Douglas SASSTO
Encyclopedia
Douglas Aircraft's SASSTO, short for "Saturn Application Single Stage to Orbit", was a single-stage-to-orbit
(SSTO) reusable launch system
designed by Philip Bono
's team in 1967. SASSTO was a study in minimalist designs, a launcher with the specific intent of repeatedly placing a Gemini capsule
in orbit for the lowest possible cost. The SASSTO booster was based on the layout of the S-IVB
upper stage from the Saturn family
, modified with a plug nozzle
. Although the SASSTO design was never followed up at Douglas, it is widely referred to in newer studies for SSTO launchers, notably the MBB
"Beta" design, which was largely an updated version of SASSTO.
sent out a series of studies on post-Apollo
launch needs, which generally assumed very large launchers for a manned mission to Mars. At Douglas, makers of the S-IVB
, Philip Bono led a team that studied a number of very large liquid-fueled boosters as a way to lower the cost of space exploration. His designs were based on an economy of scale which makes larger rockets more economical than smaller ones as the structure accounts for less and less of the overall weight of the launcher. At some point the dry weight of the launcher becomes lower than the payload it can launch, after which increases in payload fraction
are essentially free. However, this point is crossed at relatively large vehicle sizes - Bono's original OOST study from 1963 was over 500 feet (152.4 m) long - and this path to lower costs only makes sense if there is an enormous amount of payload that needs to be launched.
After designing a number of such vehicles, including ROOST and the ROMBUS/Ithacus/Pegasus series, Bono noticed that the S-IVB stage, then just starting to be used operationally, was very close to being able to reach orbit on its own if launched from the ground. Intrigued, Bono started looking at what missions a small S-IVB-based SSTO could accomplish, realizing that it would be able to launch a manned Gemini capsule
if it was equipped with some upgrades, notably an aerospike engine
that would improve the specific impulse
and provide altitude compensation
. He called the design "SASSTO", short for "Saturn Application Single-Stage To Orbit".
These same upgrades would also have the side-effect of lowering the weight of the SASSTO compared to the original S-IVB, while at the same time increasing its performance. Thus the study also outlined a number of ways that it could be used in place of the S-IV in existing Saturn IB
and Saturn V
stacks, increasing their performance. When used with the existing Saturn I lower stage, it would improve payload to low earth orbit
from 35,000 to 52500 lb (23,813.6 kg), or 57000 lb (25,854.8 kg) if the landing gear
were removed and it was expended like the S-IVB. SASSTO would thus give NASA
a short-term inexpensive manned launch capability, while also offering improved heavy-launch capability on the existing Saturn infrastructure.
SASSTO required a number of new technologies, however, which made development risky. In particular, the performance of the aerospike engine had to be considerably higher than the J-2
it would replace, yet also offer the ability to be re-started multiple times as the single engine was used for launch, de-orbit and landing. Of particular note was the final landing burn, which required the engines to be re-started at 2500 ft (762 m) during the decent. The vehicle's weight was also greatly reduced, almost by half, which would not have been trivial considering the relatively good performance of the S-IVB design.
The internal fuel tankage was considerably different than in the S-IV. The LH2 was no longer cylindrical, but spherical, and moved to the forward location in the fuselage. The LOX
tankage, originally on top of the LH2, was re-positioned into a series of smaller spherical tanks arranged in a ring below the LH2. The tanks were all moved forward within the airframe compared to the engine, all of these changes being made in order to reduce changes in the center of gravity
as the fuel was burned off. The fuselage section immediately above the engine was necked down, forming what appeared to be a larger single plug. The upper section of the fuselage, over the top of the hydrogen tank, was likewise necked down.
In order to increase the amount of LH2 being carried, given the fixed dimensions, SASSTO proposed freezing 50% of the fuel to produce a slush hydrogen
mixture. This improvement was not uncommon in designs of the era, although it was not until the 1990s that any serious development work on the concept was carried out.
The rearmost portion of the spacecraft was a single large plug nozzle, fed by a series of 36 injectors operating at 1500 psia, producing 277000 lbf (1,232.2 kN) of thrust. Since plug nozzles gain efficiency as they grow larger, the 465 sec specific impulse
(compared to the J-2's 425) was not particularly aggressive. The engine also served as the primary heat shield, actively cooled by liquid hydrogen that was then dumped overboard.
Four landing legs extended from fairings on the fuselage sides, retracting to a point about even with the "active" portion of the engine area. Four clusters of small maneuvering engines were located between the legs, about half-way from front to back along the fuselage. A series of six smaller tanks arranged in the gaps between the LOX and LH2 tanks fed the maneuvering engines.
SASSTO delivered 6200 lb (2,812.3 kg) of cargo to a 110 nmi (203.7 km) orbit when launched due east from the Kennedy Space Center
. Empty weight was 14700 lb (6,667.8 kg), considerably lighter than the S-IVB's 28500 lb (12,927.4 kg), and gross lift off weight was 216000 lb (97,976 kg). The typical payload was the Gemini, which was covered with a large aerodynamic fairing.
Re-entry manuverability was though blunt-body lifting profile, similar to the Apollo CSM. The cross-range was limited, about 230 miles (370.1 km), and there was basically no maneuverability at all on final approach. There was enough fuel for about 10 seconds of hovering and small maneuvers to select a flat landing spot. Because SASSTO was the same basic size as the S-IVB, Douglas proposed transporting it in the existing Aero Spacelines Super Guppy
after landing at either Wendover Air Force Base
in Utah, or Fort Bliss
outside of El Paso, Texas
.
in the late 1960s. Unlike Bono's version, Koelle used as much existing technology and materials as possible, while abandoning the need for the specific S-IVB sizing. The result was a slightly larger spacecraft, the Beta, that launched 4000 lb (1,814.4 kg) of payload without the use of slush fuel, advanced lightweight construction, or a real aerospike engine. As part of the Beta proposal, Koelle pointed out that even the existing S-IVB could reach orbit, with zero payload, if equipped with a high-pressure LOX/LH2 engine of 460 Isp.
Gary Hudson
later pointed out that such an engine existed, the Space Shuttle Main Engine
, using a SSME-powered S-IVB as a thought experiment to demonstrate the real-world feasibility of SSTO launchers. This study was part of his "Phoenix" series of launchers, all similar to the SASSTO.
Single-stage-to-orbit
A single-stage-to-orbit vehicle reaches orbit from the surface of a body without jettisoning hardware, expending only propellants and fluids. The term usually, but not exclusively, refers to reusable vehicles....
(SSTO) reusable launch system
Reusable launch system
A reusable launch system is a launch system which is capable of launching a launch vehicle into space more than once. This contrasts with expendable launch systems, where each launch vehicle is launched once and then discarded.No true orbital reusable launch system is currently in use. The...
designed by Philip Bono
Philip Bono
Philip Bono was a Douglas Aircraft Company engineer. He was a pioneer of reusable vertical landing single-stage to orbit launch vehicles....
's team in 1967. SASSTO was a study in minimalist designs, a launcher with the specific intent of repeatedly placing a Gemini capsule
Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....
in orbit for the lowest possible cost. The SASSTO booster was based on the layout of the S-IVB
S-IVB
The S-IVB was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine...
upper stage from the Saturn family
Saturn (rocket family)
The Saturn family of American rocket boosters was developed by a team of mostly German rocket scientists led by Wernher von Braun to launch heavy payloads to Earth orbit and beyond. Originally proposed as a military satellite launcher, they were adopted as the launch vehicles for the Apollo moon...
, modified with a plug nozzle
Plug nozzle
The plug nozzle is a type of nozzle which includes a centerbody or plug around which the working fluid flows. Plug nozzles have applications in aircraft, rockets, and numerous other fluid flows.-In Rockets:...
. Although the SASSTO design was never followed up at Douglas, it is widely referred to in newer studies for SSTO launchers, notably the MBB
Messerschmitt-Bölkow-Blohm
Messerschmitt-Bölkow-Blohm was a German aerospace company formed as the result of several mergers in the late 1960s. Among its best-known products was the MBB Bo 105 light twin helicopter...
"Beta" design, which was largely an updated version of SASSTO.
History
In 1962 NASANASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
sent out a series of studies on post-Apollo
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...
launch needs, which generally assumed very large launchers for a manned mission to Mars. At Douglas, makers of the S-IVB
S-IVB
The S-IVB was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine...
, Philip Bono led a team that studied a number of very large liquid-fueled boosters as a way to lower the cost of space exploration. His designs were based on an economy of scale which makes larger rockets more economical than smaller ones as the structure accounts for less and less of the overall weight of the launcher. At some point the dry weight of the launcher becomes lower than the payload it can launch, after which increases in payload fraction
Payload fraction
In aerospace engineering, payload fraction is a common term used to characterize the efficiency of a particular design. Payload fraction is calculated by dividing the weight of the payload by the weight of the otherwise empty aircraft when fully fueled...
are essentially free. However, this point is crossed at relatively large vehicle sizes - Bono's original OOST study from 1963 was over 500 feet (152.4 m) long - and this path to lower costs only makes sense if there is an enormous amount of payload that needs to be launched.
After designing a number of such vehicles, including ROOST and the ROMBUS/Ithacus/Pegasus series, Bono noticed that the S-IVB stage, then just starting to be used operationally, was very close to being able to reach orbit on its own if launched from the ground. Intrigued, Bono started looking at what missions a small S-IVB-based SSTO could accomplish, realizing that it would be able to launch a manned Gemini capsule
Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....
if it was equipped with some upgrades, notably an aerospike engine
Aerospike engine
The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. It is a member of the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low...
that would improve the specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
and provide altitude compensation
Altitude compensating nozzle
An altitude compensating nozzle is a class of rocket engine nozzles that are designed to operate efficiently across a wide range of altitudes.- Conventional designs :...
. He called the design "SASSTO", short for "Saturn Application Single-Stage To Orbit".
These same upgrades would also have the side-effect of lowering the weight of the SASSTO compared to the original S-IVB, while at the same time increasing its performance. Thus the study also outlined a number of ways that it could be used in place of the S-IV in existing Saturn IB
Saturn IB
The Saturn IB was an American launch vehicle commissioned by the National Aeronautics and Space Administration for use in the Apollo program...
and Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...
stacks, increasing their performance. When used with the existing Saturn I lower stage, it would improve payload to low earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
from 35,000 to 52500 lb (23,813.6 kg), or 57000 lb (25,854.8 kg) if the landing gear
Landing Gear
Landing Gear is Devin the Dude's fifth studio album. It was released on October 7, 2008. It was his first studio album since signing with the label Razor & Tie. It features a high-profile guest appearance from Snoop Dogg. As of October 30, 2008, the album has sold 18,906 copies.-Track...
were removed and it was expended like the S-IVB. SASSTO would thus give NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
a short-term inexpensive manned launch capability, while also offering improved heavy-launch capability on the existing Saturn infrastructure.
SASSTO required a number of new technologies, however, which made development risky. In particular, the performance of the aerospike engine had to be considerably higher than the J-2
J-2 (rocket engine)
Rocketdyne's J-2 rocket engine was a major component of the Saturn V rocket used in the Apollo program to send men to the Moon. Five J-2 engines were used on the S-II second stage, and one J-2 was used on the S-IVB third stage. The S-IVB was also used as the second stage of the smaller Saturn IB...
it would replace, yet also offer the ability to be re-started multiple times as the single engine was used for launch, de-orbit and landing. Of particular note was the final landing burn, which required the engines to be re-started at 2500 ft (762 m) during the decent. The vehicle's weight was also greatly reduced, almost by half, which would not have been trivial considering the relatively good performance of the S-IVB design.
Design
Although the SASSTO claimed the S-IVB as its starting point, this was a conceit, and the vehicle had little in common with the S-IVB except its size.The internal fuel tankage was considerably different than in the S-IV. The LH2 was no longer cylindrical, but spherical, and moved to the forward location in the fuselage. The LOX
Lox
Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...
tankage, originally on top of the LH2, was re-positioned into a series of smaller spherical tanks arranged in a ring below the LH2. The tanks were all moved forward within the airframe compared to the engine, all of these changes being made in order to reduce changes in the center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...
as the fuel was burned off. The fuselage section immediately above the engine was necked down, forming what appeared to be a larger single plug. The upper section of the fuselage, over the top of the hydrogen tank, was likewise necked down.
In order to increase the amount of LH2 being carried, given the fixed dimensions, SASSTO proposed freezing 50% of the fuel to produce a slush hydrogen
Slush hydrogen
Slush hydrogen is a combination of liquid hydrogen and solid hydrogen at the triple point with a lower temperature and a higher density than liquid hydrogen. It is formed by bringing liquid hydrogen down to nearly the melting point that increases density by 16–20% as compared to liquid hydrogen...
mixture. This improvement was not uncommon in designs of the era, although it was not until the 1990s that any serious development work on the concept was carried out.
The rearmost portion of the spacecraft was a single large plug nozzle, fed by a series of 36 injectors operating at 1500 psia, producing 277000 lbf (1,232.2 kN) of thrust. Since plug nozzles gain efficiency as they grow larger, the 465 sec specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
(compared to the J-2's 425) was not particularly aggressive. The engine also served as the primary heat shield, actively cooled by liquid hydrogen that was then dumped overboard.
Four landing legs extended from fairings on the fuselage sides, retracting to a point about even with the "active" portion of the engine area. Four clusters of small maneuvering engines were located between the legs, about half-way from front to back along the fuselage. A series of six smaller tanks arranged in the gaps between the LOX and LH2 tanks fed the maneuvering engines.
SASSTO delivered 6200 lb (2,812.3 kg) of cargo to a 110 nmi (203.7 km) orbit when launched due east from the Kennedy Space Center
Kennedy Space Center
The John F. Kennedy Space Center is the NASA installation that has been the launch site for every United States human space flight since 1968. Although such flights are currently on hiatus, KSC continues to manage and operate unmanned rocket launch facilities for America's civilian space program...
. Empty weight was 14700 lb (6,667.8 kg), considerably lighter than the S-IVB's 28500 lb (12,927.4 kg), and gross lift off weight was 216000 lb (97,976 kg). The typical payload was the Gemini, which was covered with a large aerodynamic fairing.
Re-entry manuverability was though blunt-body lifting profile, similar to the Apollo CSM. The cross-range was limited, about 230 miles (370.1 km), and there was basically no maneuverability at all on final approach. There was enough fuel for about 10 seconds of hovering and small maneuvers to select a flat landing spot. Because SASSTO was the same basic size as the S-IVB, Douglas proposed transporting it in the existing Aero Spacelines Super Guppy
Aero Spacelines Super Guppy
|-See also:-References:NotesBibliography*Taylor, Michael J.H. . Jane's Encyclopedia of Aviation. Studio Editions. London. 1989. ISBN 0-517-69186-8-External links:* * *...
after landing at either Wendover Air Force Base
Wendover Air Force Base
Wendover Air Force Base is a former United States Air Force base in Utah now known as Wendover Airport. During World War II it was a training base for B-17 and B-24 bomber crews. It was the training site of the 509th Composite Group, the B-29 unit which dropped the Hiroshima and Nagasaki atomic bombs...
in Utah, or Fort Bliss
Fort Bliss
Fort Bliss is a United States Army post in the U.S. states of New Mexico and Texas. With an area of about , it is the Army's second-largest installation behind the adjacent White Sands Missile Range. It is FORSCOM's largest installation, and has the Army's largest Maneuver Area behind the...
outside of El Paso, Texas
El Paso, Texas
El Paso, is a city in and the county seat of El Paso County, Texas, United States, and lies in far West Texas. In the 2010 census, the city had a population of 649,121. It is the sixth largest city in Texas and the 19th largest city in the United States...
.
Developments
Dietrich Koelle used SASSTO as the starting point for a similar development at Messerschmitt-Bölkow-BlohmMesserschmitt-Bölkow-Blohm
Messerschmitt-Bölkow-Blohm was a German aerospace company formed as the result of several mergers in the late 1960s. Among its best-known products was the MBB Bo 105 light twin helicopter...
in the late 1960s. Unlike Bono's version, Koelle used as much existing technology and materials as possible, while abandoning the need for the specific S-IVB sizing. The result was a slightly larger spacecraft, the Beta, that launched 4000 lb (1,814.4 kg) of payload without the use of slush fuel, advanced lightweight construction, or a real aerospike engine. As part of the Beta proposal, Koelle pointed out that even the existing S-IVB could reach orbit, with zero payload, if equipped with a high-pressure LOX/LH2 engine of 460 Isp.
Gary Hudson
Gary Hudson
Gary Hudson has been involved in private spaceflight development for over 25 years.Hudson is best known as the founder of Rotary Rocket Company, which in spending ~$30 Million attempted to build a unique single stage to orbit launch vehicle known as the Roton...
later pointed out that such an engine existed, the Space Shuttle Main Engine
Space Shuttle main engine
The RS-25, otherwise known as the Space Shuttle Main Engine , is a reusable liquid-fuel rocket engine built by Pratt & Whitney Rocketdyne for the Space Shuttle, running on liquid hydrogen and oxygen. Each Space Shuttle was propelled by three SSMEs mated to one powerhead...
, using a SSME-powered S-IVB as a thought experiment to demonstrate the real-world feasibility of SSTO launchers. This study was part of his "Phoenix" series of launchers, all similar to the SASSTO.
Further reading
- Philip Bono et al., "The Saturn S-IVB as a Test-Bed for Booster Recovery", Douglas Engineering Paper 3808, 6th European Symposium on Space Technology, May 1966
- Philip Bono, "The reusable booster paradox - aircraft technology or operations", Spaceflight, Volume 9 (1967), pp. 379–387
External links
- SASSTO - Encyclopedia Astronautica
- PC-compatible flight simulation for the SASSTO. Requires prior installation Orbiter general space flight simulator package, both Orbiter and SASSTO are freeware.