Wow, I used to make rocket-powered helicopters in Kerbal Space Program a few years ago by putting a "Sepratron" on the tip of the "blade" made from structural wing parts. How appropriate :)
The way these work is pretty interesting. Rather than combusting a fuel and an oxidizer as in a typical rocket, these use hydrogen peroxide that is exposed to a platinum catalyst, which causes the rapid decomposition of h202 into h20 and 02, which is then expelled through the rocket nozzle, generating thrust. This greatly simplifies the design of the rocket engine.
The German V2 rockets also used hydrogen peroxide and a catalyst (sodium or possibly potassium permanganate) to drive their turbopumps (which pumped LOx and alcohol into the engine.) Here's a pretty good 2.5 hour long documentary about the V2 turbopumps specifically: https://youtube.com/watch?v=EgiMu8A3pi0
(The H2O2/catalyst system is discussed/demonstrated around the 45 minute mark.)
The claim in this article is that the motors use 70% hydrogen peroxide with alcohol (it does not say which one), without a catalyst. I have not found any explanation of the alcohol's purpose, or whether it is oxidzed in the process.
It could be a "mixed monopropellant". The H2O2 decomposes to H2O + O2, presumably at high enough temperature to react the O2 with the alcohol (probably ethanol or methanol). As an added benefit your exhaust gas is no longer oxygen-rich, which can cause problems.
It's confusing, because at the top they say "This rockets don't use a catalyst" but further down the page they say "all this thanks to our proprietary formula of the penta metallic catalyst pack invented by Juan Manuel Lozano Gallegos from TAM."
(Side note: I saw that name and wondered where I had seen it before; turns out he's the guy Popular Science wrote about fifteen years ago for building his own peroxide-powered jetpack.)
My guess is they have a fuel line running to the rotor head, and then some sort of seal which allows for rotation and for feeding the fuel into the rotors.
I'm a little skeptical about the claim of 'performance is the same at any altitude'... while yes, the engine performance is similar at altitude, the helo performance is not (what with the fewer air bits for the blades to move around)
Yes, it's my impression that helicopters have problems with thin air reducing lift long before they lose engine power.
That's why it's easier for them to fly forward than hover at high altitudes; the forward velocity of a moving helicopter provides more airflow over the rotors and thus more lift. I don't think it's a matter of jamming more air into the turboshaft engines, which presumably could be done using larger intakes.
> it's my impression that helicopters have problems with thin air reducing lift long before they lose engine power.
I believe this is generally true, but it's certainly possible to get into trouble.
The world record for the longest autorotation was set after an engine flameout, after an attempt at the world record for highest altitude reached in a helicopter. [0]
> the forward velocity of a moving helicopter provides more airflow over the rotors and thus more lift
It's more that a helicopter in a hover (assuming little wind) must produce additional lift (i.e. more power) as it's stuck in a downdraught of its own creation. Introduce some forward movement, and the helicopter is flying through undisturbed air (or more precisely, the air entering the main rotor disc is undisturbed), which takes far less power.
This is called 'translational lift'. It kicks in at around 30 knots, and occurs at any altitude. It might be more consequential at very high altitude though, or under very heavy load, where the helicopter might have enough power for forward flight, but inadequate power for a hover. [1]
The FAA have an interesting Power vs airspeed chart, p19 of [2]. If I understand correctly, it means that if your goal is to keep a helicopter in the air as long as possible, you want to stay at around 62 knots of indicated airspeed. Fairly slow, but nowhere near a hover. (edit 'Ground effect' might also have some bearing on that question, but that's another matter.)
Very interesting, thanks for this. Do you suppose a hybrid system might be useful in special applications, like providing a power boost to rescue helicopters trying to hover above mountains?
Rather beyond my knowledge but I'll take a stab at it:
I don't think tip jets (the name for this design, oddly absent from the article) [0] are generally thought to be of practical value in modern helicopters. They have the neat advantage that they don't require a tail-rotor (the main rotor isn't driven by a drive shaft from the main body of the helicopter so there's no torque trouble) but I don't think they're all that practical. This article dates from 2010 after all.
We already have a reliable means of increasing a helicopter's power: bigger engines. Alternatively, more engines.
Heavy-lifting helicopters tend to be powered by twin turbine engines. [1][2] Even if they use unusual designs [2] the power-plant is the same as for any other serious helicopter. The enormous Super Stallion military helicopter went even further: 3 turbine engines! [3]
I imagine a hybrid design would greatly increase complexity. Helicopters use a 'sprag clutch' to permit the rotor RPM to exceed the engine RPM, but not the other way around. This allows the rotor to keep spinning in case of an engine failure. (This is the reason helicopters don't drop like bricks when their engines fail.) Perhaps there would be a way to modify the design so that the conventional engine could still contribute power even as the rockets are firing, but I imagine it would be very high in complexity.
Also, tail rotors can suffer if they aren't working in clean air, and of course this can threaten the helicopter's safety. [4] It's not something I know anything about but I imagine rockets on the main rotor could be troublesome in that regard.
For yet another wacky (but likely impractical) alternative design with no need for a tail rotor, see [5]
Lastly, google tells me rockets have been used to assist fixed-wing aircraft in takeoff. [6]
They have various other such documents, they're each a chapter of their Helicopter Flying Handbook, or you can download the whole lot as one enormous PDF. Here's the hub page:
The thing is, there's less air for the blades to move, but there's also less drag on the blades for the same reason, so they spin faster with the same power input. This ends up cancelling out the loss of lift, meaning that the performance stays roughly constant with altitude until it is high enough that the blade tips start to approach the speed of sound.
> In normal operations, and design aims to achieve this, the rotor tips do not go supersonic since when they do, there is a sudden and large decrease in performance with more power required, higher blade loads, vibration and noise.
> Think about a helicopter flying forwards. The advancing blade at its most perpendicular position experiences a relative airflow which is equal (ignoring all kinds of minor side effects) to the forward speed plus the speed of the blade. The retreating blade is experiencing a relative airflow equal to the speed of the blade minus the speed of the helicopter.
> If the blades rotate so fast that the tips are supersonic, then the main lift generating part of the retreating blade, the outer two thirds of the span, would experience such a low airspeed, for some of the span it will even be negative, that the blades will stall causing a catastrophic roll into that side. It is this phenomenon which ultimately limits the rotational speed of the blades and the maximum speed of the helicopter.
For any HN'ers in the bay area, if you're into this stuff you should pay a visit to the Hiller Aviation museum. It's right by EA on the San Carlos Airport. Hiller was a pioneer in helicopter design, and they have a ramjet tipped 'copter on display.
Here's their website: https://www.hiller.org/museum/aircraft-on-display/
doesn't this tiny rocket have very low range and bad efficiency compared to a traditional helicopter engine?
the energy density of h2o2 is close to the one of a ion lithium battery (2.7MJ/Kg) vs the traditional gasoline+o2 (13.3 MJ/Kg) [1]. It seems like more a cool trick than something which can replace helicopter engines?
but one is not carrying around a very heavy engine, gearbox and tail structure.
(no heavy tail structure because there is no rotational torque from driving the blades around against the tail rotor only gyroscopic precession when changing direction/ tilt etc)
Yes, you're right. The advantages are that it is much simpler mechanically and can be made more cheaply. But I think it is mostly just a cool novelty. I don't see how these can compete with conventional helicopters.
Terminology aside, I think the comment was accurate.
I think it would have been better to kindly correct any terminology issues you noticed without saying "nobody actually involved with helicopters calls them" twice. For example:
"While `rotary wing` is an accurate term, most people in the industry would probably call them rotor blades."
I don't think the "copter" correction was necessary, since that is in fact one of the slang terms for a helicopter and it's clear what they were referring to, whereas 'wing' could confuse someone into thinking of fixed wings on the side. But if you were going to mention it, it would have been helpful to mention what you thought would be a more appropriate term.
I'll just finish with this rule from the Recurse Center User's Manual[1] that I think applies very well to this situation:
> No well-actually's
> A well-actually happens when someone says something that's almost - but not entirely - correct, and you say, "well, actually…" and then give a minor correction. This is especially annoying when the correction has no bearing on the actual conversation. This doesn't mean the Recurse Center isn't about truth-seeking or that we don't care about being precise. Almost all well-actually's in our experience are about grandstanding, not truth-seeking. (Thanks to Miguel de Icaza for originally coining the term "well-actually.")
Might be possible do do it as a ram jet too, that would get you a lot of efficiency upsides.
Typical jet engines probably wouldn't work well however due to force but you get a fuel feed upside from the centrifugal force.
One of the major issues with helicopter design is that the effective speed of the rotor tips vary by twice the forward speed of the helicopter as it swings around from back to front to back again. The rotor going from back to front experiences a speed that is the tip speed due to rotation plus the forward speed of the helicopter. As it swings around to the front and then towards the back it experiences a speed that is the speed of the tip due to rotation minus the forward speed of the helicopter.
So if your helicopter is going at 200 mph and your rotors - which have been noted here several times need to stay below the speed of sound - are going at say 400 mph at the tip, then the rotor tip speed varies from 200 mph to 600 mph on every single rotation.
It’s hard (for me) to imagine an air-breathing engine that could operate efficiently with its intake air speed changing that much that frequently. Much less a ramjet which has no turbines or anything to even out the intake velocity and which normally wants to operate well above the speed of sound.
There was a helicopter that used a ram jet IIRC. I guess the only problem there is you need a separate starter motor to get the rotor spinning fast enough for the ramjet to take over, but most helicopters have electric starters anyways.
Ramjets do not require supersonic flow speeds. They require positive speed (they don't work at a standstill), and are usually tuned for a sweet spot. This is usually supersonic because most applications are military, where the supersonic capability is desired. That said,they usually don't produce much thrust until you get over M0.5.
Fair warning, this is probably a ridiculous idea because I'm nothing close to a mechanical/aerospace engineer.
Would it be possible to mount the ramjets to a ring around the blades along with some sort of transmission in the ring? I'm thinking setup this would allow the ramjets to function supersonically at the edge of the structure to ensure the best airflow and limit the speed of the blades. However such a transmission might be overly complex or heavy and it's possible that the ring around the blades would cause some weird aerodynamic effects.
The difference is in where the torque is generated. In a traditional helicopter, the engines (mounted to the main body) generate torque to spin the main rotor. Since the body spins the rotor, by Newton's third law, the body and the rotor spin in opposite directions. Therefore, you need a tail rotor to counteract that.
With this design, the torque to spin the rotor is generated on the rotor itself, so there is no torque effect on the airframe. Note that there is still a (rotorless) tail with fins, probably to stabilize the aircraft in forward flight.
If you want to attach a spinning object to a stationary one you need bearings, which means friction, which means torque. Nowhere near the torque of a normal helicopter, but something. I take it the double fins in the back (at the end of a long lever) can more than handle that. Or at least, as long as the bearings don’t fail.
Gyroscopic precession, though... Is that enough tail or do you have to be gentle changing directions in this thing?
> Or at least, as long as the bearings don’t fail.
If the bearings fail, counteracting torque is the least of your worries - you should land immediately before the rotor shits itself and you turn into a flaming lawn dart. From the UH-60 operator's manual:
> 9.22.10 Main Transmission Failure.
> WARNING
> If % RPM R decreases from 100% to below 96% with an increase in torque during steady flight with no engine malfunction, the main transmission planetary carrier may have failed. During a main transmission planetary carrier failure, it may be impossible to maintain % RPM R at 100%.
> NOTE
> Decreasing % RPM R may be accompanied by a drop in transmission oil pressure of 10 psi or more, and possible unusual helicopter vibrations.
> PROCEDURE
> 1. Collective - Adjust only enough to begin a descent with power remaining applied to the main transmission throughout the descent and landing.
I like to keep people guessing by alternating between ironic understatement and hyperbole. Too much BBC perhaps.
I know in low rpm devices, especially with radial forces (wheels) a bearing or race can start to go without destroying the whole assembly. To the point that bicyclists can bring in a wheel that's so far gone that the labor to fix it is twice the cost of a new wheel (I can fix this or we can get you a new pair of wheels for the same price, installed). How long do you have between the first chip and game over on a rotor?
Some how, even though "LAND AS SOON AS POSSIBLE" is in all caps, I doubt it entirely captures the urgency. If anything involved with keeping the blades attached fails, sounds like a case of any other plans you had are over and you need to be on the ground RFN while it's still a choice.
A typical helicopter has a motor between the body and the blades. Imagine using a screw gun. When you go to tighten a screw, equal force is imparted back on your hand because it's using your hand to push against.
These designs have the rocket motors pushing between the blades and air. This means that the spinning main rotor is just sitting on a bearing and the rotor is not being pushed off of the main frame.
However. You still need tail control to point it the way you want and to counteract inefficiencies in the main rotor bearing, that is why their prototypes have a tail rotor. That said, it's not As needed. If you lose the tail on a normal single blade heli, you are in for one hell of a ride. On this, a little forward movement and the vertical tail of the heli is likely enough to keep it straight.
In a normal helicopter, the helicopter is exerting a force on the rotors and thus feels an equal and opposite force that needs to be countered, either by a tail rotor or a counter-rotating rotor. This helicopter doesn't do that. If anything I'd think it might need a tail rotor to stop it from spinning in the same direction as the rotor, but I guess friction between the helicopter body and the rotor is low enough that this isn't a serious concern.
(The linked youtube video shows a version of this helicopter that does have such a tail rotor, though positioned closer to the helicopter than it might normally be.)
When the engine on top of a helicopter spins the blades, they exert an equal torque force back on the body of the helicopter. In order to just spin the blades and keep the body straight, you have to cancel that torque by using a tail rotor which creates a force way back on the tail that with the length of the tail, cancels it out.
In the case of this helicopter, the force to spin the blades comes from the tips of the blades, expelling gas out the tips will push the blade tips forward, so the body doesn't push on the rotor blades at all to spin them.
It is, the tail rotor is needed because on a conventional helicopter the main rotor is spun using a torque force from the helicopter body, through the engine/gearbox, and into the rotor. On this helicopter, the force is from the rotor through the rockets to the air, there's only a tiny friction force dragging the body along with the main rotor, and it looks like that's small enough to be counteracted by the aerodynamics.
Because the rotation is caused by a force at the rotor tips, so there is no torque on the aircraft. A conventional helicopter has an engine that is rotating a shaft connected to the rotor, so the engine is creating a torque on the shaft which causes the rotor to spin and then the shaft is creating an equal and opposite torque on the engine that is causing the aircraft to rotate the opposite way, which the tail rotor cancels.
I think Juan Lozano was making peroxide engines for early iterations of John Carmack's Armadillo Aerospace (whose website, sadly, is now only available via web archive...)
And very early on, Armadillo actually tested rotors with tip peroxide jets. Only ground tests that I know. Moved to pure rockets quickly. High test peroxide. Mixed monoprops with low grade peroxide and alcohol. Biprops. Peroxide-alcohol? Lox-alcohol? Lox-methane eventually.
That's ingenious. It's one of those rare mechanisms that are hard to come up with but obvious once you learn of them. I recommend everyone to watch this video.
(The presentation spends a lot of time explaining tail rotors, which this invention does not need. Skip to about halfway to see how it works.)
Doesn't the pitch of a helicopter blade change as it goes around the rotation? With a traditional rotor, the torque is always angular, but with these, the torque will be angular plus a cyclical vertical component, since the rocket pitch is the blade pitch. I'm wondering what the the structural and vibrational implications are of a rocket launching the blade tip up and down at 500 RPM.
OK, dumb questions - the H202 is entering the combustion chamber at what pressure? below chamber pressure,with a valve, like a pulsejet? or is there a turbopump somewhere? and what powers it? I'm guessing centripetal force once the rotors are spinning helps (but isn't there a potential evil feedback loop hiding in there somewhere?)
It doesn’t spin that fast (linear speed) close to the helicopter, and any fuel leaking out of the wings during flight will fly outwards so you would be safe inside ;-)
However, I expect they pump the fuel from the helicopter body into the wings, and I wonder how they guarantee the seal between the two. Helicopters must continuously adjust the angle of attack of the wings, so it isn’t even a matter of a single fixed axis, and the chemical properties of hydrogen peroxide won’t make things easier.
Once it's started up, it's a centrifugal pump all on its own. You don't need a positive pressure pump on the body, the fuel would be sucked out and along the blades like a siphon.
That doesn't make the seal problem much easier, but it at least does mean that if a seal were to fail, it would fail safe without spewing h2o2 everywhere.
One of the earliest efforts of this type was in 1950's, by Hiller Aircraft - and you can see the helicopter itself in the Bay Area, in the Hiller Aircraft museum. It achieved flight and was tested by US Navy & US Army.
One of concerns would be how much of it gets converted by the catalyst - if it's not 100% for any reason you've basically got a rotating sprinkler throwing HTP about.
Edit: To be fair it's not HTP, fortunately for those under the flight path...
Maybe that could be made to work, but if you're already committed to using a gas turbine of any sort, why not make it a turboshaft engine and build a conventional helicopter? Either way you have to pay for the expensive maintenance of a turbine engine. If a rocket-helicopter could have any real advantages (they're certainly cool, but I'm rather skeptical), it would surely be the simplicity of the mechanism making it cheaper to operate.
I think one of the big points is that elimination of the tail rotor could prevent 100% of the tail-rotor failure crashes. So something like 87% of crashes are related to tail rotor failures.
So I guess that would be the advantage. Also lighter because you're now missing an entire complex mechanism for the tail rotor and potentially that giant tail.
Coaxial rotor helicopters can be built without tail rotors, but my general impression is these are considered less reliable than conventional helicopters due to the added mechanical complexity of a coaxial system. But if tail rotor failure are that common, maybe coaxial rotor helicopters are worth the added complexity after all? They certainly seem more practical than rocket-helicopters.
I'm not sure about that; the Russian Ka-50 coaxial attack helicopter seems reasonably quick, certainly not the fastest helicopter ever but comparable to an Apache at least (315km/h vs 293 km/h respectively, though the Apache has a higher never-exceed speed: 350km/h vs 365km/h)
If you include compound helicopters (using horizontal pusher props or turbojets), coaxial helicopters can certainly be very quick; the S-97 can cruise at 410 km/h, but in that case you have the liabilities of a 'tail rotor' to contend with again. The S-69 could do a blistering 487km/h with it's turbojets, 289km/h without.
One to consider is the Eurocopter X3, a single rotor compound helicopter without a tail rotor. It could do 472km/h and used two tractor propellers with variable pitch to counteract the rotor torque.
Helicopters without tail rotors (Kamovs, Chinooks, etc) are typcially faster than conventional helicopters - They aren't loosing ~20% of their power to the tail.
Yeah, I've got a few gallons of 30% H202 sitting in the garage (because the 3.5% household variety was out of stock on Amazon, I ordered from a lab and we dilute it ourselves). Requires wearing some protective clothing when dealing with it. Even when diluted to around 5-10% it can cause irritation and burns. That said, it's also not shelf-stable. After about 6 months of being opened it'll just be a gallon of water. Still cheaper than buying the branded stuff.
In addition to actually bleaching things, H2O2 has vasoconstrictive properties which can sometimes cause the appearance of skin being bleached. Whenever I apply pharmacy-grade hydrogen peroxide to my skin it causes a temporary 'bleached' effect that fades after a few minutes. This is apparently a matter of small blood vessels near the surface of my skin being constricted. Wikipedia doesn't mention this effect but it's not too hard to find sources on (https://pubmed.ncbi.nlm.nih.gov/860154/). I'm not sure if large exposures could cause heart attacks though this effect, I suppose chemical burns would be the more immediate concern, but it's certainly not something I'd want to find out first hand.
Unfortunately for my morning commute (which is not happening again anytime soon anyway), their HTP jetpack has about 20 seconds of flight time and, fortunately for society at large, you can't get HTP at a gas station.
See this video for more details: https://www.youtube.com/watch?v=1Sj_jgrokww