Brikoleur

[Stock] Brikoleur's Guide to VTOL Aircraft

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Level: Intermediate/Advanced: You need to be able to slap together a plane that flies reasonably well before attempting a VTOL.
Background reading: Start with the fantastic Basic Aircraft Design tutorial in this very forum.
Craft used to illustrate this tutorial:

What's a VTOL aircraft?

VTOL stands for "Vertical Take-Off and Landing." A VTOL aircraft as discussed here is a craft that's designed to fly aerodynamically, using lift produced by lifting surfaces, but take off and land vertically. That's what this guide is all about, so we're not talking about VTOL rockets that don't make use of wings to produce lift.

We're also not discussing helicopters here, because stock kerbals have not invented the propeller, and stock propellers are a whole big topic of their own. So this guide is about atmospheric craft designed to fly by making use of lift generated by wings, which can take off and land vertically by use of downward-pointing jets or rockets. 

This guide also applies to STOL (Short Take-Off and Landing) aircraft which do their thing using downward-pointing jets or rockets, because they're pretty much the same thing. Their hoverjets just have a TWR of less than 1.0.  

Why VTOL?

Because they're fun and educational and you can. Next question?

No, seriously. Is there a point?

There are a few missions for which a VTOL aircraft is ideal. Kerbin has some biomes that are difficult to reach any other way. The same applies to Laythe, although it has gentler topography. Finally, it is really difficult to land a HTOL atmospheric craft on Duna because of the thin air: you'll be going really fast and terrain is really bumpy, so there's a huge risk of ending up as a big ball of fire, whereas it's very hard to land a conventional rocket lander precisely, like when you're aiming for your surface base. On the other hand, atmospheric craft are superb for exploring it for the very same reason – you can scout for the perfect spot for your base, then land precisely there. A V/STOL atmospheric craft built for Duna can drop you on any dime, anywhere on the surface.

But mostly, the answer is still "because they're fun and educational and you can."

yhOctmj.png

The BAK Cyclone hard at work on Duna. It's a flatbed freighter suitable for shuttling base modules to and from the surface. The cargo is near the centre of mass, but because it can shift, it's important to adjust the exact balance by tuning the power on the nose hoverjet...

The basics

At its core, a VTOL aircraft is a plain old aircraft, with downward-pointing jets that produce a TWR of > 1.0 with the vector centred on the craft's centre of mass, and some way of controlling its attitude when it is hovering, because control surfaces do nothing at an airspeed of zero. 

Getting all of this into one craft is a pretty intricate business, however. In particular, there's one constraint that needs special attention: centre of mass, and the invariance thereof, as you burn fuel. In other words, your fuel tanks need to be placed symmetrically around the centre of mass so it doesn't shift as the tanks dry, and you need to get your vertical thrust vector exactly aligned with said centre of mass. Regular HTOL aircraft can afford to be a bit sloppy with this because aerodynamic forces will effectively obliterate moderate shifts in CoM -- if your plane gets a bit more tail-happy as the tanks drain it's no problem, as long as your CoM stays ahead of your CoL. Mostly anyway. Not so with VTOLs: if the CoM shifts, you're not going to be able to land vertically anymore.  

Here's how you go about building a VTOL under these constraints.

  1. Build yourself a plane. However, don't put any fuel tanks on it yet, and empty any fuel-containing parts that you are using.
  2. Switch on the CoM and CoT overlays.
  3. Set the thrust limiter on your main engines to zero. Your CoT vector will disappear.
  4. Add enough downward-pointing jets to lift the plane, as symmetrically as you can around the CoM, in a minimum of two pods (fore and aft). (You can add more pods to the sides if your body plan permits it.) 
  5. Adjust the thrust limiter on the fore (or aft) hoverjets until the thrust vector lines up with the CoM.
  6. Add fuel tanks symmetrically around the CoM.
  7. Add RCS jets to the bottom of the craft, at the nose, tail, and wingtips. Don't forget the fuel – Vernors need oxidant, the others need monoprop. (If you're building a very small craft, you can just use a reaction wheel instead. But that's less cool.)
  8. Set up your control scheme: one action group for toggling the hover jets, another action group for toggling the main jets, plus yet another one to toggle the hover jet bays, if you're using them (as you should).

There, done. Simple, eh?

Hoverjet design

The first challenge you're likely to hit is choice of hoverjet. The second one is likely to be aerodynamics – if you just stick on some downward-pointing jets, you will find that they produce a lot of drag, which is going to be really inefficient. Your plane will be slow and have limited range, or you'll have to make it a lot bigger to brute-force your way around that limitation. The solution is to house the hoverjets in a cargo bay of some kind, with the doors opening downwards. That way you can tuck them away for normal flight, and expose them for hovering. There are lots of ways to make this work, but here are some designs I've used successfully:

  1. Juno in a Mk 1 utility bay. Stick it inside the utility bay, rotate it to point towards an opening, move it until it's completely inside.  These are easy, pretty light, and you can add more of them – within reason – for more lifting power.
  2. Array of Junos in a Mk 2 cargo bay. This needs scaffolding: you need to put something in the cargo bay that lets you attach the Junos to it. A short Mk 2 bay will fit an array of 9 Junos, and a long Mk 2 bay will fit 18. That's a lot of lifting power – three Wheesleys' worth in the bigger bay! Also a lot of parts. I hope you have a fast computer.

For rocket-powered hover, use Spark, Aerospike, or Vector (if you really need a lot of hover power). Terriers will also work on Duna. Sparks will fit in Mk 1 utility bays, the bigger ones will fit in the bigger cargo bays (Mk 2, 2.5m utility bay, Mk 3).

Giving them air

Air-breathing hoverjets need intakes.  At this point you'll probably need to go back to the plane design you started with, because air intakes are dry mass and will shift the CoM as you add them. 

Hint: The engine pre-cooler and engine nacelle are fantastic air intakes, and they can be mounted in-line or combined with other elements. You don't have to use their fuel capacity – you might want to leave them dry if they're not symmetrical to the CoM.

Hover control

The main challenge for hover control is to keep the craft horizontal. If it starts tipping in one direction, you're really likely to flip over and crash dramatically, like a tree falling over. If additionally you can give it a controlled tilt and hold it there, then it'll start accelerating in that direction, like a helicopter. This can be most helpful when transitioning to or from level flight.

Option 1: RCS

RCS will get the job done nicely, and looks cool to boot. You will need more jets at the nose and tail than on the wingtips, as there will be more forces on pitch when transitioning to or from level flight. Your choice of RCS jet is the Place-Anywhere or the Vernor. You may need to add several on bigger craft. 

Option 2: Reaction wheels

Reaction wheels will balance smaller craft just fine, but are probably insufficient for bigger ones.
 

Managing centre of mass

One of the most finicky problems with VTOL craft is managing centre of mass. In principle it's simple – just place your fuel symmetrically around the dry CoM, and centre your vertical thrust vector on it – but... how?

  1. Use wing-mounted engine pods on pylons. Engines are dry mass. Mount them on pylons on the wing, and it's easy to move them forward and back to fine-tune the CoM.
  2. Put fuel tanks outside your main stack. Wing-mounted tanks, wingtip tanks, drop tanks, and side-mounted tanks flush with the body all work, as long as they can be moved backwards and forwards relative to the dry CoM. If you don't mind a bit of clipping, you can even make the latter look pretty good by clipping them a bit in the body. It makes no functional difference, but if you consider it cheating, don't do it. 
  3. Use a long, light tail section. Long tails are good for stability anyway. If you make a long, light tail, you can adjust the balance of the craft by making it slightly longer or shorter without adding a lot of weight or making big design changes.

Body plans

I've found a few body plans to be especially amenable to conversion to VTOL. They have in common that it's easy to tweak the balance by moving things around, rather than having to add or remove pieces.

Twin-boom

The twin-boom design is one of my favourites, largely because it looks cool. In a twin-boom design, you have one hoverjet at the nose, and one in each of the booms. Light craft have a single engine at the rear of the fuselage. Larger ones have additional wing-mounted pods.

jQxygKz.jpg 

The BAK Karmilla. This one is balanced with reaction wheels. It uses six Mk 1 utility bay-mounted Junos for hovering.

eSF9l34.jpg

The BAK Drakula. A bigger twin-boom design using two arrays of 18 Junos on each boom and a single array of 9 on the nose.

Twin-Pod

A twin-pod design is similar to a twin-boom, except that it has a conventional tail extending from the fuselage. The hoverjets are housed in the big wing-mounted pods.

Ddflwhe.png

The BAK Zephyr, a rocket-powered VTOL craft designed for conducting science missions on Duna. It is entirely powered by Terriers. The absurdly big wing and control surfaces make it highly economical for high-altitude supercruising.

wyAHoF3.png 

The BAK Cyclone, delivering a station module to Duna. Note the landing area markers. The Cyclone uses Aerospikes for propulsion. Rockets are much less efficient than air-breathers, so it needs to be much bigger than a Kerbin-bound craft performing the same mission!

Control schemes and flight

To fly a VTOL craft, you need to be able to perform the following actions, which must be bound to a an action group:

  1. Toggle the hover jets
  2. Toggle the forward jets
  3. Control attitude

If you have full RCS control, you will additionally need control for that, and if your hoverjets are inside pods, you will want a control for toggling them too. 

Taking off

The procedure for a vertical take-off is as follows:

  1. Hoverjet pods OPEN
  2. Forward jets OFF
  3. RCS ON
  4. SAS ON
  5. Hoverjets ON
  6. Throttle MAXIMUM
  7. When off the ground at a sufficient altitude to clear obstacles, main jets ON
  8. When at sufficient speed for aerodynamic flight, hoverjets OFF, pods CLOSED, gear UP

The procedure for a short take-off is the same, except that forward jets and hoverjets will both be ON from the start. The craft will lift off once generated lift + hoverjet thrust overcome its mass.

Landing

To land a VTOL aircraft, approach the landing zone as you would with a regular HTOL craft, until on final approach. Then:

  1. Hoverjet pods OPEN
  2. Gear DOWN
  3. Throttle ZERO
  4. Main jets OFF
  5. Hoverjets ON
  6. Keep pitching up as you approach stall speed. When you're close to it, INCREASE THROTTLE until your rate of descent nears zero. Your airspeed will also fall.
  7. When your airspeed is low enough that aerodynamic control is getting sluggish, RCS ON, SAS ON.
  8. Control your vector primarily with pitch, and your descent rate with throttle.
  9. When your airspeed is near zero and you're above your landing spot, reduce throttle until you start descending. 
  10. Touch down, CUT throttle, CUT engines, BRAKES ON. You've landed.

pic67AX.png

...and that's it really!

I hope you've found this short tutorial useful. Have fun with your S/VTOL craft – and don't forget there are more ways to do them as well, including helicopter-like things that don't fly aerodynamically at all. My first VTOL craft was the Bumblebee, and it's still one of my favourites!

W2vlzZg.png

 

Edited by Brikoleur

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OK, I have a really pedantic question here:

Is it even possible to have a practical VTOL craft without using the unlimited fuel cheat? From my experience VTOLs guzzle fuel and trying to add more fuel means you need to add more thrust...

I've never found them to be practical (at least on Kerbin) for anything except short flights around the KSC and flying to the island runway and back.

Also, a good tip: If you use a Panther as a lifting engine, and link the 'Dry/Wet' control to the Stage action group, it allows you to have a lot of control over your lifting thrust (by toggling the Space bar). Jets are good for a certain amount of static thrust but they don't allow quick control of your vertical speed because of their slow spool time.

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My Hats off to Brikoleur, as a fellow VTOL/general spaceplane [stock] enthusiast, your tips were easy to read and right on the money!

 

You're one of the few I've read that recognized the potential of Junos in vtol vs the more popular panthers or using rotating clamp-o-tron assemblies - bigger engines have the huge design downside of shifting COM above fuselage longitudinal axis, which makes rocket placement difficult for SSTOs.  

The juno 8-pack in a mk2 bay is my go-to, plus, a single Engine nacelle will provide enough air for them all and fuel is self-contained, making the design easy to scale for larger craft.  

 

One challenge I've run into for myself with more elegant VTOL SSTOs is low-speed-hover controllability;  beyond using brute-force RCS or SAS.  I can make a design that's quite good at hovering, landing, and maneuvering around a site like a helicopter, but it'll be crap as an actual aerodynamic plane- as the control surfaces and wing setup are designed for forward flight - and for hovering I want something that has no forward-bias (ever started sliding backwards in a vtol craft designed for otherwise normal forward flight?  Things get ugly!!).   Any insights?

 

Quote

Is it even possible to have a practical VTOL craft without using the unlimited fuel cheat? From my experience VTOLs guzzle fuel and trying to add more fuel means you need to add more thrust...

Absolutely!  I have a simple Vtol SSTO that can take off with junos, hit LKO with rapiers, maybe with 100 m/s left on the rocket side, then return to spaceport and have sufficient liquid fuel for vertical landing.  Junos, Panthers, and Wheesleys are all fairly efficient at low altitude and low speed - when you said 'guzzle' i screwed up my face because when I think guzzle I think rapiers and whiplash's putting out 300 kN and just gorging on fuel.  Ill link it if interested.

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On 12/7/2018 at 1:02 PM, Stoney3K said:

Is it even possible to have a practical VTOL craft without using the unlimited fuel cheat? From my experience VTOLs guzzle fuel and trying to add more fuel means you need to add more thrust...

LTTP but anyway: the question is, practical for what?

It is certainly possible to make VTOLs that can circumnavigate the planet. Of course the cargo capacity is less than with a HTOL since you're using some of it to haul those vertical thrusters. Hovering is expensive but you don't really need all that many seconds of hover either for take-off or for landing.

I have also used rocket-powered VTOLs on Duna. You need about 150 m/s for the landing, less than that for take-off. They're obviously bigger and less efficient than conventional landers again because of the extra engines you're hauling; the advantage is being able to land them exactly where you want, while having the capability to explore the planet with atmospheric flight. 

So yes, it most definitely is possible. Whether VTOL craft are practical or efficient compared to other solutions is a different matter -- and to be perfectly honest, I'm inclined to think... not really. But they are tons of fun both to design and to fly, which for me is kind of the point of KSP.

Edited by Brikoleur

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Really nice overview and some incredible looking craft!  I have made a couple VTOLs in the past and had a lot of fun with both the engineering and flight challenges.  I've been having some trouble lately in newer with respect to air flow.  For the vertical engines, do you also need to have some vertical intakes?  I have not had much success using only horizontally oriented intakes - the engines do not have sufficient air flow to generate enough thrust to actually lift the craft.  Adding a vertical intake helps.  Have you noticed this or do you think I might just need more vertical engines when using horizontal intakes only?

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LTTP again (KSP on hiatus for a bit), but here's an answer anyway. No, the intakes don't have to be vertical. You do need rather a lot of them. Adding more engines without adding intakes will make the problem worse. If you're going for full VTOL you need to have a TWR > 1.0 at take-off weight; adding more vertical engines will make vertical landings easier and (if they're air-breathers) will make it possible to operate at higher elevations. You need enough intakes to keep them fed. You know you don't have enough if you're getting engine flame-outs on take-off or, worse, landing. So if you're getting those flame-outs, add more intakes until you don't get them anymore.

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Hi @Brikoleur, nice work! I have some really good tips and tricks to add to your tutorial, but I’m on my phone at the moment. I’ll edit this sometime this week.

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This would certainly have to be revised once Breaking Ground drops, those parts are extremely relevant for VTOL.

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Thanks to this guide I did my first ever landing on the VAB helipad! :)

On 11/1/2018 at 10:54 AM, Brikoleur said:

a craft that's designed to fly aerodynamically, using lift produced by lifting surfaces

In that vein I drag optimized the Karmilla, and added science parts. The downside is that it isn't as pretty, but it *does* top out at mach 1.8 with a humble wheesly. After the drag was sorted out, it wasn't very hard to create a dockable SSTO version.

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