Single Stage to Orbit:
A Reliable Transport System or an Unattainable Dream?

By James Meyers © 4/25/03 All Rights Reserved.  Unless otherwise cited, all work appearing here is mine.  Please contact me if you plan to use this paper as a source.

            Humankind currently has one reusable launch system capable of achieving low earth orbit (LEO). This is the Space Shuttle, and as of February 1, 2003, only three shuttles remain. Ignoring the safety issues, the Shuttle in its current method of operation cannot meet humanity’s future need for access to LEO. A new reusable launch vehicle is needed. After studying the Shuttle’s operation, Dr. Robert Zubrin, an internationally known aeronautical engineer, has concluded that there are three points necessary for a cheap reusable launch vehicle: it must “(a) have a high launch rate, (b) have a small ground staff, and (c) reuse the first stage.” He goes on to say that the only way to accomplish both (a) and (b) simultaneously is to have an extremely simple launch system, and since adding additional stages adds complexity, the simplest solution is a single stage vehicle (30). Given a higher launch rate and lower operating costs, a single-stage-to-orbit vehicle will open LEO to human occupancy. Although single-stage-to-orbit (SSTO) vehicles will never be able to replace heavy-lift vehicles or travel to other planets, they will soon be humanity’s primary method of reaching LEO.

            Though it is a magnificent machine, the Space Shuttle is poorly designed. Rather than reusing the first stage (the solid rocket boosters/external tank), it reuses the final stage (the orbiter) and lets the large external fuel tank burn up in the atmosphere (see Figure 1). The lower stages of the Shuttle stack lift 100 tonnes (110 tons) of fuel and structure to orbit, but Zubrin claims that 60 tonnes of wings, landing gear and reentry protection systems “are drag[ged] up there (at a cost of $10 million per tonne) anyway each time it flies” (30).

            A reusable single-stage-to-orbit vehicle is, by definition, a vehicle that can attain orbit around the Earth using one stage of propulsion. This means that no external rockets can be added onto the vehicle, as is done with the Space Shuttle. Because no external rockets can be added to the main body of the vehicle, a large percentage of the vehicle’s mass must be dedicated to fuel and fuel storage, which complicates the design.

            Because the inadequacy of the Shuttle design and the requirements for an SSTO vehicle are known, one can now look at various concept designs for SSTO construction. The first category of vehicle is one that employs vertical takeoff and vertical landing (VTVL). An SSTO of this design would take off like a conventional rocket, and upon returning to Earth, it would rotate so the engines face the ground. It would then land on its own rocket plume. The second category of vehicle is vertical takeoff and horizontal landing (VTHL). A vehicle of this design would operate similarly to the Shuttle, except it would only have one stage. It would take off from a launch pad and land on a runway. The third and final category of design is horizontal takeoff and horizontal landing (HTHL). There are a number of separate designs for this category. One design uses a vehicle that takes off from a runway. It then flies like a plane to an altitude where it can ignite its rocket engines. A second design is refueled by a traditional tanker aircraft in flight, which allows a lighter takeoff weight. A third design calls for the craft to be towed to a high altitude by an aircraft before it ignites its rockets.

            Because an SSTO can only have one stage, there are serious limitations that arise from the various designs mentioned above. To understand these limitations, one needs to understand a few rocketry terms. First, ΔV (“delta-V”) is the change in velocity needed to go from one state to another state. (In rocketry, this “state” means a location or an orbit where the spacecraft does not need to expend energy to maintain that location or orbit.) The ΔV from the ground to LEO is approximately 9 kilometers per second (Zubrin 46). Second, specific impulse (Isp) refers to the “number of seconds a pound of propellant can deliver a pound of thrust.” This is stated in terms of seconds (Zubrin 35). For example, the Space Shuttle’s main engines (SSMEs) have an Isp of 437 seconds (Bekey 33). Finally, every spacecraft has a total mass, which is the mass of the vehicle when fueled, and a dry mass, which is the mass of the vehicle without fuel. From these two masses, the mass fraction, that is, the dry mass divided by the total mass, can be calculated. Because the vehicle has only one stage, the mass fraction must be very low; the vehicle must be mostly propellant. This low mass fraction leads to difficulties in the construction of the vehicle.

            Since the dry mass of an SSTO vehicle must be kept low, it requires the use of high-tech materials and technologies. While these were deemed unattainable in early concept designs, many “futuristic” materials are now in common use. Using an aluminum-lithium alloy for fuel tanks instead of pure aluminum can reduce the total vehicle weight by 4%, and allow surrounding materials to be lighter as well, and it actually reduces the total vehicle weight by 23%. Also, the use of composite materials for the vehicle’s structure reduces the total vehicle weight by 45% (Bekey 34). Advanced technologies can also be incorporated into SSTO design. One of the more popular technologies is scramjet, which stands for “supersonic combustion ramjet”. A scramjet rocket acts like a combination of a regular jet engine and a rocket. Once a vehicle with a scramjet rocket breaks the sound barrier, the vehicle begins letting air enter the rocket. The fact that the vehicle is travelling at supersonic speeds pressurizes the air and allows it to react with the second half of the rocket propellant (most rockets use liquid oxygen to react with the second half of the propellant). The fact that the rocket does not need to carry the oxygen for the reaction increases the Isp dramatically. Because the rocket takes oxygen from the air, it also decreases the total weight of the vehicle and increases the mass fraction. This simplifies construction requirements.

            Using SSME performance, the mass fraction of an SSTO would have to be about 0.87, which means the structure and payload of the vehicle is 13% of the total weight of the fueled vehicle (Bekey 33). This can only be achieved with some growth margin if the majority of the above advanced materials are used. While these materials are in common use today, it is still a challenge to design or construct a vehicle with this mass fraction.
Despite these limitations, much progress has been made in the development of single-stage-to-orbit vehicles. As early as 1977, studies were conducted analyzing composite propulsion for an HTHL SSTO vehicle. Given the technologies available at that time, the study concluded that “none of the engines has performed adequately to deliver payloads to orbit as analyzed” (Martin 1). As technology progressed, it became apparent that the design and construction of an SSTO vehicle could become a reality.

            Numerous studies were conducted by both NASA and the American Institute of Aeronautics and Astronautics (AIAA). In an article written for Aerospace America in 1994, Ivan Bekey, a special assistant for NASA’s Office of Space Systems Development, discussed an SSTO vehicle that used near-future advanced technology. These technologies included composite structures and aluminum-lithium propellant tanks, both of which have been used since the article was written. The use of these technologies allowed construction of an SSTO vehicle with a mass fraction of greater than 90%. At the same time, the higher performance engines that were proposed required a mass fraction of only 87%, and, because of this, the vehicle was technically possible (33, 34, 37).

            At the time, the most common type of SSTO studied was the VTHL concept. Since the Shuttle operates in this mode, observers view it as the easiest type of vehicle to develop. One analysis studies the VTHL concept and compares it to specifications for an HTHL vehicle that employs scramjet technology, both of which use SSME baselines. The scramjet vehicle would be towed by an aircraft to an altitude where it could ignite its rocket engine. The baseline payload is 25,000 pounds lifted into an orbit that matches the orbit of the ISS. Given 1995 technology, the vehicle had a gross weight of nearly 3 million pounds. The same vehicle was then analyzed with a 2000 pound payload, but the 92% reduction in payload weight only resulted in a 43% reduction in gross weight. Finally, the scramjet vehicle was analyzed with the 25,000 pound payload, and it was found that the gross weight could be reduced to 1.37 million pounds. Although this is a significant improvement, the author notes that this gross weight “is much higher than the lifting capabilities of existing airplanes” (Shirasu et al. 597-8).

            Realizing the possibility of scramjet engines, NASA continued research on this concept throughout the late 1990s. In 1999, Charles Trefny of the NASA Glenn Research Center published a concept for a 300 pound payload HTHL SSTO vehicle that used scramjet propulsion. With this scramjet propulsion, Trefny was able to keep the dry mass fraction below 0.8 (1, 7). While this vehicle would not compete with SSTOs that launch 25,000 pounds, it could still be used to launch small satellites. It could also be useful in an emergency in space because it requires no launch pad, no tow to altitude and no mid-flight refueling, thus reducing preparation time before launch.

            In addition to this research, a massive amount of time and money was invested in the actual construction of SSTO prototypes. In 1991, the Ballistic Missile Defense Organization (BMDO) developed the first prototype vehicle under its reusable launch vehicle program. This vehicle was dubbed the Delta Clipper Experimental (DC-X). At the time, it attempted to “deploy an ABM treaty-compliant anti-ballistic missile system that [was] capable of providing a highly effective defense of the United States. . ..” As a spin-off, the SSTO technology had a goal of enabling “safe, low-cost transfer of people and cargo to and from space. . ..” This SSTO technology was funded by McDonnell Douglas, and they built the DC-X in 22 months with a $58 million contract (Hanson Para. 6). It was a one-third scale prototype of an SSTO that would use vertical takeoff and landing. Between 1993 and 1995, the DC-X made eight flights, but the body of the vehicle was cracked on the last flight during a hard landing. A year later, it flew four more flights as the DC-XA (see Figure 2), under control of NASA. On the fourth flight, an unconnected helium line caused a landing strut to fail to extend. This caused the vehicle to collapse and the liquid oxygen tank to explode, which resulted in the loss of the vehicle (The Delta Clipper Experimental Archive). Due to the lack of funding and the loss of the original vehicle, the Delta Clipper project was cancelled. Plans for the orbital Delta Clipper, an SSTO vehicle capable of placing 20,000 pounds into LEO (more than three times that of the Russian Progress spacecraft), were abandoned (Hanson; Soyuz).

            In 1994, the apparent success of the DC-X sparked NASA to initiate its own SSTO vehicle program, dubbed the X-33. Unlike the DC-X project, the X-33 would be two thirds scale, and the vehicle would go suborbital during testing. Three companies submitted bids for constructing the X-33 vehicle. Rockwell submitted a vehicle that took off and landed like the Shuttle (see Figure 4), McDonnell Douglas submitted an enlarged version of the DC-X (see Figure 5), and Lockheed Martin proposed a new lifting body design with an aerospike engine (see Figure 3) (Zubrin 32). In 1996 the Lockheed Martin design was chosen, with the promise from the company to develop the full-scale VentureStar SSTO vehicle.

            Unfortunately, the president of Lockheed Martin resigned soon after the X-33 project was awarded, and the new management clarified that there was no promise to develop the full scale vehicle. It was merely something that the company might do if conditions were right. Zubrin observes that all three aerospace giants that bid for the X-33 had a stake in the then-current launch market, and a dramatic drop in launch costs that SSTO vehicles would bring was not in their interests. For this reason he proposes that the major aerospace companies are unlikely to spend their own money on a vehicle that makes the majority of their other vehicles obsolete (33).
Lockheed’s work on the X-33 quickly began falling behind schedule. In June of 1997, “Aerospace Daily reported that ‘typical development problems’ had led to the postponement of the first X-33 test flight from March to July of 1999. . .” (Key X-33 Events in 1997). Then, in October of 1998, the first flight was delayed until December of 1999 because of the engine delivery schedule. This delay cost the project 36 million dollars and forced Lockheed to introduce “plans to cut project personnel in order to reduce escalating costs” (Key X-33 Events in 1998). While construction continued in 1998 and 1999, increasing costs brought the X-33 project under close scrutiny from Congress, and further budget cuts were made to deal with cost overruns. In early 1999, the first flight was delayed yet again. Because of a problem with the lining of the liquid hydrogen storage tank, the first flight would now occur in July of 2000 (Key X-33 Events in 1998).

            Throughout 2000, the X-33’s aerospike engine completed numerous test fires. Unfortunately, the cost overruns and design problems continued, and the first test flight continued to be postponed. According to NASA Press Release 00-157, released on September 29, 2000, the X-33 was 95% built and 75% constructed (Para. 10). In early 2001, however, the X-33 project was cancelled. No additional funds were given to the X-33 program under NASA’s Space Launch Initiative (SLI), and the contract between NASA and Lockheed Martin was terminated. NASA had spent 912 million dollars on the project, and Lockheed had invested 356 million dollars of its own (David).

            While NASA and the big three aerospace companies were beginning development of multiple SSTO vehicles, smaller venture capital companies took advantage of the SSTO craze. In 1993 Gary Hudson, who had unsuccessfully tried to develop a VTVL SSTO in the 1980s, created HMX Engineering. Since Hudson’s previous idea was almost identical to the DC-X, he created Roton (see Figure 6), a “space-launch helicopter” (Zubrin 43). Though the concept had numerous flaws, Hudson and HMX Engineering brought in a considerable amount of money. This money then brought in new ideas, and the Roton concept was modified. Instead of mounting the rockets on the helicopter blades, the new concept clustered them underneath the vehicle. There, they would rotate at 720 rpm, which would pressurize the propellants as they were fed to the rocket engines (Rotary Rocket – Summary).

            HMX Engineering, now renamed the Rotary Rocket Company, continued to develop the vehicle well beyond the dissolution of the DC-X project. They tested engines for the vehicle in 1999, but in 2002, XCOR Aerospace “acquired selected assets of the Rotary Rocket Company, including the full and exclusive rights to all technology developed by Rotary Rocket.” At that time, XCOR also announced that they had no plans to continue development of the Roton vehicle, and neither the prototype nor the landing facilities were included in the purchase (XCOR Aerospace).

            About the same time that Hudson was promoting the Roton concept, Captain Mitchell Clapp of the US Air Force was developing the “Black Horse”, a HTHL concept that employed in-flight refueling. Although the Air Force liked his concept, there were a number of flaws that made the construction of the vehicle impossible. First, he estimated a ΔV of 8.2 kilometers per second to get to LEO, instead of the more accurate 9 kilometers per second. Secondly, he estimated an Isp of 330 seconds for the hydrogen peroxide/kerosene engines, but the maximum Isp these engines had produced was 270 seconds. Lastly, the vehicle would require a tanker that operated under rocket power to refuel it. No such vehicle exists, and it is doubtful that one could be built to hold enough fuel. If Clapp’s original estimates are used, the mass fraction is 0.92 (extremely difficult). If the more realistic estimates are used, the mass fraction rises to 0.953 (impossible) (Zubrin 46).

            From these examples, it is clear that the development of an SSTO vehicle of any design is difficult and expensive. NASA has no current project attempting to develop an SSTO vehicle. The big three aerospace companies (Lockheed Martin, McDonnell Douglas, and Rockwell Aerospace) are not developing SSTO vehicles. Most venture capital companies have ceased development of their SSTO prototypes. Despite these setbacks, there is still hope for the development of a single-stage-to-orbit vehicle. This hope lies with the X-Prize.

            The X-Prize Foundation was founded in May 1996. It will award 10 million dollars to the first team that flies a spacecraft to an altitude of 100 kilometers twice within two weeks. Along with these guidelines, no more than 10% of the dry weight of the vehicle can be replaced between flights (X-Prize Competition Guidelines). Because of the limit on reusable weight, many of the concepts are single-stage vehicles. Most are not launched from the ground; instead, they are launched from high-altitude aircraft or massive balloons. While the vehicles are not required to achieve orbit to win the X-Prize, many companies plan to continue developing their vehicles until they can reach LEO.

            Scaled Composites, one of the major X-Prize competitors, unveiled SpaceShipOne on April 18, 2003. Launched from a high-altitude aircraft, SpaceShipOne will be able to reach the 100 km mark set to win the X-Prize. At this time, it is ready to begin flight testing (News). Many in the field believe that Scaled is also working on an orbital spacecraft. However, Burt Rutan, one of the major players in Scaled Composites, recently stated that “Scaled has completed 34 manned research aircraft and none were announced until they were ready to fly” (Frequently Asked Questions).

            Though NASA and the big three aerospace companies have backed out of SSTO development, smaller companies continue to make progress on the design and construction of space vehicles. When the X-Prize is won, there will be a surge in manned spaceflight. Many of these vehicles will be SSTOs, and since the vehicles will be run by private companies, they must be able to make a profit. With the lower cost of launch and support staff, as seen by the DC-X program, SSTOs will permit for-profit manned space travel that is also safe and affordable.

            Once it is seen that SSTOs provide a faster and cheaper method of launch, the government and large aerospace companies may jump on the bandwagon and develop even more SSTO vehicles. Once NASA begins using SSTO vehicles, the vehicles will be able to satisfy humanity’s need to access space. SSTOs will be able to carry people into orbit and to the ISS. They will also be able to launch mini-satellites and large communication satellites. When this time comes, single-stage-to-orbit vehicles will be humanity’s primary method of reaching low Earth orbit.

Figure 1: The Space Shuttle (Space Shuttle Basics).

Figure 2: The DC-XA in flight.  The DC-X was very similar (The Delta Clipper Experimental Archive).

Figure 3: X-33 Concept Art from Lockheed Martin (Frassanito, J., Lockheed Martin).

Figure 4: X-33 Concept Art from Rockwell (Frassanito, J., Rockwell).

Figure 5: X-33 Concept Art from McDonnell Douglas (Frassanito, J., McDonnell Douglas).

Figure 6: The Roton vehicle (Roton Rocket – Summary).

Figure 7: SpaceShipOne and White Knight, the high-altitude that will carry SpaceShipOne to a high altitude (SpaceShipOne).

Glossary

AIAA: American Institute of Aeronautics and Astronautics
BMDO: Ballistic Missile Defense Organization
DC-X: Delta Clipper Experimental
DC-XA: Delta Clipper Experimental Advanced
ΔV, delta-V: The change in velocity required by a spacecraft (Ex. ΔV to LEO is 9 kilometers per second)
HTHL: horizontal takeoff, horizontal landing
Isp, specific impulse: A standard of engine rating, based on how long one pound of propellant can provided one pound of thrust
ISS: International Space Station
LEO: low Earth orbit
NASA: National Aeronautics and Space Administration
scramjet: supersonic combustion ramjet
SLI: Space Launch Initiative
SSME: Space Shuttle main engine
VTHL: vertical takeoff, horizontal landing
VTVL: vertical takeoff, vertical landing

Works Cited

Bekey, Ivan. “SSTO Rockets: A Practical Possibility.” Aerospace America Jul. 1994: 32-37.

The Delta Clipper Experimental Archive. January 6, 1998. 12 April 2003 <http://www.hq.nasa.gov/office/pao/History/x-33/dc-xa.htm>

David, Leonard. “NASA Shuts Down X-33, X-34 Programs” SPACE.com. March 1, 2001. 13 April 2003 <http://www.space.com/missionlaunches/missions/ x33_cancel_010301.html>

Frassanito, J. X-33 Proposal by Lockheed Martin. <http://trc.dfrc.nasa.gov/gallery/photo/X-33/Small/EC96-43631-7.jpg>

---. X-33 Proposal by McDonnell Douglas. 
<http://trc.dfrc.nasa.gov/gallery/photo/X-33/Small/EC96-43631-6.jpg>

---. X-33 Proposal by Rockwell. 
<http://trc.dfrc.nasa.gov/gallery/photo/X-33/Small/EC96-43631-5.jpg>

Frequently Asked Questions. 21 April 2003 <http://www.scaled.com/projects/tierone/faq/faq.htm>

Hanson, Chris. About the DC-X. 2002. 12 April 2003 <http://media.armadilloaerospace.com/DCX/>

Key X-33 Events in 1997. 2001. 13 April 2003 <http://www.hq.nasa.gov/office/pao/History/x-33/1997.htm>

Key X-33 Events in 1998. 2001. 13 April 2003 <http://www.hq.nasa.gov/office/pao/History/x-33/1998.htm>

Key X-33 Events in 1999. 2001. 13 April 2003 <http://www.hq.nasa.gov/office/pao/History/x-33/1999.htm>

NASA Press Release 00-157. Sept. 29, 2000. 13 April 2003 <http://www.hq.nasa.gov/office/pao/History/x-33/00-157.htm>

News. April 18, 2003. 21 April 2003 <http://www.scaled.com/news/news.htm>

Rotary Rocket – Summary. 2001. 20 April 2003 <http://www.spaceandtech.com/spacedata/rlvs/rotary_sum.shtml>

Shirasu, Hiroki et al. “Analysis of Concepts for Single Stage to Orbit.” Journal of Spacecraft and Rockets 33 (1996): 597-8.

Soyuz. 12 April 2003 <http://users.commkey.net/Braeunig/space/specs/soyuz.htm>

Space Shuttle Basics. March 17, 2003. 24 April 2003 <http://spaceflight.nasa.gov/shuttle/reference/basics/index.html>

SpaceShipOne. April 18, 2003. 24 April 2003 <http://www.scaled.com/projects/tierone/photos/images/WK%20and%20SS1%20mated%20front%20left.jpg>

United States. National Aeronautics and Space Administration. An Air-breathing Concept for Single-Stage-to-Orbit. By Trefny, Charles J. NASA Glenn Research Center, Aug. 6, 1999

United States. National Aeronautics and Space Administration. An Evaluation of Composite Propulsion for Single-Stage-to-Orbit Vehicles Designed for Horizontal Take-Off. By Martin, James A. NASA Langley Research Center, November 1977.

XCOR Aerospace Acquires Rotary Rocket Assets. April 23, 2002. 20 April 2003 <http://www.xcor.com/rotaryrocket.html>

X-Prize Competition Guidelines. 2002. 21 April 2003 <http://www.xprize.org/teams/guidelines.html>

Zubrin, Robert. Entering Space. New York: J. P. Tarcher, 1999.