Black Horse: One Stop to Orbit
Black Horse: One Stop to Orbit
Robert M. Zubrin
and
Mitchell Burnside Clapp
[W3 Ed note: This article originally appeared in the June 1995<br>issue of Analog Magazine. Since this version was entered<br>by OCR software, some typographical errors may have been introduced.<br>Figures and tables have been moved slightly relative to the<br>text. (reprinted with permission of author) -DRR]
Introduction
Since the dawn of the space age, building a vehicle that can fly<br>to orbit using only a single stage has been the holy grail of<br>astronautics. The problem is that single stage to orbit flight is<br>really hard to do because the velocity change necessary to achieve<br>orbit ("Delta-v," 31,000 ft/sec, typically), including losses due to<br>aerodynamic drag, gravity, back pressure on the engines, steering, and<br>so forth, imposes vehicle-full to vehicle-empty mass ratios that are<br>difficult to achieve with current structural technology. The usual<br>approach is to seek more energetic propellants with high specific<br>impulse ("Isp" the number of seconds a pound of fuel can be made to<br>deliver a pound of thrust) values. Alternatively, people have tried<br>airbreathing approaches, which are also an attempt to achieve large<br>Isp. The first approach, ultra-high Isp rocket engines, tends to<br>involve propellants that are not very dense and are difficult to<br>handle, such as liquid hydrogen. The second way, using hypersonic<br>airbreathing jets, imposes surpassingly difficult design and<br>operations problems such as those that have afflicted the National<br>Aerospace Plane program.
Three major configurations have been proposed for single stage<br>rocket vehicles: vertical take takeoff/horizontal landing (VTO/HL),<br>such as the SSTO/R vehicle proposed by the NASA Access to Space Study;<br>vertical takeoff/vertical landing (VTO/VL), such as the McDonnell<br>Douglas Delta Clipper; and horizontal takeoff/horizontal landing<br>(HTO/HL), such as the Boeing Reusable Aerospace Vehicle (RASV) or<br>British HOTOL designs. Between the first two of these, there is no<br>obvious distinction in terms of empty weight. Credible design studies<br>appear to give similar weight estimates for similar vehicles.<br>Horizontal takeoff and landing vehicles, however, tend to be much<br>heavier for a given payload because of the unique requirements imposed<br>by runway takeoff, with wing loads at rotation and the weight of<br>landing gear being of particular concern. Because of these inert mass<br>hits, horizontal takeoff and landing vehicle designs generally tend<br>not to be pure single stage to orbit, but rely instead on sled launch<br>or auxiliary boosters to reduce gross weight.
Figure 1: Aerial propellant transfer (APT) spaceplane flight<br>plan. Because the aircraft takes off light, the mass of wings and<br>landing gear is greatly reduced.
Our purpose is this article is to discuss another approach for<br>operating spaceplanes off conventional runaways with conventional<br>facilities: Using in-flight propellant transfer to reduce the takeoff<br>gross weight of a rocket-powered aircraft, and hence its size, weight,<br>and cost. This is not an attempt to solve the single stage to orbit<br>problem by means of increasing Isp, but by decreasing Delta-v. It turns out<br>that if you begin the mission to space from tanker altitude and<br>airspeed, the amount of propellant that must be expended overcoming<br>drag and gravity losses is greatly reduced, the vehicle tankage, wings<br>and landing gear all become smaller, and everything becomes a whole<br>lot easier.
The Aerial Propellant Transfer Spaceplane
The general concept of the operation of an aerial propellant<br>transfer (APT) spaceplane is shown in Fig. 1. The spaceplane, carrying<br>only a fraction of its required propellant, takes off a runway in a<br>conventional manner using either rocket power or a set of<br>air-breathing engines and climbs to rendezvous with a tanker,<br>typically at an altitude between 20,000 and 40,000 ft., depending on<br>the spaceplane design. The tanker transfers the remainder of the<br>required propellant and departs, after which the spaceplane fires its<br>main rocket engines at full throttle and accelerates to low Earth<br>orbit. Upon reaching orbit, the payload is released, possibly to be<br>propelled to higher orbit by its own propulsion system, while the<br>spaceplane re-enters the atmosphere, glides to the vicinity of an<br>airport, and then lands in either an unpowered or rocket or jet<br>powered mode.
There are many variants possible to this basic plan, including<br>selection of propellants, propulsion systems, and refueling scheme.<br>For example, the possibility of a spaceplane using the leverage<br>offered by employing very high specific impulse air-breathing<br>propulsion above the tanker's maximum velocity of Mach 0.85 to cut the<br>rocket's required Delta-v to orbit, needs to be considered and<br>traded against the large inert mass penalties and System complexity<br>associated with such jet engines. Use of smaller jet engines for<br>takeoff, loiter, self-ferry and landing offer many...