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Engineering the Oberth Maneuver

As we saw recently with the analogy of salt grains for stars, the scale of things cosmic stuns the imagination. But we don’t have to go to galactic scale. We can stay much closer to home and achieve the same effect. Because at our current technological levels, getting even as far as the outer planets taxes our capabilities. The least explored types of planet in our Solar System are the dwarf worlds, places like Ceres, Pluto and Charon, not to mention the enigmatic Triton. It takes years to reach them.

Beyond these objects we have a wide range of other dwarfs that merit study, at distances that push us ever farther. In a description of their NIAC Phase I study, just announced as a selection for 2022, Jason Benkoski and colleagues at Johns Hopkins University look into a combination heat shield and solar propulsion system that would perform a close Solar pass and use the Sun’s gravity to slingshot outwards at the highest possible velocity. It’s a maneuver familiar to Centauri Dreams readers, and one recently examined by the Interstellar Probe team at JHU’s Applied Physics Laboratory.

Benkoski is a materials scientist who has been working with the APL team, envisioning a tight solar pass around the Sun followed by the firing of a thruster to enhance the craft’s acceleration. This will require the probe to move within 1.6 million kilometers of the Sun’s surface, actually four times closer than the Parker Solar Probe plans to reach by 2025. In a 2021 article in Johns Hopkins Magazine, Benkoski explained the concept, which will preserve the heat shield by using channels filled with hydrogen gas that are built into the bulk of the shield itself. As the article puts it:

During the probe’s searing slingshot around the sun, the gas would heat up, expand, and course through the channels that all lead to a single exhaust nozzle. “The idea is to absorb all this heat with hydrogen,” Benkoski says, “and shoot it out the back of the probe.” In this way, the cooling setup also opportunistically doubles as an engine, thus supplying the thrust needed to complete the Oberth maneuver in the first place. “It’s like hitting two birds with one stone,” Benkoski says.

Image: Graphic depiction of combined heat shield and solar thermal propulsion system for an Oberth maneuver. Credit: Jason Benkoski.

The team believes that advances in materials science and engineering make their solar thermal engine concept a workable model for development. The 20 x 20 cm prototype they designed and fabricated is at benchtop scale, using liquid helium as coolant and propellant. The new study will extend this work, taking the concept into the realm of realistic materials and propellants. No small challenge, that, given that the contemplated Oberth maneuver would subject the probe to temperatures of 2500 degrees C, enough to melt even the Parker Solar Probe’s heat shield.

Benkoski points out that neither of our Voyagers was designed for observing the interstellar medium through which it now passes, while of course the Pioneers have long since ceased to function. New Horizons remains thankfully robust but will ultimately succumb to dwindling power levels and lose communications with Earth. The numbers are daunting: The Voyagers managed 3.6 AU per year, while even a full-stack SLS (which will never fly this mission) would push a 1 tonne spacecraft only to 8 AU per year.

The latter would require not just a working SLS but a Jupiter gravity assist, limiting the fly-out direction of our probes. Hence the need for a solar Oberth maneuver, in Benkoski’s thinking, which would be capable of surviving temperatures of 2800 K and use propellants now under study to widen the range of potential mission targets:

We…therefore propose a full trade study of alternate propellants in order to determine the maximum escape velocity for a given total system mass, including spacecraft, heat shield, propellant storage, and attitude control system. The main propellants of interest include H2, LiH, Li, CH4, NH3, and H2O. Methods: First we would determine material compatibility for each propellant with respect to its proposed storage system. We then calculate the efficiency (specific impulse) as a function of temperature for each propellant using Chemical Equilibrium Analysis (CEA).

Benkoski intends to discover how the mass and storage volume of the tank scale with the quantity of propellant to produce a series of realistic tank designs, devising an equation for the heat shield area and maximum propellant fraction that can be achieved given the limitations of existing heavy boosters. We’ll see how this study fares in producing a full-scale heat shield/heat exchanger design with robust long-term cryogenic storage. A tight Oberth maneuver is not going to be easy. See Assessing the Oberth Maneuver for Interstellar Probe for some of the myriad reasons why.

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{ 23 comments… add one }
  • Mark March 3, 2022, 11:18

    I’d like to see the numbers on a fully refueled Starship already in LEO, one with no gravity assist and one with an assist at Jupiter.

    • Deanna Shaw March 4, 2022, 3:43

      Well… Jupiter’s orbital velocity is about 700 km/sec so that would be the upper ceiling for any added velocity we could steal. (Voyager gained but 10 km/sec, but was aimed ……at Saturn next…….).

      I really like this concept, combination heat shield with H2 cooling that drives the craft at a very close dive near Sol…. and I was hoping for velocity numbers from that dive too…. but I’m guessing in the neighbor hood of 500 km/sec?

      Is there any added benefit to jettisoning the shield/engine/tanks etc when no longer needed? (Nothing but the payload left)???

      Let’s see someone design a proof of concept, small scale version, loft it into LEO as a Cube Sat, send it at Sol and see how she fares.

      Maybe it will melt!

      • Adam Crowl March 4, 2022, 8:06

        Jupiter’s heliocentric orbital velocity is 13 km/s. If you skim the cloud tops the escape velocity is about 60 km/s. Do a burn and add 5 km/s you leave Jupiter’s sphere of influence at 25 km/s. Added to the 13 km/s orbital velocity it means leaving the solar system with a hyperbolic excess of 33 km/s.

      • Ron S. March 4, 2022, 9:35

        700 km/s? You’re high by about 687 km/s.

    • TheRadicalModerate March 5, 2022, 1:41

      First, you can do better from an HEEO than from LEO. It costs more propellant, but it’s not too bad and it’s operationally fairly tractable. (Not without risk if you need to hit a specific target, though: If you miss your departure burn from the HEEO’s perigee, you incur a large inefficiency.)

      I have numbers for certain specific assumptions:

      1) A deep space Starship variant that has 100t dry mass, 2000t of prop, and RVac’s with Isp=378s.

      2) Departure from a C3=-2 HEEO (400 x 385,000 km).

      3) Let’s say a 5t payload.

      4) Jupiter flyby, both with and without an Oberth burn.

      I get the following:

      Heliocentric speed after Earth departure: 48.8 km/s
      Heliocentric (not Earth) v∞ after Earth departure: 24.6 km/s
      Helio v∞ after Jupiter flyby (no Oberth): 38.3 km/s = 8.1 au/yr
      Helio v∞ after Jupiter flyby (full-tank Oberth): 63.3 km/s = 13.4 au/yr

      Before you get too excited about that last number, it requires refueling the Starship variant after Earth departure, which is very risky (gotta make a rendezvous after a screaming hot departure from perigee, with lots of tankers, all with different residuals) and really expensive (high tens to mid hundreds of tankers’ worth of prop from LEO).

      You could probably do better than this if you added a Venus flyby as well, but you’re coming close the point of diminishing returns. And you can do even better if your “payload” is a 150t gross-mass NEP system with a 5t actual payload. But with NEP, it stops being so much about delta-v and more about how long it takes to go through all the propellant, and you need a magical multi-megawatt nuke. For practical trip times, it’s hard to get much past 30 au/yr. That’s really, really fast, but it’s not fast enough to do, say, a trip to 1000 au or to a dead stop at the solar gravitational focus (542 au) within the career of a researcher.

      Still, it’s better’n a sharp stick in the eye.

      FWIW, we have a discussion on this topic going over at NSF: https://forum.nasaspaceflight.com/index.php?topic=55550.0

      • charlie March 5, 2022, 13:57

        “First, you can do better from an HEEO than from LEO. ” what is HEEO ?

        • TheRadicalModerate March 6, 2022, 0:11

          Highly Elliptical Earth Orbit. Low perigee, high apogee, which makes the speed at which the departure burn occur much higher. That gives you quite a bit of Oberth Effect, which increases the final departure speed (aka the excess hyperbolic speed).

          The downside is that you have to refuel in the HEEO, which is more expensive in terms of propellant and somewhat riskier.

        • Adam Crowl March 6, 2022, 5:46

          HEEO = Highly Eccentric Earth Orbit

  • Alex Tolley March 3, 2022, 11:48

    This approach I find more interesting from a history of technology approaches than any mission design.

    There have always been at least 2 schools of thought on protecting the surface from heat:
    1. Place a layer of resistance material between the heat source and the surface.
    2. Actively cool the surface to remove the heat.

    The first approach, familiar since school days, is to use asbestos support for glass beakers to protect from a bunsen burner flame. Asbestos and similar materials have long been used this way. Spacecraft settled on ablative materials to protect a vehicle from the heat of reentry. It is an approach used in all spacecraft including SpaceX’s upcoming starship.

    The second approach is the way rocket engines are designed, to use cryogenic propellants to cool the chambers and nozzles of the rocket engine to allow the highest temperature possible for the chamber. Reaction Engines uses LH2 to cool the hot, hypersonic air before it enters the hybrid engine, a breakthrough technology for hypersonic craft and potentially spaceplanes.

    Another thread is the use of absorbers to transfer heat from energy beams to heat propellants. Beams can be microwaves or light and have been suggested for launch vehicles and solar thermal engines. Interestingly the recent post on LTRs suggested that laser light not be absorbed and the energy transferred to a propellant but focussed directly onto the propellant in a rocket engine. The design of the engine used active cooling to allow the high temperatures needed for the high Isp when LH2 is used as the propellant.

    I think one can see that the design of active cooling of the thermal shield acts in an analogous way to the energy absorbers to heat propellants. The close approach to the sun obviates the need for solar concentrators, and neatly does some jujitsu on the heat problem and need for propulsion at perihelion in an integrated design that has been explored for other space vehicles in the past.

    In the post on the LTR (part 2) it was suggested that the design of the thrust chamber could withstand temperatures of 10,000K and was based on gas core nuclear engines that might withstand temperatures of 50,000K. With the solar surface at about 5,778 K, just how close an approach might be possible with this type of design? As sungrazing comets can potentially skim the sun’s surface, it is possible that a vehicle with active shield cooling and propulsion could theoretically make such close perihelions and thus gain the maximum benefit of the Oberth maneuver?

    • Michael Fidler March 4, 2022, 9:59

      There is one other method to keep objects cool, magnetic fields. The plasma near the sun could be deflected from the spacecraft by a large magnetic field. Why jetteson the heat shield when you could use it to go into orbit around the gas giants or Titan…

      • Alex Tolley March 4, 2022, 11:36

        You can deflect the charged particles, but not the electromagnetic radiation from the sun.

        But the use of this approach to aerobraking makes sense. Any remaining propellant both cools the shield and reduces the velocity. However, unlike the sun, most of the heat is due to the plasma generated as the craft hits the atmosphere, so a magnetic shield should work well. I recall that Slough abandoned the plasma magnet for propulsion in favor of its use as an aerobraking and reentry shield.

    • Adam Crowl March 5, 2022, 7:30

      Hi Alex
      There was a NIAC study on the heat protection needed to send a probe to the Sun’s photosphere. “Solar Surfing” was the NIAC study’s title. So it’s imaginable technologically.

      • Alex Tolley March 5, 2022, 14:58

        Solar Surfing – interesting approach. Using a highly reflective sun shield to get within 1 solar radius of the sun. So passive shielding rather than active cooling.
        Solar Surfing : Final Report on a Phase I NASA Innovative Advanced Concepts Study
        The report indicates that the reflective material is particles that are up to 3cm deep, so fairly lightweight. The shield has a high emissivity backing, that gets rid of the last 1% of the light that is not reflected. The report suggests that the shield temperature can be kept below 1000K at 1 solar radius, although it appears it would rapidly increase the temperature at closer approaches.

        Suppose such a highly reflective shield works (across all the radiation wavelengths required), since it is so lightweight, would it be worth combining this with the absorber technology? The shield would protect the craft to as close to perihelion as possible, then stowed, allowing the absorber to heat the propellant for the needed thrust to complete the maneuver, and then redeployed after the propellant is exhausted to protect the probe until it reaches a distance that is no longer in danger of being damaged by the solar radiation. This hybrid concept might allow the probe to reach the perihelion without needing to use the LH2 primarily for cooling before it reaches the desired position for the Oberth maneuver, allowing the maximum use of the LH2 for thrust at perihelion.

        The Breakthrough Starshot sail is to withstand very high laser energies with extremely high reflectance. But these are for monochromatic light. Is there any possibility that these foils can handle a wider range of wavelengths with high reflectivity?

        Dave Nixon’s comment below is analogous to ablative shielding which is effectively what comets do.

  • James Webb (no, not THAT one) March 4, 2022, 16:28

    Should have asked Arthur C Clarke while we had the chance. He described it all in “Rendezvous With Rama” (which I’ve just been re-re-reading)!

  • Michael March 4, 2022, 17:00

    Could a diamond lens work, the serious amount of light will be bent away from the sensitive parts of the craft and the focal point of the solar radiation can be on the propellant chamber.

    • Alex Tolley March 4, 2022, 21:25

      Diamonds melt and boil before 5000K. Therefore it seems that diamond lenses would not be useful in this regard. Maybe they could be cooled with LH2 in a way that still allows them to focus the light?

      • Michael March 5, 2022, 2:48

        That’s still better than the 2500 C talked about and helium as a coolant is probably better as it won’t react with the carbon of the diamond. Nano lens are getting better and better now so weight costs are reduced significantly.

      • Michael March 6, 2022, 18:17

        A nice thing about using say a frenzel lens is that the light can be concentrated onto a very small rocket motor lowering the weight and this also allows us to be further out from the sun lowering the temperature on the lens material. Should be easy to test as well.

        • Alex Tolley March 6, 2022, 21:24

          If you are talking about Fresnel lenses for STR (ie even LTR) to heat propellant (but probably nowhere close to the sun, then yes I like that type of lens too. Because it is flat, it can be spin-stabilized to keep it flat. 2 contra-rotating lenses would eliminate the gyroscopic effect. The downside is that w/o 2ndry mirrors, they have to be kept aligned with the illumination source – e.g. the sun – which means a limited direction of travel. If a large, flat solar sail was added, then the directions of travel are increased, and the craft can be a hybrid solar sail and STR. It should be possible to keep the sail and the 2 Fresnel lenses with mass and aerial density approaching 3 sails, with the rocket engine and payload as the minor part of the total mass, and the propellant mass dependent on the mission.

          I will say that the LTR post by Higgins did intrigue me in one regard; using the propulsion stage just to accelerate the craft out of orbit and then releasing the payload on an unpowered trajectory. It makes me wonder if that idea should be developed further for STR/LTR boost stages to send a solar sail on its way quickly out of a gravity well. A Fresnel lens for the boost stage would perhaps be designed either more like a donut to allow the propellant to be exhausted back through the lens, or for the exhaust to be directed by confinement through the lens before being released.

          • Michael March 7, 2022, 16:09

            The fresnel or meta designed lens can be laid as troughs with the spacecraft components in the shadow areas so the motors are perpendicular to the lens surface.

            • Alex Tolley March 7, 2022, 22:23

              I don’t understand what you are saying. The Fresnel lens must be positioned between the motor at the focus and the light source (the sun). Assuming one intends the initial journey to be away from the sun the exhaust must pass by the lens in the direction of the sun. For a circular or annular lens, this means that the exhaust must pass through the lens. One could add structure so that the heating chamber at the focus passed the hot propellant in parallel to the lens (in 2 opposite directions) and then perpendicular to the lens after the lens periphery was reached. The exhausts could be rotated and gimballed to provide thrust in most directions. Unlike mirrors, you don’t get any choice in offsetting the lens – it must be aligned between the light source and the propellant heating chamber.

              But if you have a better way to do this, then please describe it or link to a drawing/image that shows what you mean. [You can post your own drawings on any image hosting site and link to that URL.]

              • Michael March 9, 2022, 14:58

                If the lens concentrates the light onto the parallel exhaust chamber the craft will be pushed towards the sun. If the lens concentrates the light on a chamber that is perpendicular to the lens it can be used to slow the craft down to fall into a close pass of the sun. Perhaps a two stage system is required, with a laser system to drop it fast inwards to start with and ejection of part or all of the components to lighten the load on the next drop.

  • Dave Nixon March 5, 2022, 3:03

    Spinning the craft would reduce the incident solar energy per unit area by a factor of pi and also simplifies attitude control. Another idea is to use many layers for the heat shield and make a short fat craft that gradually gets thinner as layers are destroyed.

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