|Star-Raker (right), a single-stage-to-orbit space plane, parks next to a 747 at a conventional airport. Image credit: M. Alvarez/Rockwell International.|
Elsewhere in this blog, I have described the 1970s joint NASA/Department of Energy Solar Power Satellite (SPS) studies (see "More Information" below). Had even a single SPS been assembled, it would have been by far the largest human construction project in space; it would have weighed more than 100 times as much as the 420-metric-ton (460-U.S.-ton) International Space Station. The SPS studies envisioned assembly of two such satellites per year between 2000 and 2030, bringing the total number in the SPS constellation to sixty.
NASA envisioned boosting SPS components to low-Earth orbit (LEO) in the payload bays of massive reusable launch vehicles. One such launcher, Boeing's winged, two-stage Space Freighter, would have weighed about 11,000 metric tons (12,125 U.S. tons) at liftoff and delivered about 420 metric tons (463 U.S. tons) to LEO. For comparison, the two-stage Saturn V rocket used to place 77-metric-ton (85-U.S.-ton) Skylab into LEO weighed about 2800 metric tons (3086 U.S. tons) at liftoff.
The Space Freighter would have risen vertically from a launch pad and pointed itself generally toward the east. As its first stage, the Booster, expended its propellants, it would have separated. The second stage, the Orbiter, would then have ignited its engines to complete its climb to LEO. In orbit, it would have maneuvered to rendezvous and dock with a large space station designed specifically for handling SPS cargo modules.
The Space Freighter Booster would have been a fully reusable winged vehicle closely resembling the Space Freighter Orbiter. After Space Freighter Orbiter separation, the Space Freighter Booster would have turned, deployed jet engines, and flown to a long, wide runway at its launch site.
To begin return to Earth, the Space Freighter Orbiter in LEO would have separated from the cargo-handling space station, then would have turned its tail forward and ignited rocket motors to slow down, lowering its orbit so that it intersected Earth's atmosphere. Following a fiery reentry, it would have landed on the runway near its launch pad.
After launch pad, Orbiter, and Booster refurbishment, the two Space Freighter stages would have been hoisted vertical. After the Orbiter was placed atop the Booster's nose, a cargo module would have been loaded into its payload bay. The Space Freighter would then have been moved to a launch pad to begin another flight. Launching parts for two SPS into LEO in a year would have required about 240 Space Freighter launches, or about one launch every 36 hours.
In October 1977, a team of 14 Rockwell International engineers studied a Space Freighter alternative. The Star-Raker space plane, 103 meters (310 feet) long with a wing span of about 93 meters (280 feet), would have carried a maximum of 89.2 metric tons (98.3 U.S. tons) of cargo into LEO. More than 1100 flights would have been required each year to support the SPS program, or about one launch every eight hours.
In its fully developed form, however, Star-Raker would have had important advantages over Space Freighter which might have made its required high flight rate feasible. For example, it would have begun its flights to LEO by taking off horizontally from a conventional 2670-to-4670-meter-long (8000-to-14,000-foot-long) runway at virtually any civilian or military airport capable of supporting 747 or C-5A Galaxy cargo planes. No specialized launch and landing site would have been required.
Every bit as important, Star-Raker would have been capable of flying routinely between such airports. The Rockwell team explained that this would "reduce the number of operations required to transport material and equipment from their place of manufacture on Earth to [LEO]." For example, rolls of solar cell blankets would not need to be shipped by train, barge, or plane to a specialized launch and landing site; they would, potentially, need only be transported to a local airport for Star-Raker pickup.
David Reed, an engineer at North American Rockwell (NAR), as the company was then known, originated the Star-Raker concept in 1968, as NASA began earnest efforts to develop a reusable Space Shuttle. Key elements of the concept had been proposed — and rejected — earlier in the 1960s decade. These included wings packed with lightweight structurally integral tanks holding liquid hydrogen fuel and liquid oxygen oxidizer and a complex jet engine/rocket engine propulsion system.
The 1968-1969 study determined that, as it burned the propellants in its wings and maneuvered through ascent from subsonic speed to Mach 6 (six times the speed of sound), aerodynamic pressure on its structure would become excessive. This led NAR to examine wing designs developed in 1970 for the proposed (and subsequently abandoned) U.S. Supersonic Transport program.
A "tridelta flying wing" design appeared to solve the pressure problem; by then, however, NASA had narrowed its Shuttle design requirements, excluding Star-Raker from consideration. NAR continued Shuttle studies and became Shuttle prime contractor in July 1972.
Rockwell revived study of the tridelta flying wing Star-Raker as SPS studies ramped up in 1976. The Star-Raker study that began in October 1977, led by Reed and performed for NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama, continued into late 1978, yielding the design described in this post.
The 1977-1978 study benefited from computer modeling that enabled Rockwell to further refine Star-Raker wing shape and flight profile. It also allowed Reed's team to take more fully into account the benefits of propellant-saving "lifting ascent."
Star-Raker's propellants, liquid hydrogen and liquid oxygen, were not typically found at airports in 1977-1978; this remains true in 2020. The Star-Raker study team might have assumed that airports would evolve to provide them by the time SPS cargo flights began in 2000. This would, perhaps, not have been an unreasonable assumption, given that the 30-year SPS program was expected to create a lucrative new industry spanning the continental United States.
|One Star-Raker takes off as another undergoes airport servicing. With its landing gear extended, Star-Raker ground clearance would have been 1.52 meters (five feet). Image credit: M. Alvarez/Rockwell International.|
For the 1977-1978 study, however, they hedged their bets by assuming that liquid hydrogen fuel would be available at airports only in sufficient quantities for airport-to-airport subsonic air-breathing jet engine Star-Raker flights. Liquid oxygen would, of course, not have been required. Flights to LEO, which would have needed both propellants in large quantities, would have begun on a runway at NASA's Kennedy Space Center (KSC) in Florida, at Vandenberg Air Force Base, California, or at any other launch sites the U.S. might have deigned to establish.
The propellant tanks in Star-Raker's wings would have been approximately conical in shape. They would have extended from the space plane's body to its wing tips and been designed to strengthen the wings with minimal weight penalty. They would have been reinforced with regularly spaced "cell web" walls. Foam-filled glass-fiber honeycomb would have surrounded the tanks, defining Star-Raker's shape.
The Rockwell team described in detail a Star-Raker flight from KSC to 556-kilometer-high (345-mile-high) LEO and back to a U.S. airport. It would have begun with arrival at KSC of a Star-Raker space plane loaded with cargo bound for LEO at the end of a subsonic flight from a conventional airport.
Following a limited airplane-type checkout, crews would have installed three sets of jettisonable orbital-takeoff main landing gear, each with eight wheels, and pumped liquid hydrogen and liquid oxygen propellants into Star-Raker's tanks. Fully loaded with propellants and cargo and with its orbital-takeoff gear attached, Star-Raker would have weighed about 1935 metric tons (2130 U.S. tons).
Star-Raker would have lifted off from the runway at a speed of 415 kilometers per hour (260 miles per hour) under "supercharged afterburner" power from its 10 multicycle jet engines. The Rockwell team explained that it had consulted with leading jet engine manufacturers to arrive at its jet engine design; these included General Electric, Pratt & Whitney, Aerojet, Marquardt, and Rocketdyne. The resulting engine was more a wish list than a firm design, though it was an informed wish list.
The Rockwell team envisioned four operational cycles for its jet engine ranging from conventional turbofan to ramjet. Liquid hydrogen would have been used to cool the engine and then burned as fuel. Large, slot-shaped inlets on the underside of Star-Raker's wings, arranged in two groups of five on either side of the space plane's body, would have funneled air to the engines, which would have been mounted at the wing trailing edge. The inlets would have been equipped with "ramp" doors that could close partially or fully to moderate or halt airflow.
Shortly after leaving the ground, the space plane's crew would have dropped the three sets of orbital-takeoff landing gear (they would have lowered to the ground on parachutes for recovery and reuse), then would have retracted its nose and main landing gear. The space plane would then have switched its jet engines to turbofan power, climbed to 6100-meter (20,000-foot) cruise altitude, and increased its speed to Mach 0.85. It would have turned due south and, over the next hour and fifty minutes, flown directly to Earth's equator.
Star-Raker would have flown to the equator and turned east so that it could get a boost from Earth's rotational velocity, which at our planet's midriff can, in theory, add about 1600 kilometers (1000 miles) per hour to the orbital velocity of ascending launch vehicles.
In addition, and more importantly, the turbofan flight to the equator would have amounted to a plane-change maneuver; that is, it would have enabled Star-Raker to reach equatorial LEO without performing the rocket-propelled plane-change maneuver in LEO required if Star-Raker flew directly to orbit from a non-equatorial launch site, such as KSC. The Rockwell team hoped that this would save propellants, enabling an increase in cargo weight.
Following the eastward turn, the space plane would have climbed to 13,710 meters (45,000 feet) under supercharged afterburner power, then would have begun a shallow dive to 11,280 meters (37,000 feet). During the powered dive, a propellant-saving maneuver, Earth's gravity would have helped it to break the sound barrier and accelerate to Mach 1.2.
Star-Raker would then have begun ascent to orbit in earnest, with a supersonic climb to 29 kilometers (18 miles). During this phase, the space plane's jet engines would have throttled up to "full ramjet" power, accelerating it to Mach 6.2. Throughout its climb to orbit, Star-Raker would have maneuvered to put to good use lift provided by its wings.
Upon reaching Mach 6.2, the three rocket motors in Star-Raker's tail would have ignited, adding rocket power to ramjet power. The three engines, with a combined thrust of 1.45 million kilograms (3.2 million pounds), would have drawn liquid hydrogen from a sturdy tank located at the aft end of the long, narrow Star-Raker cargo bay. The tank, to which the engines would have been mounted, would have served as the load path that would have distributed their thrust to the space plane's structure.
At Mach 7.2, Star-Raker would have switched to full rocket power. As it throttled up the rocket motors to full thrust, it would have shut down the jet engines and closed completely their air inlet doors.
When Star-Raker reached a 51-kilometer-by-556-kilometer (32-mile-by-345-mile) equatorial orbit, the main rocket motors would have shut down. At apogee, the high point in its orbit, the crew would have ignited the twin advanced Orbital Maneuvering System (OMS) engines at the base of its tail to raise its perigee (orbit low point) and circularize its orbit. Upon attainment of circular equatorial orbit, Star-Raker would have used the OMS to maneuver to a rendezvous with the SPS cargo-handling space station.
|Star-Raker in low-Earth orbit. Image credit: M. Alvarez/Rockwell International.|
The weight of cargo Star-Raker could carry would depend on its mission profile. For the profile described here, cargo weight delivered to orbit would have amounted to only about 48.6 metric tons (53.6 U.S. tons). The aerodynamic flight to the equator under jet power, meant to steal some of the Earth's rotational energy and avoid a plane change maneuver in LEO, had under close examination turned out to be expensive.
The Rockwell team proposed improving the equatorial profile's payload performance by loading liquid oxygen at the equator, either during flight using a new-design tanker aircraft, or after a landing at an equatorial facility with an adequate runway, orbital-takeoff gear attachment and recovery capability, and ability to provide liquid oxygen. Either approach would, however, have complicated Star-Raker operations.
To unload cargo, Star-Raker would have swung its nose, which would have contained its crew compartment, sideways out of the way, exposing one end of its six-meter-high-by-six-meter-wide-by-43-meter-long (20-foot-high-by-20-foot-wide-by-141.5-foot-long) cargo bay. The bay's arched ceiling would have made it a point of structural strength, not weakness, in the Star-Raker design.
The crew would have moved to the rear of the crew compartment to assist with cargo transfer. Windows at the rear of the two-deck crew compartment would have provided a 121° field of visibility.
The Rockwell team did not describe its cargo transfer system in any detail, though it is clear that Star-Raker would not have docked in the conventional sense. Brief mention was made of a transfer rail system in the cargo bay that would have linked to equivalent rails on the space station.
Return to Earth would have begun with cargo bay closure. After moving away from the space station, the crew would have turned Star-Raker so that its tail faced in its direction of orbital motion, then would have fired its OMS engines to slow down.
Maximum deceleration during the unhurried shallow-angle reentry would have reached no more than 2.3 gravities. Star-Raker would, in general, have experienced reentry temperatures lower than the Space Shuttle Orbiter, though nose and wing leading-edge temperatures were expected be somewhat higher. The higher leading-edge temperature was attributable to its relatively blunt shape.
The Rockwell team proposed two types of reusable Thermal Protection System (TPS) for Star-Raker. Both would have been mounted on an outer facing sheet covering a honeycomb layer. The honeycomb layer would in turn have been attached to an inner facing sheet covering the honeycomb core that surrounded the propellant tanks.
The first TPS design closely resembled that baselined for the Space Shuttle Orbiter. Ceramic tiles individually molded and milled to match Star-Raker's curves would have been glued to fabric strain-isolator pads affixed to the outer facing sheet.
The second TPS design, similar to one developed for the B-1 Bomber, was more complex. Metal panels — titanium-aluminum for low-temperature areas and "superalloy" for high-temperature areas — would have been attached to the outer facing sheet using flexible standoffs. The standoffs would have permitted the overlapping panel edges to slide over each other as they grew hot and expanded or cooled and contracted. Foil-wrapped thermal insulation blankets affixed to the outer facing sheet would have provided additional thermal protection.
Both TPS designs would have included a system for detecting breaches in the TPS. The Rockwell team provided no details of its design and did not describe what the crew might do if a breach were detected.
|Star-Raker on approach. Image credit: M. Alvarez/Rockwell International.|
When Star-Raker slowed to Mach 6, it would have begun cross-range maneuvers designed to shed energy and slow it to Mach 0.85. The crew would then have opened the inlet ramps and started "some" of its jet engines.
The Rockwell team provided the space plane with enough liquid hydrogen to permit a 556-kilometer (345-mile) subsonic cruise and two powered landing attempts. Landing velocity would have been about 215 kilometers per hour (135 miles per hour). At wheels stop at an airport capable of supporting a cargo 747 or a C-5A Galaxy, Star-Raker would have weighed about 281 metric tons (310 U.S. tons).
Star-Raker weights given in this flight description are based on data the Rockwell team generated in the period spanning December 1977-January 1978. In February-March 1978, NASA MSFC and NASA Langley Research Center (LaRC) in Hampton, Virginia, reviewed the Rockwell team's Star-Raker weight numbers.
The NASA centers found that Rockwell's estimates were low if "normal" technology were assumed and high if "acceleration" (advanced) technology were assumed. Whereas Rockwell had placed Star-Raker's "dry" weight with orbital-takeoff gear at 293.5 metric tons (323.5 U.S. tons), MSFC/LaRC determined that, with normal technology and a 10% cushion for weight growth during development, Star-Raker would weigh 407.6 metric tons (449.3 U.S. tons) without propellants; with advanced technology and the cushion, it would weigh only 257.6 metric tons (284 U.S. tons).
The Rockwell team and NASA MSFC engineers met in May 1978 to try to reconcile the weight estimates. They made one important change in Star-Raker's flight profile: they abandoned the subsonic flight to the equator in favor of a KSC launch and direct climb to a 556-kilometer (345-mile) LEO inclined 28.5° relative to Earth's equator (that is, the latitude of KSC).
The NASA and Rockwell teams settled on a Star-Raker weight without propellants (but with orbital-takeoff gear and 10% cushion) of 330.4 metric tons (364.2 U.S. tons). As it began ascent to orbit on a KSC runway, the space plane would have weighed 2280.5 metric tons (2514 U.S. tons). Of this, Star-Raker's maximum weight, 89.2 metric tons (98.3 U.S. tons) would have comprised cargo for the SPS project.
Independent Research and Development Data Sheet, Project Title: Earth-to-LEO Transportation System for SPS, Rockwell International, 15 December 1978.
"Star-Raker: An Airbreather/Rocket-Powered, Horizontal Takeoff Tridelta Flying Wing, Single-Stage-to-Orbit Transportation System," SSD 79-0082, D. Reed, H. Ikawa, and J. Sadunas, North American Rockwell Space Systems Division; paper presented at the American Institute of Aeronautics & Astronautics Conference on Advanced Technology for Future Space Systems in Hampton, Virginia, 8-11 May 1979.