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Ernst Stuhlinger: Ion Propulsion to Mars

If you’re a Centauri Dreams regular, you’re familiar with Adam Crowl, an Australian polymath who is deeply involved in the ongoing Project Icarus starship design study. Adam maintains a blog called Crowlspace where interesting and innovative ideas emerge, some of them related to earlier work that has been largely forgotten in our era. A recent post that caught my eye was on Ernst Stuhlinger’s ‘umbrella ship,’ a kind of spacecraft that, when introduced to the world on Walt Disney’s 1957 TV show Mars and Beyond, surely surprised most viewers.

The umbrella ship, as Adam notes, looks nothing like what readers of the famous space series in Collier’s (1952-1954) had come to associate with manned travel to other worlds. Wernher von Braun was then championing massive rockets to be engaged in the exploration of Mars, an exploratory operation that would send a fleet of vessels to the Red Planet. Unlike tiny capsules of the kind we used to reach Earth orbit and explore the Moon, these would be large vessels to be sent in great numbers. The expedition would be described by its designer in a book von Braun wrote in 1948 called Das Marsprojekt (translated into English in 1953).

What von Braun depicted and what both Collier’s and Disney immortalized was a fleet of ten spacecraft that would send 70 crew members to Mars, the spacecraft to be built in Earth orbit using reusable space shuttles. While von Braun radically revised the plan in 1956 and scaled it back substantially, Ernst Stuhlinger was working with an entirely different concept.


Image: Ernst Stuhlinger’s Umbrella Ship, built around ion propulsion. Notice the size of the radiator, which disperses heat from the reactor at the end of the boom. As Adam notes in his blog piece, the source for this concept was a Stuhlinger paper called “Electrical Propulsion System for Space Ships with Nuclear Power Source,” which ran in the Journal of the Astronautical Sciences 2, no. Pt. 1 in 1955, pp. 149-152 (online version here). Credit: Winchell Chung.

No chemical rockets for Stuhlinger. While von Braun envisioned his fleet using a nitric acid/hydrazine propellant, Stuhlinger was interested in electrical propulsion, producing thrust by expelling ions and electrons instead of combustion gases. He noted in the paper that using chemical reactions to produce thrust created a high initial mass as compared to the payload. To reduce this mass problem, he saw, it would be necessary to increase the exhaust velocity of the propellant. Accelerating propellant particles by electrical fields made the numbers more attractive, as the paper notes in its summary:

A propulsion system for space ships is described which produces thrust by expelling ions and electrons instead of combustion gases. Equations are derived from the optimum mass ratio, power, and driving voltage of a ship with given payload, travel time, and initial acceleration. A nuclear reactor provides the primary power for a turbo-electric generator; the electric power then accelerates the ions. Cesium is the best propellant available because of its high atomic mass and its low ionization energy. A space ship with 150 tons payload and an initial acceleration of 0.67 x 10-4 G, traveling to Mars and back in a total travel time of about 2 years, would have a takeoff mass of 730 tons.


Image: Wernher von Braun and Ernst Stuhlinger discuss the Umbrella Ship concept at Walt Disney Studios. Credit: NASA MSFC.

Adam works out the details, drawing from the Stuhlinger paper itself and deriving some quantities through his own work. We get a payload, including landing vehicle and crew habitat, that is about 20.5 percent of launch mass, an impressive figure indeed. We’re also saddled with low acceleration, as you would expect. The Umbrella Ship would take about a year to reach Mars, while a chemically propelled ship as analyzed by Stuhlinger would make the journey in about 260 days. The longer the travel time, the greater the hazard, which was in many ways unknown to Stuhlinger, as Adam comments:

These days we wouldn’t want a crewed vehicle spending weeks crawling through the Van Allen Belts, but back when Stuhlinger computed his trajectory and even when the design aired, the Belts were utterly unknown. Now we’d have to throw in a solar radiation “storm shelter” and I’d feel rather uncomfortable making astronauts spend two years soaking up cosmic-rays in interplanetary space. Even so, the elegance of the design, as compared with the gargantuan Von Braun “Der Mars Projekt” for example, is a testament to Stuhlinger’s advocacy of electric propulsion.

But what an interesting design to emerge in the 1950s, and it’s ironic given the above remark that when Explorer 1 was launched in 1958, Stuhlinger was at the controls of the timer that, in those relatively primitive days of space technology, handled rocket staging. Explorer 1 was the satellite that discovered the Van Allen belts in the first place. A German infantryman (he was wounded outside Moscow and later served at Stalingrad), Stuhlinger joined the German V-2 effort and worked closely with von Braun, later coming to the United States as part of Operation Paperclip. In the 1950s, he actively collaborated with von Braun on the Disney films Man in Space, Man and the Moon and Mars and Beyond.

Stuhlinger would spend a great deal of time on ion thrusters using either cesium or rubidium vapor, accelerating positively charged ions through a grid of electrodes. Today, he is considered a pioneer of ion propulsion, well known for his book Ion Propulsion for Space Flight (McGraw-Hill, 1964). He would serve as director of Marshall Space Flight Center’s Space Science Laboratory until 1968 and later as MSFC’s associate director for science, going on to become a professor at the University of Alabama in Huntsville and a senior research assistant with Teledyne Brown Engineering. Ernst Stuhlinger died in Huntsville in May of 2008.


Comments on this entry are closed.

  • Alex Tolley July 21, 2015, 13:57

    It is a magnificently asymmetric design that almost jars the mind. Taking a quick peek at my copy of the “Mars and Beyond” segment, I see that the idea still bears the Von Braun hallmark of a fleet of ships to explore Mars. One can only imagine the cost.

    IIRC, ion engines were just in the earliest stages of conception at that time, so his design was highly speculative.

    The specs called for a lot of power, 114MW of which 23MW was usable electrical power. That would be large even today, and we would be hard pressed to deliver that with a solar array. But the specific power is low for today’s technologies.

    But overall I like that design, although I would prefer to substitute solar power for the nuclear.

    It would be interesting to run the numbers for the ship without the lander, using more contemporary technology, to see if the design might be feasible for a Mars flyby and return.

  • Hop David July 21, 2015, 15:50

    Looks like Stuhlinger imagined a nuclear power source with a honking big radiator to dump waste heat. I believe a solar power source could more watts per kilogram. A.k.a Alpha — alpha’s a very important consideration for ion drives as well as ISRU. I talk about the needed for a better alpha:

  • galacsi July 21, 2015, 16:53

    Out of the 730 tonnes of the spaceship , 189 are dedicaced to the atomic powerplant. I suppose the radiator (The Umbrella) is counted inside these 189 tonnes. Anyway solar energy is much simpler and lighter. The spaceship will need very big solar panels and presently we don’t have much experience in this matter but I don’t see that as a real problem.Then we have something like the Nautilus X project !

  • Dana Andrews July 21, 2015, 17:01

    The one year manned trip time can be cut down to ten months and the power reduced from 23 MWe down to about 1 MWe if the spacecraft spirals out from earth and transfers the crew onboard just before it does a lunar flyby.

  • Geoffrey Hillend July 21, 2015, 17:02

    If we use a nuclear reactor we might as well use VASIMR and get to Mars in only 39 days with today’s technology. Ion power with a nuclear reactor as a power source was certainly cutting edge technology in 1957 and safe when you consider that the other nuclear option was a solid core nuclear reactor propulsion by simply heating the fuel by sending it straight through the reactor(around the combustion chamber first to cool it and then through the reactor. )

    The solid core type engine was a graphite reactor with fuel rods to control the reaction rate. The Uranium 235 is inside the graphite walls I recall. The whole thing emits radiation and takes weeks to cool down, so it is dangerous and a radiation shield is needed for the passenger compartment or engine. Wherner Von Braun’s book, Space Frontier, 1969 verision.

  • Adam July 21, 2015, 17:12

    Hi Alex,
    Substituting solar for nuclear *might* seem more user friendly – that reactor is on the end of the boom for a very good reason – the numbers are rather daunting. Stuhlinger’s specific power for his drive system is ~96 W/kg, which would be very challenging for a purely solar system. His ion drive assumed an efficiency of ~90%, which is still a bit more advanced than our present ion-drives (typically ~60-70%.) Then there’s the inverse square law drop off between here and Mars to contend with. A solar thermal system can probably achieve 25-30% conversion efficiency though it then has a pointing machinery penalty.

    Even so, Stuhlinger also advocated solar ion-drives, which I believe Paul has blogged on before (i.e. Frank Tinsley’s “Cosmic Butterfly” depiction, which is a Stuhlinger design.)

  • Al Jackson July 21, 2015, 17:49

    Of course Ernst Stuhlinger did work on the Mars Project when von Braun was writing it in 1948, so did another space visionary Krafft Ehricke. As I recall Stuhlinger was a member of the group that discussed manned space flight at Peenemünde and it there that von Braun encouraged him to study ‘electric’ space flight, which led to his monograph many years later.
    It is my suspicion that when von Braun planned out the space flight part of the Disney program he was reworking The Mars Project, in 1955, into what became The Exploration of Mars published in 1960. He streamlined the Mars Project down to 12 crew and two ships.
    The Mars Project had been done in Colliers April 30, 1954 so von Braun felt it was time for something different.

  • ljk July 21, 2015, 19:40

    These spacecraft were presented on television in 1957 by Disney with their program titled Mars and Beyond:


    The companion comic book online, which includes the Umbrella ship flotilla mission:


  • ljk July 21, 2015, 22:00

    Winchell Chung’s page on the Mars Umbrella Ship here:


    Lots of details and diagrams.

    Other Stuhlinger Mars spaceship designs may be found here on this fascinating and detailed page of realistic rocket designs:



  • Hop David July 22, 2015, 1:41

    Geoffrey Hillend, the 39 day VASIMR trips are fiction. They assume an alpha that’s not doable.

  • Michael Spencer July 22, 2015, 7:36

    Hey, what’s that thing in Stuhlinger’s left hand? :-)

    Those were the days. I was never really very good with my ‘slip stick’- born in ’49, I’m on the electronic cusp- but I do recall seeing some of the older guys whiz around the thing maniacally. Truly a wonder to behold.

    I was given one on my 18th birthday, a very high-end K+E device of made of bamboo and plastic. And now I can’t even remember the names of the different scales.

  • Alex Tolley July 22, 2015, 13:15

    I think 96W/Kg would be very challenging for nuclear power too. Just launching such a reactor today is an issue, something that was not considered in the 1950’s. At least with solar you have a safe system that is technologically advancing.

    Like other space exploration schemes of the time, the size and scale is breathtaking. Today we would want to scale it all down to make it affordable. But no question the design was innovative, arguably more so than the “Butterfly”.

  • Hop David July 22, 2015, 16:54

    Near term thin film photovoltaics offer 250 watts per kilogram. Some folks (like Jeff Greason) believe a kilowatt per kilogram is doable with thin film photovoltaics. I am wondering though if that includes support structure and gimbals to point solar array towards the sun. For these thin film photovoltaic alphas I’m getting a mental image of acres of Seran Wrap.

  • Geoffrey Hillend July 22, 2015, 17:40

    Quote by Hop David: “Geoffrey Hillend, the 39 day VASIMR trips are fiction. They assume an alpha that’s not doable.” This is incorrect. Fiction is something that is impossible. All VASIMR needs is 200 megawatts of power and a power supply on the ground can easily be built on the ground which can deliver that but one small enough to launch into space is the problem. This problem can be solved in the future by a new type of reactor which does not use steam driven turbines for power. https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket

    recently, Ad Astra was awarded a 10 million dollar contract by NASA over three years to make VASIMR flight ready and produce thrust continuously for over 100 hours non stop. It will be put on the international space station at some point to keep its orbit from decaying.

  • Hop David July 22, 2015, 22:21

    Geoffrey Hillend, you mention a huge number of watts (which is a problem but not an insurmountable one). That’s the numerator of the alpha quantity. But you make no mention of how many tonnes this power source masses (alpha’s denominator). Please learn what alpha is. Once again the 39 VASIMR trips require a magic power source. See http://forum.nasaspaceflight.com/index.php?topic=1139.msg600043#msg600043

  • Adam July 23, 2015, 5:06

    To clarify an interesting argument on the merits of VASIMR, the oft-referenced “39 days to Mars” scenario had the following specifications: payload 22 tons, power-plant+engine: 100 tons (meaning a bracing specific useful power of 2 kW per kg), Launch Mass ~600 tons, Arrival speed at Mars 6 km/s.

    No solid core nuclear reactor in space can produce 200 MW of power in such a low-mass system. The Ad Astra study posited a Gas Core or Plasma Core reactor using MHD power conversion – none of the technologies alone are without some experimental experience but in combination it’s unknown, unproven territory.
    As much as we’d like to believe in magic wands, such exotic fission reactors aren’t really viable options at present. Developing it on Earth for power *might* be worthwhile, but mandatory before we can hope to use it in space.

  • Adam July 23, 2015, 8:37

    Here’s the breakdown of the Stuhlinger Mars-ship:

    Propellant: 365 tons – cesium or rubidium [NB: don’t suggest xenon – only 6 tons of the stuff is extracted commercially per year]
    Engine+Radiator+Reactor: 215 tons
    Payload: 150 tons – includes mini-von Braun Wheel Station as crew habitat, landing rocket, sounding rockets and two “bottle-suits”.
    Specific Impulse: 84 km/s
    Thrust: 485 N

    For power-limited thrust systems using continuous acceleration trajectories, a mass-ratio of 2 is the gold standard.

    The assumed jet-power is 20.5 MW, with a 90% efficient ion-drive. The reactor produces ~114 MW of heat and converts that into ~22.8 MW of electricity. The difference is dumped as heat via the umbrella.

    The engine mass is only a small fraction of the whole – the reactor+radiator comprise most of the alloted 215 tons. If a solar array could be lighter, per kilowatt of power supplied, then it would also need to provide less power for the same acceleration performance. The real mass limitation on space solar power systems is not the arrays as such, but the power distribution system. Handling multi-MWs of electrical power means a LOT of wiring is involved. If only we had low mass superconductors…

  • Robert Clark July 23, 2015, 8:48

    The ion propulsion disadvantage is the low thrust. This means a long transit time. However, Hall effect thrusters put out more thrust and can shorten the transit time to weeks instead of months IF you have sufficient power. The point is we have the technical means to provide that power by using solar concentrators:

    Sunday, March 16, 2014
    Short travel times to Mars now possible through plasma propulsion.

    Bob Clark

  • Hop David July 23, 2015, 9:49

    Thank you Adam for a lucid and informative post.

    In anticipation I want to point out the difference between thermal and electric watts.

    When I question VASIMR it’s common for defenders to cite power sources with better than 2kW per kg specific power. But the examples give output in thermal watts. An ion engine needs watts in the form of electricity.

    The nuclear electric power plants earth side must dump large amounts of waste heat. The massive cooling towers have become an icon for nuclear power: http://wpmedia.o.canada.com/2011/03/5556-simpsons-nuclear-reactor-2501.gif

    In space there are no neighboring streams to carry away waste heat, in fact vacuum’s a good insulator. An interesting part of the Stuhlinger design is the large umbrella heat radiator.

    Looking at Crowlspace … A very good blog!

  • Michael July 23, 2015, 14:00

    @Adam July 23, 2015 at 8:37

    ‘Propellant: 365 tons – cesium or rubidium [NB: don’t suggest xenon – only 6 tons of the stuff is extracted commercially per year]’

    Also Iodine and now with the ‘stagecoach’ idea water.



  • Adam July 23, 2015, 17:25

    Hi Michael,
    Alex Tolley’s Stage Coach (aka Space Winnebago, as I prefer to call it) uses electrothermal thrusters with relatively low Isp compared to Stuhlinger’s ion-drive. Water is sub-optimal for ion-drives for many reasons, chiefly because the acceleration grid is exposed to hot oxygen chemical attack and would erode rapidly. Indirect heating, via helicons, and oxide engine chambers (eg. plain old glass) make water a viable propellant, but needs far more energy to ionize for higher velocities.

    As I noted in my follow up comment above, the problem isn’t array mass or reflectors or the solar cells themselves. There are many options to minimise that side of the mass equation. The real problem, limited by the inherent power carrying capacity of wires, is the power distribution of multi-megawatts. That provides a rather hefty floor value of the mass of any large SEP system. A fundamental materials improvement is required to overcome that issue.

  • Paul D. July 23, 2015, 23:08

    Very high specific power electrical systems would have to be beamed, I suspect. One could imagine beaming UV light across a few AU with emitter and receiver not TOO large.

  • Eniac July 23, 2015, 23:48


    Handling multi-MWs of electrical power means a LOT of wiring is involved. If only we had low mass superconductors…

    Or, we could use many smaller engines distributed between the solar panels, such that the power never needs to be bundled.

    Here are a few promising-looking thrusters:


    Plasma thrusters have a big advantage over ion thrusters in that they can use pretty much anything as propellant, including water. Their specific impulse is comparable, and can mostly be chosen to be optimal for the mission. Apparently the MIT team has built plasma thrusters from glass bottles and aluminum cans….

  • Adam July 24, 2015, 8:27

    Hi Michael

    Thanks for the iodine Ion drive link. Very interesting. I’ll pass it on to a friend who’s designing a CubeSat.

  • Michael July 24, 2015, 15:14


    ‘Handling multi-MWs of electrical power means a LOT of wiring is involved. If only we had low mass superconductors…’

    Superconductors can be heavily massed and still be economical as they can carry much higher currents that are hundreds of times higher than normal conductors.

    As for the Iodine info you are welcome.