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Assessing the Oberth Maneuver for Interstellar Probe

I notice that the question of ‘when to launch’ has surfaced in comments to my first piece on Interstellar Probe, the APL study to design a spacecraft that would be, in effect, the successor to Voyager. It’s a natural question, because if a craft takes 50 years to reach 1000 AU, there will likely be faster spacecraft designed later that will pass it in flight. I’m going to come down on the side of launching as soon as possible rather than anticipating future developments.

Two reasons: The research effort involved in stretching what we can do today to reach as high a velocity as possible inevitably moves the ball forward. We learn as we go, and ideas arise in the effort that can hasten the day of faster spacecraft. The second reason is that a vehicle like Interstellar Probe is hardly passive. It does science all along its route. By the time it reaches 1000 AU, it has returned massive amounts of information about the interstellar medium, our Sun’s passage through it, and the heliosphere that protects the Solar System.

All of that is germane to follow-on missions, and we have useful science data all the way. So I’m much in favor of pushing current technology into stretch missions even as we examine how to go faster still with the next iteration, the one that would succeed Interstellar Probe.

Getting Up to Speed

How fast can we travel now, as compared to 1977, when we launched Voyagers 1 and 2? We know we can reach 17 kilometers per second with 1977 technology because that is what Voyager 1 is doing right now. Interstellar Probe advocates would like to see something in the range of 95 kilometers per second as a way of making the 1000 AU journey in 50 years. That’s still, I suppose, within the lifetime of a researcher, but not by much, and it’s heartening to me that we’re extending the boundaries into a frank admission of the fact that some missions may be launched by one generation, maintained by another, and brought home by a third.

I always assumed we had an ace up our sleeves when it came to ramping up Voyager speed levels. Moving close to the Sun and making a propulsive burn at just the right moment seemed a sure way to exploit that deep gravity well and fling a probe outward at high velocity. The idea first appeared in Hermann Oberth’s Wege zur Raumschiffahrt (Paths to Spaceflight), which was published in 1929 in Germany. At the time, Oberth was also working as a consultant on the Fritz Lang film Frau im Mond (The Woman in the Moon), which would popularize the idea of rocketry and space travel. In fact, Oberth would dedicate Wege zur Raumschiffahrt to Lang and actress and screenwriter Thea von Harbou.

The authors of the Interstellar Probe 2019 report note in their extremely useful appendices that Oberth’s thinking on the maneuver that would be named after him anticipated in many ways the idea of using a gravity assist that was developed in the 1960s by Michael Minovitch. His thought experiment involved an astronaut on an asteroid 900 AU from the Sun. The astronaut, apparently quite long-lived, wants to go to a star some 1015 kilometers away (roughly the distance of Regulus). His asteroid has an orbital speed of 1 km/s and an orbital period of 27,000 years.

I won’t go into this in huge detail because it’s laid out so well in the report’s appendix (available here). But Oberth’s setup is that the target star is in the orbital plane of the asteroid, and he assumes the astronaut has a rocket that can produce a velocity change of 6 km/s. Sun, asteroid and target star are in a line in that order. He asks: What is the fastest way to reach the star?

Using the rocket alone reaches it in 5,555,000 years. Waiting for 20,000 years to add the asteroid’s orbital velocity to the velocity of the ship reduces that to 4,760,000 years. But Oberth realizes that the best answer is to use the rocket to move opposite to the asteroid’s motion, falling in toward the Sun to reach 500 km/s at perihelion, then using the remaining rocket fuel to boost the speed a bit further. He ultimately gets 70.9 km/s moving out of the Solar System, and his transit time is now reduced to 470,000 years. Thus the ‘Oberth maneuver’ enters the literature.

A spacecraft launched from Earth has to lose the heliocentric angular momentum of Earth’s orbit to fall toward the Sun in order to make the Oberth maneuver possible, the most efficient method being a direct trajectory from Earth to Jupiter, a retrograde gravity assist at Jupiter, and a long fall back to perihelion, at which point a kick-stage provides the further propulsive burn. All of this, including of course the thermal issues raised by putting the payload into such proximity to the Sun, has to be weighed against a straight gravity assist at Jupiter, with no close solar pass, when contemplating how best to accelerate the Interstellar Probe for the journey.

Image: This is an image of Parker Solar Probe as envisioned by Goddard Media Studios at NASA’s Goddard Space Flight Center in Maryland. It’s the closest I could come to what a close solar pass would look like, though it lacks the propulsive element of the Oberth maneuver. Credit: NASA GSFC.

Oberth in Today’s Terms

When I contacted Interstellar Probe principal investigator Ralph McNutt (JHU/APL) about these issues, he pointed out that the Mission Concept Report for the entire project would be made available on the probe website in the first week of December. Putting what the report will describe as the Solar Oberth Maneuver (SOM) through the severe filter of engineering capabilities with today’s technologies is a major priority of this report, and the results McNutt conveyed make it clear that my enthusiasm for the concept has been unjustified.

Unjustified, that is, in terms of a spacecraft being designed, as this one must be, around current technologies. Remember that we’re talking about a mission with a specific timeframe, one with a launch in the early 2030s, meaning that the materials and techniques to build and fly it have to be within range today. The Oberth maneuver at the Sun may have possibilities for us down the road. But today’s engineering constraints make the issues stark. As McNutt told me in an email:

…after a very careful look and relying on the same people, including the mission system engineer, who worked the thermal protection system (TSA) for Parker Solar Probe (PSP) we have concluded (1) the SOM offers no advantage over prograde gravity assists in rapid escape from the solar system for a “technology horizon” in the 2030’s and (2) there is no obvious “path” to changing this conclusion for the foreseeable future.

Image: Ralph L. McNutt Jr., chief scientist for Space Science at the Johns Hopkins University Applied Physics Laboratory and principal investigator for Interstellar Probe. Credit: Johns Hopkins University.

In other words, going to Jupiter straightaway, with no Oberth maneuver, is just as workable, and as we’ll see, avoids a series of thorny problems. One issue is the need for thermal protection, another the demand of launching a payload sufficiently large, one that would incorporate not only the propulsive stage for operations at perihelion preceding the long cruise, but would also include the science instrument package and the necessary high gain antenna that would be needed for data downlink at the distances the probe is envisioned to reach. We have to work within the constraints of present-day launch systems as well as existing engines for the kick.

On thermal issues, the Interstellar Probe team worked with Advanced Ceramic Fibers, an Idaho-based company, on ultra-high temperature material studies, the question being how one could take existing thermal protection as found on the current Parker Solar Probe mission and extend it into the range needed for the Solar Oberth Maneuver. But shield mass, said McNutt, is only one consideration. A ‘ballast’ mass is also required to keep the center of gravity moving along the engine centerline as the propellant burns down during the maneuver.

These issues of mass are critical. Let me quote McNutt again:

The real problem is the mass of the thermal shield assembly – multiple shields plus the supporting structure – to shield just the kick stage itself, even with no Interstellar Probe spacecraft. We’ve adopted solid rocket motors (SRMs) with specific impulses approaching 300s with masses of up to ~4,000 kg (Orion 50XL). In that case, we have an engineering solution that closes on paper, has all of the design margins included, would require specialized design work (> ~10’s of millions and multiple years of dedicated effort) and ends up with about the same performance (flight distance after 50 years) as a prograde Jupiter gravity assist, but with significantly more inherent risk, both in development and in the actual execution of the burn at the Sun itself. Bottom line: it may be doable with an investment of significantly more time and money, but it offers no advantage, and, therefore, we have concluded it would be a poor trade.

Within the upcoming report will be the 181 staging scenarios the team examined by way of reaching its conclusions about the Solar Oberth Maneuver. It becomes clear from the synopsis that McNutt gave me that existing technologies are simply not up to speed to realize the potential of the SOM, and even extending the technologies forward to nuclear rocket engines and greatly enhancing the performance of today’s launch vehicles would not change this fact. To make the Oberth maneuver at the Sun into a viable option, it appears, would take decades of work and demand billions of dollars in new investment. Best to shelve Oberth’s concept for this mission, though I suspect that future technologies will keep the concept in play.

Where to next with Interstellar Probe? If we rule out Oberth, then the two scenarios involving a Jupiter gravity assist remain, the team having considered other options including solar sails and finding them not ready within the needed timeframe. The first is a ‘passive’ flyby, in which every rocket stage is fired in an optimized launch sequence. The second is a powered gravity assist, in which a final kick-stage is reserved for use at Jupiter. We will see what the upcoming report has to say about these options, balancing among outbound speed, complexity, and mass.

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{ 24 comments… add one }
  • steven Torry Rappolee October 6, 2021, 14:33

    What would be the trades for this?
    Ice Giant mission
    Jupiter sun flyby but the solar flyby is no closer than Venus
    Ice Giant missions are in need of more launch date/trajectory options
    Would this open up any?
    Note this is not an interstellar probe

  • Alex Tolley October 6, 2021, 15:46

    Question about the chart on page 11-17 of the online document.

    The chart x-axis; “Perihelion in solar radii from the Sun’s center”.

    The axis scale starts at 0, and the data plots to ~ 0.5 Rs. Since the axis is from the center of the Sun, not the surface, this implies that the craft would be inside the Sun – i.e. impossible. Is the x-axis label incorrect? If not, what is the explanation?

    • Michael Million October 8, 2021, 16:04

      The Barycenter is near the surface of the Sun so not inside.

      • Alex Tolley October 8, 2021, 18:32

        Barycenter of what? The sun and…? Surely not the sun and the spacecraft. If the barycenter is the 0 point, why not make this clear in the x-axis label?

      • Alex Tolley October 8, 2021, 18:57

        The Sun-Jupiter barycenter is outside the sun’s surface. A sun diver mission using Jupiter to dive back to the sun would need the perihelion to be between the sun and Jupiter. Ordinarily the craft would achieve perihelion on the other side of the sun, so the distance to the barycenter would be greater than 2Rs.

  • Michael October 6, 2021, 15:54

    I once thought about a sundiver probe which had a long thin streamer sail which unfolded edge on approaching the sun, from two rolls of it, it then opened up using the sunlight to split open the sail and expose it to the sun quickly accelerating it greatly with no use of fuel other than sunlight.

    • Andrei October 7, 2021, 14:08

      There has been some people who have calculated that such a maneuver would yield a tremendous good result. The 100 km per second for a TAU mission described here is well within the possible.
      The sunlight will be extremely faint at 1000 AU, and I guess the sail would be ejected at some point. If there’s any solar sail missions to KBO’s the sail might not be discarded but reused to focus sunlight either on a stirling engine or even used in other ways. That’s something for possible mission planners to think about, that if we already have tagged along a sail that far – there might be useful ways to repurpose it at a later stage.

  • Dave Moore October 6, 2021, 16:01

    I think I’ve got a way of doing the Oberth maneuver, if not for this generation space probes, then the next.

    Currently under development are electro-thermal microwave steam drives. These use solar-power to microwave water to a high temperature to provide thrust.

    Initially, the efficiency of these thrusters will be about the same as a nuclear thermal rocket, i.e. 600 sec. With the development of higher temperature materials like Thorium Oxide (m.p. 3,390 °C) they hope to reach the ISPs of ion thrusters.

    The scenario I envisage would be that the interstellar probe using solar cells for power would spiral out from low Earth orbit. Most of this probe would be water tank. It would make its way to Jupiter, using a Venus-Earth gravity assist and use Jupiter to drop it in towards the sun. As it approached the sun and the temperature on its heat shield approached 3000°C, it would then pump water through the heat shield, cooling it to allow for a closer approach to the sun. The very high temperature steam would be then fed through the existing rocket nozzle system providing good thrust at high ISP.

    Not being a rocket engineer, I have no idea how well this would work.

    • Alex Tolley October 7, 2021, 10:11

      With the development of higher temperature materials like Thorium Oxide (m.p. 3,390 °C) they hope to reach the ISPs of ion thrusters.

      Do you have a useful reference for this?

      BoE calculation:
      Rocket exhaust velocity (Ve) is proportional to sqrt(T/M) [where T = combustion temperature and M = molecular weight of exhaust]

      Assume that the Thorium Oxide material helps to double the exhaust temperature, that only increases Ve by 1.4x. Unless the exhaust molecular wt changes between a LH2/LOX SSME with Isp = 450s, then Ve reaches 630s.

      NTRs with H2 propellant have Isp ~ 800-900s.
      If mol wt of SSME exhaust = 6 [(O + 2H)/3 and NTR = 2 [H2]
      Ve -> sqrt(3) -> 1.7x -> Isp 765s
      Ve =>sqrt (3) => 1.7 => Isp = 780s LH2/LOX average mol wt = 6,

      If mol wt of SSME exhaust = 9 [(O + H2)/2 and NTR = 2 [H2]
      Ve -> sqrt(4.5) -> 2.1x -> Isp = 950s

      Combining T and M inprovements Ve => sqt(2*4.5) => 3x => Isp 1350s

      Ion thrusters have higher Isp than this. Wikipedia: Ion Thruster.

      Hence I am interested in a reference that indicates why my BoE calculations are so far off what might be achieved with what is being described as advanced solar thermal rocket.

      • torque_xtr October 8, 2021, 20:09

        There could be a serious limitation to water microwave thruster, because at some temperature water will dissociate and cease to absorb many microwave wavelengths (absorption is reduced for simple molecular fragments). If there are no suitable high-power microwave sources which are absorbed both by H2O molecules and hydroxyl radicals, then Isp will have a limit comparable to that of H2+LOX engines. Or, it could go straight for plasma torch mode, but this is different story…

        • Alex Tolley October 9, 2021, 10:01

          Some proposals H2O plus microwaves includes using electric/magnetic acceleration of the ions once the water has dissociated to a plasma to boost the Isp. Microwave only systems will get you an Isp up to perhaps 900s. Solar thermal systems have lower Isp. There are also microwave systems that heat an absorber plate that in turn heats a propellant that gets you a similar Isp to chemical rockets and has been proposed for launchers, but they have the added complication of needing a high energy microwave beam source that can closely track the flightpath of the launcher.

      • Dave Moore October 9, 2021, 16:35

        This is thruster I had in mind:
        https://en.wikipedia.org/wiki/Microwave_electrothermal_thruster

        Here’s a paper in their current state of development:
        https://www.researchgate.net/publication/3166116_The_microwave_electro-thermal_MET_thruster_using_water_vapor_propellant

        It’s downloadable, and as you can see from it, it’s highly scalable, and has a ISP of 800 sec. Further development of it does not appear to be a great problem.

        I pulled this off my mental shelf as it uses the same propellant as my cooled thermal shield idea.

        The cooled thermal shield would have an ISP similar to that of a nuclear thermal rocket, but a better thrust to weight ratio as it wouldn’t have the nuclear reactor.

        • Alex Tolley October 9, 2021, 23:15

          This looks like one of the sources Brian McConnell and I used as the propulsion for our Spacecoach concept.

          • Dave Moore October 10, 2021, 13:36

            I always thought it was the best proposal for space propulsion, and my computer drive is littered with various proposals for space rockets.

            Your biggest competitor appears to be the VASMIR rocket, but I regard electrothermal steam propulsion as superior (at least at lower powers) as it appears to be simpler mechanically, more scalable, and relies on water for a propellant, which is more easily storable than Hydrogen especially for long trips.

            When I wrote my first post on this matter, I was thinking of TransAstra, a space mining project, but their rocket is a solar thermal one, which has applicability to the Oberth maneuver.
            https://www.transastracorp.com/space-propulsion

            Trans Astra has a website full of lovely graphics, but I not sure they are going anywhere at least in the short term as their proposal is to bring a 100 tons of water back from an asteroid. Trouble is Elon Musk, using brute force methods with his giant starship rocket, will be able to lift a 100 tons of water into low Earth orbit quite cheaply.

  • ljk October 6, 2021, 17:13
  • gbaikie October 6, 2021, 22:29

    “In other words, going to Jupiter straightaway, with no Oberth maneuver, is just as workable, and as we’ll see, avoids a series of thorny problems. One issue is the need for thermal protection, ”

    That what I thinking in regard to the other post. Or just use Jupiter {and get close to Jupiter- not sure how close is “normal” distance- and get the Oberth effect accelerating at Jupiter. And rather have any radiation shielding added than the thermal protection added}.
    But I would add needing refueling in orbit technology as part of the mission- and so have bigger payload and leaving Earth to Jupiter, faster.

  • Robin Datta October 7, 2021, 17:05

    If an adeuqate black hole could be found, the issues of heat and an atmosphere might be avoided.

  • torque_xtr October 7, 2021, 17:59

    From a more distant point of view, the challenges of shielding for sundiver Oberth maneuver don’t look so insurmountable with current-tech-only. The Parker Solar Probe shield needs to be so robust because of long exposure and repeated thermal loading. On the contrary, during SOM there is only a single pass, with several days of heating comparable to PSP’s, and several hours-to single day of much harder peak load. I can imagine a structure comparable to that of JWST shield but made from reflective tungsten foil, with sun-facing layer a thickest one to compensate for sublimation. Or just some kind of ablative shield, like carbon aerogel panel, or composite-phenolic like used on Galileo entry probe… which faced harder loads – more than twice as hot as Sun’s *surface* together with huge ram pressure and g-forces for several minutes, compared to 3000-4500 K in vacuum for several hours. There will be the need to compensate for mass loss and reactive force, but it’s hard to be a sundiver without becoming a little comet and experiencing non-gravitational forces!
    SOM could also enable studies of near-solar environment not accessible to PSP…

  • Patient Observer October 7, 2021, 21:10

    Hey! If Planet 9 is discovered fairly soon, that would be a destination in itself sufficient to justify the mission. It may be as distant as 650 AU per YouTube (really) so far enough away to drive the development of high velocity space probes. It would be a tantalizing target. It could have multiple moons; perhaps even a moon similar to Europa heated by tidal interactions with a liquid ocean beneath the ice. Without a mysterious destination, the mission planner would be accused of spending tax payer’s money on a mission to nowhere. It would be a flyby but a lot of data can still be harvested as proven by New Horizons.

  • Bill October 8, 2021, 7:04

    I’m surprised no one has mentioned starshot as an option for such a mission. I suspect that’s because of the proposed timeline and the fact that starshot is still not much more than a concept at this point but these sorts of missions are ideal to learn what is possible.

    • Alex Try October 9, 2021, 3:51

      I am sure none remind Starshot – because understand that this project will never be implemented. Even if implemented (do not believe that), by default , Starshot as it is described – cannot return any usable data, because cannot carry any useful payload.
      Starshot is nonsense, by the way as well as planned “interstellar” mission discussed here.
      Voyagers had some interesting fly-by targets , this discussed mission has no target, i.e. nonsense.

      • Alex Tolley October 9, 2021, 12:41

        If you read Lubin’s early proposal (2015) for high power laser sails, the idea was that lower velocities with larger payloads could be done. IOW, the 1gm version was only needed for achieving 0.2c.

        As an example, on the eventual
        upper end, a full scale DE-STAR 4 (50-70 GW) will propel a wafer scale spacecraft with a 1 m
        laser sail to about 26% the speed of light in about 10 minutes (20 kgo accel), reach Mars (1 AU) in
        30 minutes, pass Voyager I in less than 3 days, pass 1,000 AU in 12 days and reach Alpha Centauri
        in about 20 years. The same directed energy driver (DE-STAR 4) can also propel a 100 kg payload to
        about 2% c and a 10,000 kg payload to more than 1,000 km/s.

        Is a 10MT payload with a velocity of 1000 km/s good enough for a science mission? ;) We can argue over the likelihood of his propsed DE-STAR 4 (50-70 GW) phased laser arrays ever being developed, but the smaller systems could be with DoD funding for anti-satellite missions (as well as the “cover story” of planetary defense). I wouldn’t bet against China developing such a system either, given their prior experiments.

        Source: A Roadmap to Interstellar Flight

        • Alex Tru October 10, 2021, 3:23

          Alex, in my comment I related only to my vision of the “breakthrough starshot” project future and results.
          This project is dead beast and nonsense by default, from it’s own definitions.
          I do not deny possibility to use lasers as propulsion – but this technology will not be developed by Starshot, anyone else can do that, but not this funny project.

          • Alex Tolley October 10, 2021, 12:32

            I get what you are saying. Starshot Breakthrough as funded by Milner is just a early development to prove that the idea is workable and that we could, with full funding, send a diminutive probe to Proxima and maybe get some data back.

            Bear in mind that the approach, laser propelled sails was predicated on Lubin’s work on lasers, that was presented at the Starship Century conference as a way to do planetary defense against asteroids. Personally, I interpreted this as away to present a proposal to develop very powerful lasers for military purposes but in a palatable way. I think the highly reflective sails to monochromatic laser light came a little later (but my memory fails me on this point). As teh laser arrays are the major cost of such probes, the only US agency capable of funding it is teh DoD. As I don’t see them particularly interested in interstellar probes, nor even planetary defense except as a civilian rationale, they would want such lasers for miliray purposes – as demonstrated by earlier laser experiments.

            Lubin’s roadmap is just one path to bootstrap laser arrays for various missions preparatory to interstellar missions. I am quite interested in beamed propulsion, whether for sails of for electric engines, as well as supplying energy to machines and bases across the solar system. (Perhaps it is a failing due to being engrossed in Dan Dare, or the various sci-fi scenarios of laser arrays near Mercury beaming power back to Earth and elsewhere. Beamed power has a history going back to at least Nikola Tesla, and I have seen ideas for powering aircraft, as well as the powersat era that became the economic backbone to O’Neill’s space habitats.)

            Lastly, I wouldn’t write off gram scale space probes just yet. Miniaturization continues apace, and we now know that even tiny signals can be amplifield by using gravity lenses. So a probe doing a fast flyby of a system might capture enough pixels and other data that could be transmitted back to Earth by maneuvering the craft to Proxima’s needed focal line to earth as it passes beyond Proxima.

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