Ask Hackaday: Quadcopter in Near Space?

Your mission, should you choose to accept it, is to send a quadcopter to near space and return it safely to the Earth. Getting it there is not that difficult. In fact, you can get pretty much anything you want to near space with a high altitude weather balloon. Getting it back on the ground in one piece is a whole other ballgame.

Why does someone need to do this? Well, it appears the ESA’s StarTiger team is taking a card out of NASA’s book and wants to use a Sky Crane to soft land a rover on Mars. But instead of using rockets to hold the crane steady in the Martian sky, they want to use…you guessed it, a quadcopter. They’re calling it the Dropter.

quadcopter on mars

At first glance, there seems to be a lot wrong with this approach. The atmosphere on Mars is about 100 times less dense than the Earth’s atmosphere at sea level. How do props operate in these conditions? Testing would need to be done of course, and the Earth’s upper atmosphere is the perfect place to carry out such testing. At 100,000 feet, the density of the stratosphere is about the same as that of the Martian surface atmosphere. AND 100,000 feet is prime high altitude balloon territory.  Not to mention the gravity on Mars is about 38% of Earth’s gravity, meaning a 5.5 pound model on Earth could accurately represent a 15 pound model on Mars.

With all of these facts taken into consideration, one can conclude that realistic testing of a scale model Martian quadcopter is within the grasp of the hacker community. We’ve seen some work on high altitude drones before, but never a quadcopter.

Now it’s your turn to do something no one has ever done before. Think you got what it takes to pull such a project off? Let us know what your approach to the challenge would be in the comments.

Hmmm, desert planet, ground vehicle being delivered by a flying transport. Anyone else getting a [Frank Herbert] vibe from this?

62 thoughts on “Ask Hackaday: Quadcopter in Near Space?

  1. While I applaud the idea, at least in the US this is a non-starter. Altitudes between 18k and 60k are controlled, and you need a Mode C transponder and 2-way radio communication equipment, an IFR flight plan, and an ATC clearance to enter it. Either that or a waiver from the FAA.

    Anyways, with their attitude and rulemaking stance towards model aircraft as of late, I don’t see them issuing any waivers for this kind of activity. They unilaterally want model aircraft out of airspace where they could interfere with manned craft.

    A more serious note though, this poses several interesting problems. Props that were designed for earth atmosphere density aren’t going to work nearly as well (if at all) at FL1000. Since propellers are airfoils, you’ll need to take some design cues from high-altitude aircraft: long, wide blades with a lot of surface area. The ability for the aircraft to generate lift out of pure rotation will be paramount, as the component of transitive lift won’t be available until forward (rather than downward) momentum is achieved.

    A better way to test this out initially would be to use a vacuum chamber, similar to the ones the FAA uses to give pilots a taste of what hypoxia feels like, to test the airfoil design in low-density atmosphere environments.

    Fun stuff. I’ll have to think about this and come up with some ideas.

    1. those problems aside, i’d say use a multi-stage setup, first a gas ballon to get it to a manageable speed, then use the electric model jet engines in place of propellers. seems fairly simple really. would need some covers here and there but over all the real problem seems to be the regulations on everything.

      1. Since model jet engines typically are engineered (fuel delivery/mixture wise) to operate in a sea-level environment, would a model jet engine be able to compress the air enough to run? I think your fuel mixture would end up awfully rich if you plan on trying to compress enough air to feed a jet engine at that altitude…

    2. Well knowing average quad battery life, I’d be inclined to have the unit just drop out of the sky totally out of control until about 10,000ft (altimeter actively monitored) then activate and slowly stabilise, initially just to counter high spin rates, then coming downwards with a little lateral motion to avoid vortex ring states in all four props.
      But I guessed this challenge is about having control at such altitudes really…

      1. You can do better than that – with the proper motor control, you could actually *charge* on the way down, using controlled autorotation with the props spinning in reverse. Running out of power definitely wouldn’t be an issue (although you’d probably need a braking resistor to soak some of the energy).

        1. Depends, if you’re aiming for autorotation getting the rotors freewheeling (putting the motor on ‘open circuit’ instead of a braking resistor) to increase their RPM might work better to slow the quadcopter down during its descent. It’s a balancing act between the ‘driving’ and ‘driven’ areas of the rotors to get an acceptable rate of descent so putting to much mechanical load on the rotor shaft would reduce the amount of lift generated.

      1. They also apply for FAA exemption for the area and airspace to prevent the possibility of a mid air collision. Just putting up a rocket randomly will get you jailed.

        Weather balloons also do this, but they are both passive and have a very low weight limit, so have their own rules. Payload maximum density of 3 oz/sq inch (so they’ll be easily digestible) and less than 6 lbs (again, no large parts) with an overall weight of less than 12 lbs.

        http://www.chem.hawaii.edu/uham/part101.html is one reference I found to the rules.

        1. Not to mention, there are specific FAA rules for quad-copters and other “drones” which I think might be ignorant of altitude (which is to say, they would be still in effect if you’re 100 or 100,000 feet off the ground).

  2. I think its a horrible idea both due to what loather says, and also the potential of it falling on someone. But really, talk to NASA, get a grant to do a contest in a controlled area. Range or ocean.

  3. “Not to mention the gravity on Mars is about 38% of Earth’s gravity, meaning a 5.5 pound model on Earth could accurately represent a 15 pound model on Mars.”

    Isn’t this a bit backwards? Shouldn’t it be LIGHTER on Mars?

      1. Mass vs weight. You’d be able to lift 15 lbs worth but it would act completley different, thanks to inertia. You’d have to program the flight controller differently too.

        1. I think that that the comparison was largely centered around lifting strength, due to the thin air. The difference in mass could probably be accounted for in software with less trouble than ensuring it has sufficient lift at that air pressure.

    1. It’s just worded kind of funny. They’re saying that a quad copter that weights 15 pounds on earth would weigh roughly 5.5 lbs ( or 5.7 by my calculator) on mars. So to test on earth you should use a 5.5 quad copter to account for the gravity difference. When it comes time to send it to Mars you could scale up to 15lbs.

  4. Vortex ring state (power settling) makes this a serious challenge along with tip speed not exceeding MACH. Long wide blades at slow rotation speeds would need to counter act that, but at the expense of weight and power margin. Of course the longer the blade, the higher the tip speed.

  5. I’d start by looking at large area, single bladed airscrews.
    Make them variable pitch to keep the RPM’s as low as you can.

    It’s a bummer about all those annoying/inconvenient laws and regulations!

    Here in Australia you can get permission//authorization for a HAB, but it would seem CASA don’t like the idea of an autonomous aircraft returning to it’s launch point!

    How a HAB payload making an uncontrolled decent is safer than a small glider, be it fixed wing or gyrocopter, is some how safer defies reason!

    1. Re: uncontrolled descent

      They’re thinking of predictability. It’s an uncontrolled descent, but within a predictable corridor in space, which they can route around. If it has wings or blades, the corridor becomes larger and the time of flight is increased, so there’s greater potential for conflicts.

      It’s a very conservative policy and it certainly isn’t optimal considering the technology available, as you pointed out, but there is a logic to it.

      1. nothing wrong with a single bladed airscrew, have a look at the competition rubber motor models that use a single blade prop.

        Waaaay back before n-copters became a thing I was flying a tethered tricopter, 3 single bladed props powered by full size, geared down Mabuchi 510 motors.

        MEMS gyroscopes hadn’t been thought of yet and the Murata peizio ones were something like rocking horse poop and hen’s teeth to find.

        The beast flew reasonable well, hovered nice and flat, ok it drew 15A doing it, but as a proof of concept it was pretty good.

  6. I agree about using the test chamber.

    They are basically asking for an aircraft that is 40 times more effective at generating lift than is ordinarily used. I’m sure it’s possible with some combination of increased rotor diameter, chord, and RPM. While I understand the attraction of rotorcraft there’s a noticeable lack of high altitude helicopters because so much of the blade is exposed to low energy air and can’t generate much lift, unlike fixed wings where the entire wing is exposed to the same airspeed.

    For those interested in rotorcraft development, check the links in this post http://www.eng-tips.com/viewthread.cfm?qid=378776

    1. this is why I think a single blade rotor would be the way to go, none of that nasty “advancing blade” hassle to worry about.

      Increasing rotor RPM won’t help though, keep them nice and slow to stop the tips breaking the sound barrier and minimize tip vortex interaction.

  7. Do we know why the ESA is opting for this quad approach vs tried-and-true rockets? Perhaps if we knew more about their motivation we’d be able to know more about how to make it work.

    1. They aren’t building a Mars quadcopter. The story has gotten muddled.

      The team’s task was to build a system that could visually identify and navigate to a safe landing site, then deliver the rover onto that site. The quadcopter was just a convenient piece of kit they could modify for their purposes.

      So the project is really about navigation and control, rather than quadcopters. The Martian Quadcopter is still good fodder for an Ask Hackaday post, though. It certainly got everyone talking. :)

    2. I would guess it’s because the quadrotor (or any other electrically-powered flying machine) is reusable – the quadrotor could, in addition to the rover, also carry itself a few instruments and a solar panel. Even if it only has a flight time of five minutes and takes a week to recharge afterwards, that gives it a huge advantage at, say, surveying the area to best direct the rover towards interesting samples, or gives it a huge range if it wanders off somewhere by itself.

      Fixed wing would definitely be more practical in a lot of ways, but you would need VTOL for it to work without having found a safe landing site in advance. The mechanical simplicity of a multirotor would be a huge advantage over, say, a traditional helicopter design.

      TL;DR: Because rockets run out of fuel.

      1. I’d also expect the Quad-rotor sky crane to be equipped to auto-gyro initially. Should be able to ditch a few parachute stages and insure the landing battery is topped off. Though I wouldn’t be surprised if the power system for a Martian sky crane was a hydrozene rocket engine and turbine. (maybe even tip-jets)

        1. But if you’re not going to make the skycrane reusable, why include the mechanical complexity of rotors/turbines instead of using the parachutes + rockets approach that Curiosity took? If it’s just about having more control over the landing site and the skycrane is totally disposable, you’d probably be better off adding a fixed wing stage for controlled gliding and then using parachute and/or rockets to wipe off the last of the horizontal velocity before landing, no?

          1. Simplicity. If you aren’t NASA, I can imagine a rocket system might be a little daunting to build than a quad-copter, as quad-copters are pretty well figured out by a wide range of people, with direct control over thrust, the possibility of off the shelf parts, easier testing, laxer-ish regulations (plus the possibility of rockets exploding ect, ect). If you can slow this thing down to an airspeed where you can just find a spot and set it down, why not use something that’s easier to control, build and get through red tape with? (the FAA sure as hell doesn’t care about quad-copters when they’re flying around on mars, but might someone might object to you sending rockets full of explosive fuel there…)

  8. Welcome to todays episode of “drunken physics (Jagdschloss Kräuterlikör) with rumburack”

    Potential energy is E = m g h.

    So lifting some 10 kg up 30 km stores

    E = 10 kg * 9,81 m/s/s * 30000 m = about 3 MJ into it.

    Energy density (J/kg) of batteries:

    – conventional: 250 Wh/kg (1 MJ = 2778 Wh)
    – experimental: 1000 Wh/kg

    So the battery weight is between 33 and 8 kg. Which does not work, because we need to lift it up, too.

    Lets switch to liquid hydrogen: that stores 120 MJ/kg (german wikipedia) or 141 MJ/kg (english wikipedia), so let us buy the English hydrogen.

    1 kg is more than enough to do the job. We only need some piston engine and a reboiler. Anyone with a whiteboard?

    1. I love german’s sense of humor.

      A weather ballon can reach up to 53 km. I even renember that redbull challenge of a guy jumping from near space into the earth.

      100,000 ft = 30.48 km

      So reaching that distance “could***”* be as simple as realising a ballon.

      How about returning safely to earth? Well you must have some pretty bad memory or a short attention span. Renember the redbull challenge? The guy used a parachute.

      About the quadcopter, you might carry it above the weather ballon + parachute thingie. It doesnt even have to be on.

      If the quadcopter is not required, you may just shoot a bullet into the sky and wait for it to fall back.

      1. The goal is not to return to earth; If this thing were to “return to earth” when at the same air pressure on mars, then it would end up deep, deep underground. The goal is to make a quad-copter that would have the strength and control such that once delivered to mars (at some not-inconsiderable velocity) it could slow down before hitting the surface (below a certain air pressure in the on-earth tests) provide enough lift to not crash, and be controllable enough to find a landing spot for the payload mass it has to lug through the entire venture on it’s belly. (which might make generating enough lift to hover difficult, never mind slowing down)

    2. Presumably in Martian atmosphere you’d need to carry your own O2 as well. Plus some beefy pressure vessels to contain both gasses, at sufficient pressure to keep them liquefied. So going to hydrogen as an energy source appears to incur weight penalties several times the actual hydrogen weight. Still far better than batteries. And opens up the possibility for jet propulsion, rather than what seem like some infeasibly large rotors. I like it.

    1. While efficient, those probably would not produce enough energy fast enough (wattage) to be able to fly a quad-copter, especially in a severely reduced atmosphere, with a payload. (never-mind stopping the rest of the way upon reentry, after whatever parachutes are in use have done their jobs)

  9. Live testing seems way premature until some vacuum chamber testing is complete. Napkin calculations say a Quad on Mars will have the lifting power of a wet paper bag. You’ll need a huge rotor diameter, times four, plus the structural weight to hold the rotors apart, the motors, the batteries, and the payload. Testing on earth, even at altitude is a poor substitute, since the size needed in Earth gravity would need way more structural mass then on Mars.

    All this (and many other ideas) has been engineered to death by Nasa, and rejected, hence their tests in the pacific for their “flying saucer looking” decent ring balloon (LDSD) which will most likely be their next mars decent vehicle.

  10. I’m going to interpret the challenge as the craft needs to be able to recover from a free-fall and then hover at 100,000 feet.

    For the recovery part:
    Let’s say we have a quad. The propellers are easily the largest (and therefor most draggy) part of the airframe. Get you’re center of mass far below the plane of the props, and with a well tuned FC the quad should be able to catch itself pretty easily.

    For the hovering part:
    Lift and drag are directly proportional to the pressure. They are proportional (roughly) to the wing area. So for propellers that would work in 0.01atm, we’re looking at something about 10 times larger than on the surface of Earth. Obviously these would have to be redesigned to be well suited to the target pressure. But I would say that this size is actually quite feasible with some light CF props and a large spindly CF frame.

    For the testing part:
    High altitude weather balloons are regularly launched in the US. Lift up to 105,000 feet. Release. Self stabilize by 100,000 feet and hover for a couple of minutes. Then cut the throttle and free-fall all the way down for a parachute recovery. Obviously a radio link with the quad probably won’t be legal, but it’s a fairly simply autonomous mission.

    (please let me know if any of this is blatantly false)

  11. I remember a number of years ago the author of the X-Plane flight simulator added the ability to fly your planes on Mars, using NASA’s laser terrain map of the planet and known data about atmosphere, gravity, etc. You could load up a 747 on top of Olympus Mons and the simulator would do its best to represent the actual performance of the airframe in that environment.

    Of course, it turns out that nothing designed for flight on Earth has a hope of getting into the air on Mars. Even if you fudged it a bit and gave the plane the correct amount of thrust (since of course no oxygen-breathing combustion engine would work), the air is too thin and the planes are too heavy. The simulator included a couple of theoretical designs that were able to get off the ground, at which point you discovered all kinds of wacky behavior. For instance: because of the thin air, even in a sailplane-like design you can’t take off until ~500 knots ground speed (50 knots indicated)…but once you’re in the air, you have 500 knots’ worth of inertia but 50 knots’ worth of air on your controls. So you rip around faster than Chuck Yeager, but when it comes time to turn it’s like trying to redirect a charging elephant with a leaf blower.

    More info: http://www.x-plane.com/adventures/mars.html

  12. Lol, anyone looked at the altitude record of conventional helicopters? Its 12400m while flying forward and something like 9000m (Everest) while hovering in downwash. Quadcopters have smaller rotor diameter, thereby less efficient rotors.

    The U2, highest altitude subsonic plane and idealized model what the best subsonic rotor could do, goes only up to maybe 25km. Atmosphere there is still 2.5x denser than at 30km, the altitude at which the pressure approximately corresponds to mars @ ground level.

    So pretty unrealistic to do that with a quad copter with a significant payload.

    1. What about using multiple quad-copters, flying in formation…?

      Wait, just realized how dumb that would be. Not only would it be useless for trying to slow down, but maintaining that kind of coordination would be difficult even just for short hops across the surface carrying the load.

  13. Instead of a quad, a two rotor stacked coaxial. Push the blade area out toward the tips, with just thin/flat sections toward the center. That area of the blades can’t contribute much lift in the thin atmosphere so make it as light as possible while being strong enough to take the lift force of the tip-weighted area.

    A possibility for compactness of the blades during launch and travel to Mars, crease a thin blade of spring steel then roll it up, like a reverse snap bracelet that is made to revert to straight when released. The creases would keep the blade rigid while the light material allows for fast RPM with less mass and inertia to require more power. Properly designed they could automatically counter tendency to twist like a curved tape measure, or that could be exploited to alter pitch with rotor RPM.

    Make them bi-metallic and electricity could be run through the blades to alter pitch and dihedral. No mechanical linkages and swashplates required.

  14. a drop-blimp might be a more robust and technically simpler solution. Inflate a bag, and steer that around. Heck, almost no need for a separate rover. Just land, sample, grab some ballast sand if necessary, vent some hydrogen (if this could find water it would provide its own lift gas) and move onto the next site. Compress the lift gas down and collect the bag for hunkering down in bad wind conditions. then reinflate and move on.

    1. How much helium volume would you need at that air pressure to slow down in the less dense air above, and stop nearer to the ground? I suppose it could be done with weather-balloon-like technology, but you would probably need to lug a heavy helium cylinder much of the way to mars, at least until you started to enter the atmosphere at a speed that wouldn’t rip your balloon-parachute apart…

      …though I guess it might make for a nice airbag for landing if multiple weather balloons were arranged around the spacecraft? They’d probably burst on impact if uncovered though.

      Part of me thinks this plan might be far more feasible than the quad-copter idea, if helium (or hydrogen) storage en-route could be solved.

  15. Sounds like at least a cool school project for a pongsat. They now also have a 3D printed 5cm cube that could house some type of an experiment whereby lift force could be tested for various blade configurations at a given altitude.

    http://www.pongsat.co/ Nice video. I’ve seen some of the past launches on the pongsat site, but this one at the store is nice. With the Propeller mini, it looks like a lot of the work is already done.

    From their site, next high altitude balloon flight to 100,000 feet is April 25th, 2015. Looks like they have room for 10 more cubes.

  16. Why not just inflate a weather balloon from a canister when it reaches the atmosphere, then slowly deflate it with a valve until it descends. If it starts moving too fast, then a few more puffs from the canister. Basically a hot air balloon style flight. This of course doesn’t allow for accurate positioning, which might be what they want from the quad.

  17. So, do they want an accurate lander? If so, current quad control systems will not really work well since they rely on GPS locationing to determine where they are in 3D space. On Mars there is no GPS and accelerometers and other earth type sensors will not work that well. Accurate landing will be difficult and will need some other means of navigation. Obviously, if one know the drop coordinates and where it needs to land in relation to the drop coordinates, one can program that. At 57-million miles away, things are rarely that accurate. Hence, it will be very difficult to land it in the intended location, unless the NASA orbiter drops a transponder at the intended location. For a test on earth it will be easy to get it to the intended location since we do have GPS available. The real test will be to land a copter at a specific location without the use of GPS. Gliding it down from altitude, slowing it down to a manageable speed and altitude and deploying the quad safely is not that difficult. I already have a flight system design that can do that. My 10-year old boys and myself have been working on a system to take a quad up to 130,000ft and drop it, using a glider and some other things.The trick will be to find the landing location without using GPS location. Cool problem. If we have a beacon, we can have it go there.

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