Advanced Space Propulsion Study - Antiproton and Beamed Power Propulsion

AFAL TR-87-070

R. L. Forward

OTHER PROPULSION CONCEPTS

In addition to the major effort on antiproton annihilationpropulsion and the minor effort on beamed power propulsion, anumber of other advanced propulsion concepts were investigatedduring the contract. Those studies carried far enough to bepresented in papers or reports were investigations of theconcepts of metallic hydrogen as a high energy rocket fuel,tether space transportation systems, and unconventionalapplications of solar sails. The latter two systems arediscussed in a paper, "Exotic Propulsion in the 21st Century,AAS-86-409, presented at the 33rd Annual Meeting of the AmericanAstronautical Society held in Boulder, Colorado from 26-29October l986.

RENEWING RESEARCH ON METALLIC HYDROGEN

In a prior study⁷ carried out in 1983 on alternate propulsionenergy sources, page 7-7 summarized the conclusions of a task tolook at the feasibility of using metallic hydrogen for advancedpropulsion. Metallic hydrogen is a postulated high energypropellant that releases its energy when the atomic metal isconverted into gas molecules. The estimated specific impulse is1700 s and the specific density is 1.15. It has been estimated⁸ that pressures of 1.9 to 5.6 Mbar [megabars or millionatmospheres] or equivalently 0.19 to 0.56 TPa [terapascals] wouldbe needed to produce metallic hydrogen. There are sometheoretical speculations that once metallic hydrogen is produced,it will be metastable and will remain in the metallic form whenthe pressure is released, while other theoretical estimates castdoubt on any metastable state.

At the time the previous study was completed in September1983, the highest continuous pressure that had been obtained inthe laboratory was 0.5 Mbar or 0.05 TPa. Thus, therecommendation of the previous study was that research onmetallic hydrogen should "wait for the development of new highpressure machines that can produce steady megabar pressures."

In a recent issue of Science magazine, two reports indicatethat perhaps the time has come to reevaluate the desirability ofcarrying out research on the formation of metallic hydrogen. Inthe first paper, ⁹ scientists at the Geophysical Laboratory,Carnegie Institution of Washington, have assembled a diamondanvil press that attained static pressures of 0.21 to 0.55 TPa(2.1 to 5.5 Mbars). These are an order of magnitude higher thanhad been achieved before and are the theoretically predictedpressure levels where metallic hydrogen should be formed.⁸ Ifthese pressures can be attained with molecular hydrogen in thesample chamber, then either metallic hydrogen will be formed orthe theory will have been proved wrong. Either result will be ofgreat scientific importance.

In the second paper,¹⁰ scientists from Sumitomo ElectricIndustries in Japan, manufacturers of carat-sized single-crystalsynthetic diamonds, demonstrated that their synthetic diamondscan be used as a pressure generator. Although their firstattempt was terminated at a pressure of 68 GPa (0.68 Mbar) by thefailure of one of the diamond anvils, their results suggest thatsynthetic diamond can be effectively used for pressuregeneration. There are a number of advantages to syntheticdiamonds. First, their cost is extremely low. Six half-caratdiamonds cost only $130. Second, they can be grown to largesizes in a repeatable fashion, although long bars and plates areeasier than thicker lumps. Third, the synthetic diamonds can bemade with controlled impurity levels, especially nitrogen, and itis known that substitutional nitrogen improves the strength ofdiamond. Thus, as the synthetic diamond growth process matures,it should be possible to make large, inexpensive, relatively highvolume, very high pressure presses. As a result, carrying outthe research and scaling up the process does not look formidable.

I therefore recommend that the Air Force look once again atcarrying out in-house and sponsored research on metallichydrogen. This would first involve the construction and use ofhigh pressure diamond anvil presses to demonstrate the feasibilityof making metallic hydrogen. Then, if this research issuccessful, further fundamental research should be carried out onthe properties of the metallic hydrogen as a function oftemperature, pressure, time, applied magnetic field, nuclear andatomic spin state, purity, and type of chamber wall material. Ifthis research is successful, then engineering studies should bestarted to determine the feasibility and cost of scaling up theresearch results to achieve production quantities of metallichydrogen for testing as a rocket fuel.

TETHERED SPACE TRANSPORTATION SYSTEMS

The concept of a stairway to heaven is a constant themethroughout mythology. In the Bible, Jacob dreamed that there wasa ladder set up on the Earth, and the top of it reached toheaven, and the angels of God were ascending and descending onit. From the far East came tales of magicians who could toss theend of a rope into the air, where it would stay, hanging fromseemingly nothing. Then there is the ancient children's story of"Jack and the Beanstalk." It turns out that it may be possibleto make these fairy tales come true.

Out at the very special distance of 36,000 km from thesurface of the Earth (about six Earth radii), there now existdozen of satellites in geostationary orbit. Suppose somefriendly giant in one of those satellites were to let down a longcable - 36,000 km long. If the cable were strong enough to holdits own weight, then it would reach down to the surface of theEarth. It would be a Skyhook, a magic beanstalk in reverse.Given adequate supplies stashed along the way, a lightweightspacesuit, and enough time, Jack would be able to climb intospace instead of having to use a rocket.

One of the first persons to think of the Skyhook concept wasthe Soviet engineer and popular science writer Yuri Artsutanov. ¹¹He unfortunately published the idea as a popular article in theSunday supplement section of Komsomolskaya Pravda (YoungCommunist Pravda) in 1960, where it was ignored by the West andsubsequently reinvented a number of times. The final versions ofa Skyhook system have two cables, one 36,000 km long going downto the surface of the Earth, and another 110,000 km long goingoutward to a ballast weight in order to keep the center of massof the system at geostationary altitude. Theoretically, byputting a sufficient taper in the cable, it can be made ofanything. In practice, there is no material presently availablestrong enough and light enough to make an Earthgoing cable withan acceptable taper ratio.

A breakthrough in the production of long fibers of singlecrystal graphite or diamond might change the picturesignificantly. For example, actual measurements of tiny graphitewhiskers show a tensile strength of 2.1xl0⁶ N/cm² or 3x10⁶lb/in.². With that strength, a 1 cm² cable of crystallinegraphite could lift almost a 1000 km length of itself in thegravity field of the Earth. With a taper of 10:1, a graphitecable could be built to go all the way out to geostationaryorbit, and beyond. The starter cable, which must be assembled inspace and lowered down to the Earth, would have a mass of about900 tonnes, have a diameter of about 1 mm at the Earth's surface,and be able to lift only 2 tonnes. This would suffice for aboot-strap operation that would allow more cable to be hoisted upfrom the ground.

If the Skyhook design used a number of cables arranged in ahollow structure, then electrified tracks could be built insidethe structure. As each car climbed the skyhook from the Earth'ssurface into geostationary orbit, it would consume an appreciableamount of electrical energy. The cost of the electricity, $2/kghauled into orbit, would be much less than the cost of usingrockets, which is presently $5000/kg. Cars continuing beyond thegeostationary point would be pulled along the cable by the everincreasing centrifugal force. The cable cars would have to braketo keep from flying out too fast. If the braking were done by anelectric motor, the braking energy could be turned intoelectricity and used to raise the next cable car up from theground. On reaching the ballast mass, the cable car would be150,000 km from the center of the Earth and moving with atangential velocity of 11 km/s. If the cable car were to let goof the cable at just the right time, the car (now turnedspacecraft) would be able to coast to Saturn on a minimum energyorbit or travel rapidly to all the other planets nearer thanSaturn.

There is another version of the Skyhook that I call theRotavator. It uses a cable that is much shorter than thegeostationary orbit Skyhook. The Rotavator rotates as it orbitsabout the Earth, the ends of the cable touching down near thesurface. This concept was also the brainchild of YuriArtsutanov, ¹¹ who published it as a popular article in themagazine Znanije-Sila (Knowledge-is-Force). The magazineillustrator's title drawing for the article shows a huge wheelrolling over the surface of a small Earth - an apt illustrationof the concept since the rotating cable acts like a pair ofspokes rotating inside an invisible wheel. It was Moravec, however, who published the first technical paper ¹² on theconcept.

The Moravec design for a Rotavator uses a 4000-km-long cable.This is one-third the diameter of the Earth, but only one-ninththe length of a 36,000 km geostationary Skyhook. The taper for aderated graphite cable would be about 10:1. To be able to lift a100 tonne cargo into space it would have to mass about5400 tonnes. The central portion of the cable would be put intoan orbit that is 2000 km high with a period of 120 min. Thecable would be set to spinning at one revolution every 40 min.Six times each orbit, once every 20 min, one of the ends of thecable would touch down into the upper regions of the Earth'satmosphere. Because of the large dimensions of the bodiesinvolved, the ends of the cable would seem to come down into theupper atmosphere nearly vertically, with almost no horizontalmotion. At touchdown the end of the cable would approach andleave the Earth with an acceleration of 1.4 Earth gravities.Counting the one gee field of the Earth itself, there would be atotal acceleration at liftoff of only 2.4 gees, less than that ata Shuttle launch. Since even a stiff cable would have somestretch to it, there would be almost a full minute available fortransferring cargo and passengers. After riding on the Rotavatorfor 20 min, you would be at the peak of the trajectory andtraveling at 13 km/s. A payload released with this velocitycould arrive at the orbit of Mars in 72 days and reach Venus in41 days.

A Lunar Rotavator could be made with presently availablematerials, like. the superfiber Kevlar made by DuPont. A3700 tonne Kevlar Rotavator around the moon would be able to liftand deposit 100 tonnes every 20 min. Rotavators could also beput on all the smaller planets and moons. Similar spinningcables in solar orbits between the planets¹³ could act astransfer points or "velocity banks" to cut the travel timebetween tie planets in the solar system. As long as more mass isdropped inward down the gravity well of the Sun than is goingout, no energy source would be needed to operate thisinterplanetary space transportation system once it was set intomotion.

It won't be long before the first tethers will be flying inspace as a Shuttle experiment.¹⁴ NASA and the Piano SpazialeNazionale of the Consiglio Nazionale Delle Ricerche (PNS/CNR) ofItaly issued a joint Announcement of Opportunity OSSA-l-84 in 15April 1984 for a Tethered Satellite System. A NASA built tetherwill be used to "troll" an Italian scientific satellite 100 kmdown from normal Space Shuttle altitude into the upper reaches ofthe atmosphere at 150 km. The Italian satellite mass is 500 kgand is spherical in shape with a tail to keep its aerodynamicinstrument pointing in the forward direction. This half-tonnesatellite will be supported by a very thin metallic or syntheticline I to 2 mm in diameter and 100 km or longer in length.Although the satellite mass is 500 kg, the tension expected inthe cable is only 200 N (40 lb).

Once the NASA engineers have flown one or more of thesesystems without incident, then some of the more risky tetherexperiments can be attempted. A payload can be sent upwards manyhundreds of kilometers from the Shuttle on a tether.^{15, 16} If thepayload is released from the end of the tether, it will fly up toa higher elliptical orbit. The peak of this orbit could be highenough to catch onto a tether hanging down from a space stationin geostationary orbit. Longer tethers could even launch apayload into an Earth-escape trajectory.

There is a serious problem with single strand tethers -cutting of the tether by meteorites or space debris. ¹⁷Multistrand, cross-Iinked, fail-safe tether designs are needed.If this design problem can be solved, then as stronger hightensile strength materials become available, we may see thisexotic propulsion system leap from the pages of the fairy talebooks and send us bounding through the solar system on seven-league boots.

UNCONVENTIONAL MISSIONS USING SOLAR SAILS

The concept of solar sailing appears to have been firstconceived by Russian space enthusiasts Tsander and Tsiolkovskiback in the early 1920s. The most complete review¹⁸ of the stateof the art in solar sailing was carried out by a group at JPLback in 1973. Unfortunately, not much has been done since then.

A solar sail works by photon pressure from sunlight (theamount of pressure from the solar wind is negligible incomparison to the photon pressure) . When light reflects from thesurface of a body, the momentum of the light is reversed indirection. As a result, the body experiences a forceproportional to the power in the light divided by the speed oflight,

Where the factor 2 assumes normal incidence for the light on thereflective surface. Since the solar power flux at 1 AU is1.4 kW/m², the solar force per unit area is about 9 N/km². Thesolar sail can be steered by tilting the sail to vary thedirection of tie resulting force vector. If the sail is in anorbit around the Sun, it can move outward by directing the forcevector so that the sail speeds up, flying outward from the Sun.By tilting the sail so that it slows down, the sail will fallinward to the Sun. Since the maximum force is achieved when thesail operates near the Sun, most solar sail trajectories forinterplanetary missions tend to first go inward to do their planechanges before heading out toward their target.

The 1976 JPL study¹⁸ was a design for a solar sail to carryout a Halley rendezvous mission (as distinct from the high speedflyby missions that were actually carried out) . The vehiclestructure used masts and rigging to deploy a square sail 800 m(½ mile) on a side. The sail was made of 1-µm-thick aluminum-coated Kapton, weighing 1.2 g/m². The total flight vehicle masswas 3150 kg, with a payload bus of 861 kg. The optimum missiondesign would take 200 days to go inward to the Sun, where thesail would "crank" its orbit around the Sun for 225 days until itwas going in a retrograde orbit matched with the retrograde orbitof Halley's Comet, then the sail would arc out in a long 506-dayelliptical trajectory until it caught up and made the rendezvouswith the incoming comet. The sail would be dropped, and thepayload bus would stay with Halley through its entire trajectoryaround the Sun (and hopefully be revived 75 years later).

A 1 g/m² sail was a low risk project in 1976. Today we coulddo much better. One approach would be to replace the mast andspars with a rotating structure that would use ballast weights tomaintain a wire support structure in tension. The 1976 JPLHeliogyro design¹⁸ is one example; the 1979 Drexler highperformance solar sail design¹⁹ is another. It consists of ahexagonal structural mesh of wires held in tension by rotation.Each triangular section of a meter or so on a side would hold anunbacked ultrathin film of aluminum. For a l0-km-diameter sailwith an area of 8x10⁷ m², the nonfilm structural mass isestimated as 0.03 g/m², while a thin film 30-nm-thick would mass0.08 g/m², for a total areal density of 0.11 g/m². Such a solarsail would accelerate at almost 0.01 gees.

You can't make the reflective sail much thinner without itsbecoming transparent. But it still might be possible to decreasethe mass per area without losing too much in reflectivity. Asany radar engineer knows, you do not have to make a radar dishout of solid metal. You can make it out of chicken wire if theholes in the chicken wire are much smaller than a wavelength ofthe microwaves being used. Thus, it might be possible to makesolar sails significantly lighter than present theoretical limitsif we poke holes in the sail, as long as the holes aresubmicrometer in size so they are smaller than a wavelength ofvisible light. Techniques exist in the laboratory to make a thinperforated sail. Focused ion beams have already demonstrated thecapability to make holes smaller than 0.1 µm, well below solarlight wavelengths. Crossed holographic gratings have alreadybeen developed in photosensitive resists and used to make arraysof square posts with 0.2 to 0.5 µm spacing. The use of apositive rather than a negative resist would produce a squaregrating with similar-sized square holes. Such a thin, perforatedsail could be produced using a plastic backing so that it couldbe handled and deployed. The plastic would be the type thatwould disintegrate in a short time under solar ultraviolet, leaving the perforated film. Once these high performance sailsbecome a reality, they can be considered for exotic trajectoriesthat are impossible with any other form of propulsion.

One of the potential applications of an ultrathin perforatedsolar sail is using light pressure from Sun to levitate theorbit of a geostationary satellite up out of the equatorialplane.²⁰ At present, the only geostationary orbits are thosealong the equator at 35,800 km altitude (42,200 km from thecenter of the Earth) . Although geostationary spacecraft can beseen at the Arctic and Antarctic Circles (depending on the localhorizon topography), they cannot be used by ground stations nearthe poles.

If a spacecraft were supplied with a lightweight sail, itcould use the sunlight to supply a constant force in the polewarddirection This would levitate the orbit out of the equatorialplane and the spacecraft would orbit about a point determined bythe relative magnitude of tie Earth gravity forces and the solarlight pressure forces. The amount of displacement north or southof the equatorial plane is limited to a few hundred kilometersfor unfurlable Kapton sails, and a few thousand kilometers forvery thin solid-aluminum~film sails. By perforating the sail,however, we can improve the displacement distance significantly.Figure 4 shows a geostationary orbit that is levitated by theconstant solar pressure 13,000 km northward from the equatorialplane, about twice the radius of the Earth.²⁰

The levitated orbit is noticeably displaced in the directionopposite to the Sun. (This effect was noticed on the Echoballoon satellite.) By varying the sail angle with the seasons,the levitated orbit can be kept synchronous with the Earth'srotation. The time of year chosen for Figure 4 is at summersolstice, where the Sun angle is the worst for providingnorthward thrust. In this worst case example, the position ofthe satellite is not truly geostationary. As seen from the northpole, it moves +/-1.7 deg. about its nominal elevation angle of 9.3 deg.The development of perforated solar sails and their use to createlevitated orbits would not only relieve the pressure on thelimited number of positions along the equatorial geostationaryorbit, but would for the first time provide a true geostationarycommunications capability to the militarily important polarregions of the Earth.

Since the solar sail never runs out of fuel, it.can be usedto carry out exotic missions that are not conceivable usingrocket propulsion. For example, a solar sail can hover in space,completely ignoring the usual constraints of orbital mechanics.One example is a shadow sitter that stays permanently in theshadow cone of a planet. ²¹As shown in Figure 5,the shadow cone of the Earth extends out to 217 Earth radii. An Eclipsat with asail made in the shape of a ring could then be placed to sit onthe shadow cone, with its payload hanging in shadow. From thisvantage point, the payload would see the Sun in a constantperfect total eclipse. From here scientific instruments couldcontinuously monitor the solar corona for solar storms ofimportance not only to solar physicists but also to communicationsystems and space travelers. Since the gravity field of theEarth at 217 Earth radii is only 20 microgees, even a sail with a]970 technology areal density of 1 g/m² could levitate a heftyscientific payload.

Since the source of the propulsion for solar sails is theSun, they naturally work better closer to the source. Inaddition, the gravity field of the Sun varies as 1/R² and thesolar flux varies as 1/R². Thus, once we get away from thegravity field of the planets, the performance of solar sails inthe gravity field of the Sun is independent of distance. For asail to be able to hover in the sunlight from the Sun, its mass-to-arearatio must be less than a certain value. This isdetermined by the balance between gravitational attraction andlight pressure repulsion,

where M is the mass of the Sun, m is the mass of the sail, andthe total light power P is given by the solar flux S times thearea A of the sail. For the distance of the Earth from the Sunof R = 1 AU = 1.5x10¹¹ m, the solar flux S = 1400 W/m². Withthese numbers we can calculate the mass per unit area of sail andpayload needed to just levitate the sail in the gravity field ofthe Sun:

One exotic mission for solar sails around the Sun would be toset up a set of scientific Sunwatchers to continuously monitorthe changes in the solar surface. As is shownin Figure 6, we could place a number cf high temperature sails in hoveringorbits around the Sun One could be placed over each pole to constantlymonitor what is going on in the polar regions.¹⁹ Others could beplaced in solar synchronous orbits with a period matching that ofthe solar rotation at the latitude of interest. Note thatbecause of the constant propulsive thrust capability of the solarsails, the resultant force vector can be used to cancel out aportion of the solar gravity pull, making the effective netattraction any value that you want, independent of the orbitalradius. Thus, the orbital period can be decoupled from theorbital radius, and a 25-day solar synchronous orbit can have anyaltitude from the solar surface that is convenient.(Unfortunately this trick won't work for communication satellitesaround the Earth. The Sun is off to one side of the Earth andnot at the center of the Earth.)

These are only a few of the exotic missions that can beperformed with the only rocket ship that never runs out of fueland lasts almost forever. Obtaining most of the more interestingmissions, however, requires the development of high performancesails - probably perforated thin films. What is needed are goodmeasurements of the performance of real films as a function offilm thickness, hole size, and ratio of hole area to total area.We also need engineering studies of how these films might befabricated, deployed, and supported during operation.