ). The thinner the wings the less you have to worry about that, of course - and sweep too, some highspeed aircraft can have straight wings because they're extremely thin ( not sure how well that works with FAR's wing profile ).

I'll try and do it on two at some point, should be manageable with a lot more area ruling.

Edited July 14, 2016 by Van Disaster

[SyzygyΣE]

@Van Disaster

Wow those graphics are beautiful. I wish my laptop could run that without crashing instantly. 

Anyway, I actually don't have the J85. It's true I use AJE but I think playing with RP-0 modifies it a bit. The engines on my plane is Rolls-Royce Nene Mk.102. That's the best one I have for the amount I've progressed in the tech tree. So, with the Mass-Strength Multiplier, does it vary based on the purpose of the plane? I'm assuming for slow flying, subsonic planes, there's no need to turn it up high, but for supersonic planes were you may need to make a sharp turn or something of that nature, it's better to turn it up for greater joint strength?

On another note, I started over and tried to apply what you and @Empress Neptune said in order to design a smaller and not-so-complex plane. I'm still pretty much a beginner at FAR but at least I understand the general principles better with your explanations. And, good news: after tweaking the new plane for hours finally I can reach 350m/s in level flight. Since it is a new plane, I've created a new thread for it. You are welcome to have a look and see if there are some factors I haven't performed the best in. The two of you have been incredibly helpful. 

[Van Disaster]

I'd think the Nene is roughly equivalent ( 20ish kN thrust ). Joint strength matters mostly with transient loads, and highly aerobatic craft which load up large g-forces ( like, say, combat craft or less structurally sound high-speed craft ). You'll probably never need it above 0.6 unless you build something that pulls 25g at mach 1... I usually put it down to 0.3 for general use just in case the craft ends up spinning or something equally violent, and usually going lower doesn't really save that much.

Edited July 14, 2016 by Van Disaster