Duncan Lunan

Rotorcraft in Space

Just under 40 years ago, in April 1983, the late Chris Boyce sponsored a mailshot to local newspapers and magazines in which I offered a 3-part series of articles.  The first segment was ‘The Sky Above You’, which has appeared in many places since and has been running in Orkney News since 2018.  The second was ‘Beginners’ Astronomy’, which I had begun for the Glasgow Parks Astronomy Project in 1979, and most recently has been an updated 90-part series for Orkney News ending at Christmas 2022.  But the oldest part was ‘Space Notes’, which I had been writing since 1977 for Spectrum, which became Astronomy Quarterly in 1979 and continued for several more years.  Subsequently it ran as ‘Science Notes’ in Dream and New Moon magazines, edited by the late Trevor Jones, sadly ending with his early death in 1992.  Most recently, between 2004 and 2018 it ran in all 30 issues of Jeff Hawke’s Cosmos, edited for the Jeff Hawke Club by William Rudling.  Much as I enjoy writing ‘The Sky Above You’ and have enjoyed rewriting and expanding ‘Beginners’ Astronomy’, I’ve missed writing on current news in detail and ranging more widely, so it’s good to be taking it up again.

I’ve written about flight in non-terrestrial atmospheres with fixed-wing and flex-wing aircraft, in ON, July 4^th, Nov. 27^th and Dec. 4^th 2022, but so far have said nothing about rotorcraft and very little about balloons and airships  (though that may change, if current plans for Venus materialise).  But with rotorcraft, that’s an omission that it’s time to put right.

The Venus settlers of Patrick Moore’s young adult novel World of Mists  (1956)  used helicopters for transport  (Fig. 1), because that version of Venus was water-covered below the clouds, except for scattered islands.  I don’t remember how they were propelled but probably they were electrical, because the atmosphere was being terraformed to make it breathable for humans, and presumably internal combustion engines couldn’t function either.  On Venus as we now know it to be, rotors would have no advantage because the atmospheric pressure at the surface is 90 times Earth’s, and there’s no solar power available until well above the 1-atmosphere level in the clouds.  Radar images appear to show wind streaks and there are imaginative proposals for land-yachts, windsurfers and wind-turbine rovers, but for flight, balloons and dirigibles will have the edge.

I’ve found online another Patrick Moore novel with an almost identical helicopter on the cover, Peril on Mars  (1958).  Again I don’t know what the propulsion is, but I would guess it’s electrical as the characters are again portrayed in spacesuits  (Fig. 2).  Back then it was known that the atmosphere was mostly carbon dioxide, but surface pressure was assumed to be about the same as at the summit of Mount Everest – in the low gravity of Mars, perhaps not impossibly far above the ceilings of helicopters which have been used for mountain rescue in the Alps and Himalayas.  But as we now know, the surface pressure on Mars is much lower than that, about 1% of Earth’s, equivalent to ours 20 miles up.

However, as the late Prof. Terence Nonweiler said, “The helicopter is the most inefficient flying machine we have, but we accept that in exchange for its capabilities to do things which fixed-wing aircraft can’t, like rise and descend vertically, hover and fly backwards.”  And one of those capabilities is to spin the blades faster, fast enough to generate lift even in the Martian atmosphere.

The Mars helicopter which eventually became known as ‘Ingenuity’  (Fig. 3)  was designed by the Jet Propulsion Laboratory of California Institute of Technology in Pasadena  (still not part of NASA, but essential to it).  It was carried on the underside of the Perseverance rover  (Fig. 4), with an underslung cover to protect it from the rigours of the descent below the hovering, rocket-powered Skycrane for the final touchdown on January 18^th, 2021.  The touchdown site in Jezero crater, which shows clear signs of flooding in the ancient, watery past of Mars, has been named after the science fiction writer Octavia E. Butler.  Ingenuity’s cover was dropped at the end of March  (Fig. 5), and after deploying the helicopter’s undercarriage  (Fig. 6), it was set down on the surface on April 1^st, surviving its first night unprotected on Mars on April 3^rd  (Fig. 7). 

After close-up study  (Fig. 8), the rotor blades were unlocked on April 8^th  (Fig. 9)  and after ground tests, the first flight of any aircraft on another planet took place on April 19^th   (Fig. 10).  The site was named ‘Wright Field’ in honour of the Wright Brothers’ first flight  (Fig. 11), and carried a piece of fabric from the original Wright Brothers’ flier.  The second flight took place on April 22^nd  (Fig. 12).

Unlike Perseverance, which is nuclear powered, Ingenuity has solar cells and flies when the Sun is overhead  (Fig. 13). 

Because of its fragility, it wasn’t expected to last for more than five test flights  (Figs. 14 and 15). 

Things went so well, however, that by the fifth flight Ingenuity was looking for a new ‘landing field’ which would allow Perseverance to move on,  By the 8^th flight, on June 15^th 2021, Ingenuity had reached its fourth landing site  (Fig. 16), and was actively exploring an areological  (Mars equivalent of geological)  region called ‘Seitah’, plotting rock ridges and patches of soft sand which it would be dangerous for the rover to cross  (Fig. 17).  By that time it had become a major component of the mission, and despite one or two wobbles in flight, due to glitches in software, it was showing no signs of wear and tear and looked likely to continue indefinitely.  After its 9^th and 10^th flights, an official Flight Log was created for it  (Fig. 18).

In 2022 Perseverance was heading back towards its landing site, to establish a cache of samples for future return to Earth.  On its 26^th flight, on April 19^th, Ingenuity photographed the re-entry backshell and parachute to see how they were surviving Martian conditions  (Fig. 19).  Winter was approaching, and thickening dust in the atmosphere was limiting the solar power available, so the helicopter was temporarily grounded after its 29^th flight to conserve power.  When the worst was over it took to the air again, but sadly the Chinese rover Zhurong, powered down for the same reason, has not been so lucky and has yet to ‘phone home’, long past the date its onboard timer was set for.  By its 37^th flight on December 17^th, 2022, Ingenuity had flown over 7 miles, totalling over an hour in the air.  By the end of January 2023, it was up to 41 flights and still going strong.

Perseverance has meantime planted its first 10 samples in a zig-zag pattern, for easier recovery, at a site called Three Forks.  The original plan was for them to be retrieved in due course by a specialised European rover engagingly called ‘Fetch’  (Fig. 20), for delivery to a rocket which would take them up to the Earth Sample Return Vehicle in orbit  (Fig. 21).  But Ingenuity’s success has caused a rethink.  The Fetch rover has been discarded, and the samples will now be taken to the return vehicle by Ingenuity and Perseverance acting in concert.  In case Ingenuity does not survive till then, two duplicate helicopters will be brought to Mars by the carrier rocket.  The date for the retrieval has still to be set.

In the Solar System, the possibilities for off-Earth rotorcraft are limited to Venus, Mars and Titan, the largest moon of Saturn.  Mercury has no atmosphere, the gas giants’ winds are too strong and turbulent, and the atmospheres of Io, Triton and Pluto are all too thin to provide lift.  But the situation on Titan is interesting:  Titan is almost the largest moon in the Solar System, with a diameter only 100 km less than Ganymede’s, and surface gravity half that of Earth’s.  The atmosphere is nitrogen and very cold, just 100 degrees K, and the surface pressure is twice what it is here.   

Robert Zubrin, the architect of the proposed ‘Mars Direct’ manned missions, designed the NIFTE  (Nuclear Indigenous Fuelled Titan Explorer)  which would have a mass of 8 tonnes and would require only 4 square metres of wing at 160 kilometres per hour, “(practically no wings at all!)”, or 100 square metres at 32 kph..  (Robert M. Zubrin, ‘The Case for Titan’, Ad Astra, June 1991).   NIFTE could release a range of 10 kg sub-probes with rotors, lifting cells or other combinations.  In October 2005 NASA’s Outer Planets Assessment Group proposed a Titan Orbiter Aerorover Mission, using an airship 14 metres long and able to travel round Titan’s equator in two weeks  (Keith Cooper, ‘Return to Titan’, Astronomy Now, January 2007).  The University of Idaho proposed a Titan aircraft called AVIATR  (Aerial Vehicle for In-situ and Airborne Titan Reconnaissance).  The allusion is to the biopic of Howard Hughes, rather than to James Cameron’s Avatar and its coming sequel.  Global Aerospace Corporation, jointly with Northrup-Grumman, has proposed VAMP, a hybrid glider and ‘aerobot’ blimp in the shape of a flying wing, initially for Venus but adaptable for Titan.

There were joint studies by NASA and ESA for a Montgolfière hot-air balloon with a radioactive isotope power source, and NASA’s Innovative Advanced Concepts office added the Titan Aerial Daughtercraft  (TAD), a small helicopter which would return periodically to the balloon to recharge its batteries  (Fig. 23).  The success of Ingenuity on Mars has again led to a major rethink and the helicopter is now an official programme, more substantial, landing solo  (Fig. 24) and with its own power source  (Fig. 25).  Currently it’s scheduled for launch in 2027, to reach Titan in 2034  (Fig. 26).

Because the surface air density is twice that on Earth, “a human standing on Titan could fly by strapping wings on to his or her arms in the manner of Daedalus and Icarus”  (Robert M. Zubrin, ‘The Case for Titan’, op cit).  My colleague Gordon Ross, inventor of the Dick-Dick inflatable aircraft for Mars and the flex-wing Waverider  (see ‘Flight in Non-terrestrial Atmospheres’, ON Dec. 4th 2022)  came independently to the same conclusion, looking for a way to explore Titan without generating enough heat to disrupt the environment, and coming up with a one-person vehicle rather like the ‘ornithopters’ of Frank Herbert’s Dune novels  (Fig. 27).  Similar ones feature in a short documentary  (3 mins 50 secs)  by Erik Wermquist called Wanderers, accessible online, with text by Carl Sagan.  (Not to be confused with a documentary on Bolton Wanderers which Google thought I must be looking for.)   Interestingly, if the gearing problem could be solved for Titan, ornithopters could be made to work on Earth, at last, with modern compact power supplies and high-strength, lightweight materials.  Leonardo da Vinci wasn’t wrong, just a long way ahead of his time!  

But there is a problem in making such an elaborate, flexible design work at such low temperatures.   Ice pebbles around the Huygens lander on Titan could be seen melting in the heat which it emitted.  A vehicle much above the temperature of its surroundings might trigger some very strange chemical reactions, spoiling its scientific usefulness if not actually dangerous – “one small step for a man”  (Fig. 28)  might turn into a giant leap indeed.

Still, the late Prof. Terence Nonweiler, inventor of the Waverider spacecraft  (ON Nov. 27^th 2022)  would have approved.  Human-powered flight was another interest of his, praised by Arthur C. Clarke in The Challenge of the Spaceship.  Nonweiler’s work on it influenced the design of the Gossamer Albatross, which achieved the first human-powered crossing of the Channel in 1979, and that in turn influenced the design of Gordon’s inflatable Dick-Dick for Mars.  In this field what goes around comes around, and with Ingenuity going around so successfully on Mars, and Dragonfly heading for Titan in four years’ time, we can expect to see a great deal more happening.