Rotor Kites as Alternatives to Parachutes

[Kerbin Dallas Multipass]

@Mattasmack

That proves my point

I don't see why a capsule with a rotor would not be able to convert downwards speed into forward speed. It's the same principle after all.

If adding weight to the actual rotor blades helps autorotation landings... lets put weight on them! Rocket fuel!

Let's store the rocket fuel for the last few meters of descent in the hollow blades. This way we get full benefit of autorotation all the way down and we get slowed down to a relatively slow speed. Then we fire the rockets in the blades to get a perfect landing. We get away with much smaller blades and less weight because the effects combine. We also still don't need a tail rotor because both autorotation and rocket powered blades are torqueless.

[shynung]

For something like an Orion capsule with rotors, the fact that the rotors wouldn't extend much past the body of the capsule itself (i.e. the capsule would block most of the 'driving region' of the rotors) is an additional problem for vertical autorotation.

If the blades could be made to be telescopic or otherwise fold itself along its length, this issue could be avoided entirely, at the cost of further complexity.

Though, if we're going to go tipjet rotors anyway, I have an idea: use monopropellant. Engines using them are relatively simple, and have been made in similar size as RCS thrusters.

[Mattasmack]

@Mattasmack

That proves my point

I don't see why a capsule with a rotor would not be able to convert downwards speed into forward speed. It's the same principle after all.

If adding weight to the actual rotor blades helps autorotation landings... lets put weight on them! Rocket fuel!

Let's store the rocket fuel for the last few meters of descent in the hollow blades. This way we get full benefit of autorotation all the way down and we get slowed down to a relatively slow speed. Then we fire the rockets in the blades to get a perfect landing. We get away with much smaller blades and less weight because the effects combine. We also still don't need a tail rotor because both autorotation and rocket powered blades are torqueless.

Sure, and the reentering capsule sure doesn't have a problem with starting near the ground. But how well do you think something shaped like a capsule with stubby little rotors will autorotate? My guess is, not very well.

I agree about putting rockets on the rotor tips for power. Its the most sensible way of powering the rotors, since it doesn't require counteracting any significant torque on the capsule body. (And I said so too a page or two back.) The high-inertia rotors were a concept for easing the transition to autorotation; if you're powering the rotors for landing there's no particular need for extra mass there. I don't know if there's otherwise any advantage to holding the fuel in the rotors rather than elsewhere.

And now we're getting back to a Soyuz. It descends passively (unpowered) under parachutes, but its descent speed is too high for a comfortable landing so it adds rockets to slow it down at the last second. Now you have a system that descends passively under rotors, but needs rockets to slow it down at the last second.

Shynung earlier said that an advantage of the rotors is that it gives more control over where the capsule lands ... but how much control there is depends on the glide ratio of the autorotating capsule, and I think that will be lousy. Heck, at one point there was a concept of having a Gemini capsule return under a parasail, and I think that would do better and could be completely unpowered.

If the blades could be made to be telescopic or otherwise fold itself along its length, this issue could be avoided entirely, at the cost of further complexity.

Though, if we're going to go tipjet rotors anyway, I have an idea: use monopropellant. Engines using them are relatively simple, and have been made in similar size as RCS thrusters.

Hmm, has there ever been a helicopter (or similar vehicle) with telescoping or hinged rotors? I'm not aware of any. I assume it can be done, but it doesn't sound easy.

Monopropellant, sure. Roton's test vehicle used hydrogen peroxide tip rockets, and I think Armadillo Aerospace experimented a bit with them as well, though they never got close to putting them on a vehicle.

[Tarantulae]

I only just recently stumbled across the fact that you can autorotate in KSP. When I googled to see if this was a novel discovery, I learned that it, sadly, was not. However I found this thread. I am a helicopter pilot. Kerbin Dallas Multipass you are correct that autorotation can be done with no forward motion. The height-velocity diagram you posted is a chart that is produced and issued in the manufacturer handbook for helicopters. It shows you what profile to take on takeoff so that in the case of an engine failure, you can autorotate to a successful landing. It is not a landing chart, it is for takeoff.

Normal autorotations are a trade of energy. There is no torque on the system because the air you are falling through provides the turning force to the rotor disk. You are trading your potential energy in the form of altitude for lift which is used to keep the blades turning. Autorotations are done with forward speed so that when you land, you can trade your momentum forward for more energy to use to cushion the landing.

The problem I see with a capsule based rotor system, is that helicopter blades stall below a certain RPM. If the capsule is reentering the atmosphere, the rotors would have to be spun up using some system to above stall speed to provide meaningful lift. Rotor blades also get a lot of strength from their RPM. If the capsule was falling in atmosphere when the blades were deployed, they would likely bend up and not be able to spin up to a useful RPM. Likewise, deploying them before entering the atmosphere would mean they would burn up in the upper atmosphere long before they had enough air to provide slowing lift.

[shynung]

The problem I see with a capsule based rotor system, is that helicopter blades stall below a certain RPM. If the capsule is reentering the atmosphere, the rotors would have to be spun up using some system to above stall speed to provide meaningful lift. Rotor blades also get a lot of strength from their RPM. If the capsule was falling in atmosphere when the blades were deployed, they would likely bend up and not be able to spin up to a useful RPM. Likewise, deploying them before entering the atmosphere would mean they would burn up in the upper atmosphere long before they had enough air to provide slowing lift.

Deploying rotor systems before entering the atmosphere is just as effective as deploying a parachute in the same place; not only it would not survive reentry, they'd probably break off the capsule long before it. The rotors are meant to be deployed deep into the atmosphere, after the heatshield has done its job.

If the rotor could be feathered, it is possible to deploy them at high speeds with less bending, but this requires even more complicated engineering. Alternatively, tipjets on the rotor blades could pre-spin it at deployment, then let it autorotate after reaching a useful RPM. At final approach, said tipjets could power the rotor for some seconds of powered flight, to cushion the landing.

[K^2]

That proves my point. The reason you need altitude if you lack forward motion, is that you use that altitude to gain the forward motion! Inside the red area on the left you don't have enough height to get up to the auto-rotation airspeed, and you just fall out of the sky.

Forward air speed isn't necessary for auto-rotation in general. It's necessary for helicopters, because of the fairly high loading and the vortex state. It's the same reason why you should never increase collective sharply while in hover. If you are designing something specifically for descent, however, you can build a rotor that will auto-rotate just fine without any horizontal speed.

And yeah, with a rotor, you can flare before hitting the ground, so you can have a gentle landing even if your descent speed is pretty high. But to be honest, parachutes with retro-rockets seem more reliable.

[Seret]

Purely for burning off kinetic energy I don't really see rotors offering any major advantage over a simple system like parachutes and retro rockets. Where it would come into its own is if you wanted a system that offered the ability to pick your landing spot, although that also increases the weight and complexity required in your rotor hub. Might be useful for precision landing probes on planets with an atmosphere though.

[bs1110101]

Here's a thought, put the rcs on the tips of the rotors, they can be used as tip jets on landing and the rotors can be used in space to make rotating more efficient.

[Tarantulae]

If the rotor could be feathered, it is possible to deploy them at high speeds with less bending, but this requires even more complicated engineering. Alternatively, tipjets on the rotor blades could pre-spin it at deployment, then let it autorotate after reaching a useful RPM. At final approach, said tipjets could power the rotor for some seconds of powered flight, to cushion the landing.

The rotor would have to have the ability to be feathered to control the rate of descent, but usually this is limited to some degree, not a full 90º. I'm not sure how well trying to enter autorotation from feathered non-rotating blades works though.

Forward air speed isn't necessary for auto-rotation in general. It's necessary for helicopters, because of the fairly high loading and the vortex state. It's the same reason why you should never increase collective sharply while in hover. If you are designing something specifically for descent, however, you can build a rotor that will auto-rotate just fine without any horizontal speed.

And yeah, with a rotor, you can flare before hitting the ground, so you can have a gentle landing even if your descent speed is pretty high. But to be honest, parachutes with retro-rockets seem more reliable.

Sorry, but it is not necessary for helicopters to be moving forward to autorotate. It is better to have forward airspeed so that you can convert the momentum into rotor rpm to flare at the end for a softer landing. Vortex ring state only is a concern when power is applied to the rotor system, it is not a concern during autorotation. I do agree that parachutes with rockets is a simpler more reliable system that would also be lighter with less chance for error.

[astropapi1]

So, today I tested this out...

Turns out it's a really neat idea. Realism aside, I managed to fly it around the KSC and land it on top of the control tower with more than enough electric charge.

[shynung]

So, today I tested this out...

http://i.imgur.com/USCu9Ka.png

Turns out it's a really neat idea. Realism aside, I managed to fly it around the KSC and land it on top of the control tower with more than enough electric charge.

Cool! How did you make this? Firespitter?

[astropapi1]

Cool! How did you make this? Firespitter?

Yep! The retractable electric rotor, to be exact.

[K^2]

Sorry, but it is not necessary for helicopters to be moving forward to autorotate. It is better to have forward airspeed so that you can convert the momentum into rotor rpm to flare at the end for a softer landing. Vortex ring state only is a concern when power is applied to the rotor system, it is not a concern during autorotation. I do agree that parachutes with rockets is a simpler more reliable system that would also be lighter with less chance for error.

You don't get settling with power, because you need power for that, but you always have a vortex ring. When autorotating, it eats into your driven region. If you try to offset that with the collective, you stall your rotor. So your sink rate is going to be significantly higher if you don't have any horizontal motion.

[Javster]

Yep! The retractable electric rotor, to be exact.

Why did I not think of this? I MUST TRY!

[Tarantulae]

You don't get settling with power, because you need power for that, but you always have a vortex ring. When autorotating, it eats into your driven region. If you try to offset that with the collective, you stall your rotor. So your sink rate is going to be significantly higher if you don't have any horizontal motion.

Adding collective would cause your driven region to increase, while reducing your driving region. It also will greatly increase drag, which is what will stall your rotor once the RPM reduces enough.

Yes, forward airspeed helps you descend slower, to a point. From what I know, most helicopters it is around 40-70 knots. Also, if you are in a vertical autorotation, you can still increase your airspeed as you autorotate for the landing. Its not like you are stuck in vertical or forward autorotation with no chance to change it.

[K^2]

Of course. The question was, why it's recommended for helicopters to autorotate with some minimal forward speed, and if that's going to be a problem for rotor kite approach to capsule landing, which is what I was trying to address. My point was that helicopter is designed for powered flight, which doesn't make it an ideal vehicle for autorotation, and hence the much softer landing with forward speed. If your goal is to build a replacement for parachute, you can design around that.

[X10Z]

I suppose it could work, but any number of accidents could occur to the module in orbit, causing rotors to be damaged or not deploy.

[shynung]

I suppose it could work, but any number of accidents could occur to the module in orbit, causing rotors to be damaged or not deploy.

Parachutes and heatshields face the same problem. Remember Columbia?

[Tarantulae]

Of course. The question was, why it's recommended for helicopters to autorotate with some minimal forward speed, and if that's going to be a problem for rotor kite approach to capsule landing, which is what I was trying to address. My point was that helicopter is designed for powered flight, which doesn't make it an ideal vehicle for autorotation, and hence the much softer landing with forward speed. If your goal is to build a replacement for parachute, you can design around that.

Of course. The question was, why it's recommended for helicopters to autorotate with some minimal forward speed, and if that's going to be a problem for rotor kite approach to capsule landing, which is what I was trying to address. My point was that helicopter is designed for powered flight, which doesn't make it an ideal vehicle for autorotation, and hence the much softer landing with forward speed. If your goal is to build a replacement for parachute, you can design around that.

When you autorotate, if you speed up, your rpm slows down, if you slow down, your rpm speeds up. At the bottom of an autorotation, you flare, which greatly reduces your forward airspeed while at the same time providing enough energy in the rotor system for you to pull some pitch and cushion the last couple feet of landing.

Here is a pretty good video showing starting an autorotation at 0 airspeed, gaining forward airspeed during the auto, and landing with a ground run of 5-10 feet.

[X10Z]

That is true, I suppose, but I would rather trust my life to a system that is known to function properly MOST of the time. Of course, it would be much better to ditch rockets altogether and use spaceplanes, which can land where you want them to and be reused. While Spacex is trying to make rockets easier to reuse, spaceplanes work very well right now.

[K^2]

Tarantulae, I'm not sure what your point is. There was never a question that you can trade altitude for air speed. The question was if this is absolutely required for a rotor kite to work as a landing system fora capsule. I'm perfectly aware of how this works for a helicopter.

[Armchair Rocket Scientist]

Tarantulae, I'm not sure what your point is. There was never a question that you can trade altitude for air speed. The question was if this is absolutely required for a rotor kite to work as a landing system fora capsule. I'm perfectly aware of how this works for a helicopter.

In theory, descending vertically with the rotors at negative pitch would cause them to spin up, and the high drag would keep the descent fairly slow. Suddenly increasing the pitch would slow the vehicle down for landing as suggested in the OP. In practice, I'm not sure if the rotors would have enough momentum for that kind of maneuver to work.

The thing is, there's no real reason not to descend with some forward speed. According to this, the Apollo missions all landed at most 5 km from the planned site. This isn't accurate enough to hit a runway, but the cross-range capability provided by a rotor kite probably would be. A capsule could probably land with a decent bit of forward airspeed without tipping over, especially if the support structure for the rotors doubled as skids, giving the vehicle a very wide based. Even if the capsule was coming down directly above the landing site, it could just glide down in circles.

[K^2]

In theory, descending vertically with the rotors at negative pitch would cause them to spin up, and the high drag would keep the descent fairly slow. Suddenly increasing the pitch would slow the vehicle down for landing as suggested in the OP. In practice, I'm not sure if the rotors would have enough momentum for that kind of maneuver to work.

Autorotation is done with positive pitch, and it works perfectly fine with vertical descent. I know, it can be difficult to accept that the rotor with positive pitch does not stall, but it does not. I strongly suggest you look up some explanations of that on-line. Wiki page on autorotation or autogyros might be a good place to start.

[Tarantulae]

The point is that descending with forward speed in autorotation does not mean you are landing with forward speed. You descend with forward speed, then trade that forward speed for RRPM in the flare to cushion the landing with almost no forward speed on touchdown.

[kellven]

There's a whole bunch of problems here.

1. Rotors are heavy.

2. Rotors with high inertia are the ones that autorotate well. They have weights added to the rotors specifically for that purpose.

3. Heavy is bad ultimate evil for spaceships.

4. Go to orbit without chutes, deorbit, pay attention to your airspeed at altitudes. Preferably write it down, especially when you hit about 250m/s. See how many more seconds it takes for you to impact, and write that down.

5. Why 250 m/s? 0.8 mach asl is the absolute maximum you're going to sustain with a rotating aerofoil design without going into transonic blades and all that fun.

6. First thing you've got to do now is deploy that rotating contraption without snapping the blades/mast and/or destroying your attitude control. If that's done, you still aren't in the position of a helicopter autorotating from a very high hover, you've got a helicopter with zero rotor momentum and a stupidly high negative axial velocity. There is no real way to explain why without going through the whole BET/momentum derivation of rotor thrust, but that is a very, very bad spot to be in.

7. Remember the time to impact you wrote down earlier? Subtract the time it takes to deploy your contraption, and time it takes for the rotors to get up to speed without snapping off. That's about how much time you have left to gain control of your descent, negate 250m/s descent speed, pick a landing area, align, initiate glide, and flare. No rush or anything.

8. Assuming we're still considering the 1D (zero lateral velocity) scenario, an autorotating rotor will behave exactly like a parachute of the same diameter. What that means is you're better off with the chute anyway, as the parachute is going to be simpler, lighter, and much more capable of safely deploying at the airspeed you need it to.

9. If we're considering the 2D(or 3D) scenario where you want to pick your landing spot, you still gain absolutely nothing with an unpowered rotor over a steerable chute. All the real navigation has to be done by carefully planning the deorbit.

10. In order to gain anything with a rotor, it needs to be powered, which needs an engine, which is very heavy. You are much, much better off with a well planned deorbit, small flight control surfaces for course correction, and the tiny amount of propellant you'll need to land with rockets. In other words, this thing, or this thing.

I know coptas are cool, I had one in KSP a couple years ago, but they just don't have any purpose in spaceflight.

If you really want to drown yourself in calculus, find a textbook on helicopter aerodynamics, and figure the math yourself, it's the only way to really understand the why.

Edited April 24, 2014 by kellven