Tier One is Scaled Composites' program of suborbital human spaceflight using the reusable spacecraft SpaceShipOne and its launcher White Knight. The craft are designed by Burt Rutan, and the project is funded 20 million US Dollars by Paul Allen. In 2004 it made the first privately funded human spaceflight and won the 10 million US Dollars Ansari X Prize for the first non-governmental reusable manned spacecraft.
The objective of the project is to develop technology for low-cost routine access to space. Tier One is not itself intended to carry paying passengers, but it is envisioned that there will be commercial spinoffs, initially in space tourism. The company Mojave Aerospace Ventures was formed to manage commercial exploitation of the technology. A deal with Virgin Galactic could see routine space tourism, using a spacecraft based on Tier One technology, starting as soon as 2009.
The design concept is to air launch a three-person piloted spacecraft which climbs to slightly above 100 km altitude using a hybrid rocket motor and then glides to the ground and lands horizontally. Scaled Composites lists the following components of the program:
Tier One's spacecraft, Scaled Composites model 316, known as SpaceShipOne, is a spaceplane designed to:
The fuselage is cigar-shaped, with an overall diameter of about 1.5 m. The main structure is of a graphite/epoxy composite material. From front to back, it contains the crew cabin, oxidizer tank, fuel casing, and rocket nozzle. The craft has short, wide wings, with a span of 5 m and a chord of 3 m. There are large vertical tailbooms mounted on the end of each wing, with horizontal stabilisers protruding from the tailbooms. It has gear for horizontal landings.
The overall mass of the fully-fueled craft is 3600 kg, of which approximately 270 kg is fuel and 1400 kg is oxidizer. Thus the dry mass is 2000 kg.
Originally the nozzle protruded from the back, but this turned out to be aerodynamically disadvantageous. In June 2004, between flights 14P and 15P, a fairing was added, smoothly extending the fuselage shape to meet the flared end of the nozzle. On flight 15P the new fairing overheated, due to being black on the inside and facing a hot, black nozzle. The fairing softened, and the lower part crumpled inwards during boost. Following that flight the interior of the fairing was painted white, and some small stiffening ribs were added.
The craft has a single unsteerable and unthrottleable hybrid rocket motor, a cold gas reaction control system, and aerodynamic control surfaces. All can be controlled manually. See the separate section below concerning the rocket engine.
The reaction control system is the only way to control spacecraft attitude outside the atmosphere. It consists of three sets of thrusters: there are thrusters at each wingtip to control roll, at the top and bottom of the nose to control pitch, and at the sides of the fuselage to control yaw. All thrusters have redundant backups, so there are twelve thrusters in all.
The aerodynamic control surfaces are designed to operate in two distinct flight regimes, subsonic and supersonic. The supersonic flight regime is of primary interest during the boost phase of a flight, and the subsonic mode when gliding. There are separate upper and lower rudders, and elevons. These are controlled using aviation-style stick and pedals. In supersonic mode the trim tabs are controlled electrically, whereas the subsonic mode uses mechanical cable-and-rod linkage.
The wings can be pneumatically tilted forwards into an aerodynamically-stable high-drag "feathered" shape. This removes most of the need to actively control attitude during the early part of reentry: Scaled Composites refer to this as "care-free reentry". One of the early test flights actually performed re-entry inverted, demonstrating the flexibility and inherent stability of Burt Rutan's "shuttlecock" design.
This feathered reentry mode is inherently far safer than the behaviour at similar speeds of the only comparable craft previously built, the Space Shuttle. The Shuttle undergoes enormous aerodynamic stresses and must be precisely steered in order to remain in a stable glide. (Although this is an interesting comparison of behaviour, it is not an entirely fair comparison of design concepts: the Shuttle starts reentry at much higher speed than SpaceShipOne, and so has some very different requirements.)
An early design called for a permanently shuttlecock-like shape, with a ring of feather-like stabilising fins. This would have made the spacecraft incapable of landing independently, requiring mid-air retrieval. This was deemed too risky, and the hybrid final design manages to incorporate the feathering capability into a craft that can land in a conventional manner. The tiltable rear sections of the wings and the tailbooms are collectively referred to as "the feather".
The landing gear consists of two widely-separated main wheels and a nose skid. These are deployed using springs, assisted by gravity. Once deployed, they cannot be retracted inflight.
The spacecraft is incapable of independent takeoff from the ground. It requires a launch aircraft to carry it to launch altitude for an air launch.
The parts of the craft that experience the greatest heating, such as the leading edges of the wings, have about 6.5 kg of ablative thermal protection material applied. The main ingredient of this material was accidentally leaked to Air and Space. If it flew with no thermal protection, the spacecraft would survive reentry but would be damaged.
There is an acknowledged "known deficiency" with the spacecraft's aerodynamic design that makes it susceptible to roll excursions. This has been seen on SpaceShipOne flight 15P where wind shear caused a large roll immediately after ignition, and SpaceShipOne flight 16P where circumstances not yet fully understood caused multiple rapid rolls. This flaw is not considered dangerous, but in both of these flights led to the achievement of a much lower altitude than expected. The details of the flaw are not public.
The core of the spacecraft avionics is the System Navigation Unit (SNU). Together with the Flight Director Display (FDD), it comprises the Flight Navigation Unit. The unit was developed jointly by Fundamental Technology Systems and Scaled Composites.
The FDD display data from the SNU on a colour LCD. It has several distinct display modes for different phases of flight, including the boost phase, coast, reentry, and gliding. The FDD is particularly important to the pilot during the boost and coast phase in order to "turn the corner" and null rates caused by asymmetric thrust. A mix of commercial and bespoke software is used in the FDD.
The spacecraft cabin, designed to hold three humans, is shaped as a short cylinder, diameter 1.5 m, with a pointed forward end. The pilot sits towards the front, and two passengers can be seated behind.
The cabin is pressurised, maintaining a sea level breathable atmosphere. Oxygen is introduced to the cabin from a bottle, and carbon dioxide and water vapour are removed by absorbers. The occupants do not wear spacesuits or breathing masks, because the cabin has been designed to maintain pressure in the face of faults: all windows and seals are doubled.
The cabin has sixteen round double-pane windows, positioned to provide a view of the horizon at all stages of flight. The windows are small compared to the gaps between them, but there are sufficiently many for human occupants to patch together a moderately good view.
The nose section can be removed, and there is also a hatch below the rear windows on the left side. Crew ingress and egress is possible by either route.
Tier One's launch aircraft, Scaled Composites model 318, known as White Knight, is designed to take off and land horizontally and attain an altitude of about 15 km, all while carrying the Tier One spacecraft in a parasite aircraft configuration. Its propulsion is by twin turbojets: afterburning J-85-GE-5 engines, rated at 15.6 kN thrust each.
It has the same cabin, avionics, and trim system as SpaceShipOne. This means it can flight-qualify almost all components of SpaceShipOne. It also has a high thrust-to-weight ratio and large speed brakes. These features combined allow it to be used as a high-fidelity moving platform flight simulator for SpaceShipOne. White Knight is also equipped with a trim system which (when activated) causes it to have the same glide profile as SpaceShipOne; this allows the pilots to practice for landing SpaceShipOne. The same pilots fly White Knight as fly SpaceShipOne.
The aircraft's distinctive shape features long, thin wings, in a flattened "W" shape, with a wingspan of 25 m, dual tailplanes, and four wheels (front and rear at each side). The rear wheels retract, but the front ones, which are steerable, are permanently deployed, with small fairings, referred to as "spats", in front. Another way to look at the overall shape is as two conventional planes, with very thin fuselages, side-by-side and joined together at their wingtips, with the cockpit and engines mounted at the point of joining.
Although White Knight was developed for certain roles in the Tier One program, it is a very capable aircraft in its own right. Scaled Composites describe it as a "high-altitude research aircraft".
Hybrid rocket motor
Tier One uses a hybrid rocket motor supplied by SpaceDev, with solid hydroxyl-terminated polybutadiene (HTPB, or rubber) fuel and liquid nitrous oxide oxidiser. It generates 88 kN thrust, and can burn for about 87 s.
The physical layout of the engine is novel. The oxidiser tank is a primary structural component, and is the only part of the engine that is structurally connected to the spacecraft: the tank is in fact an integral part of the spacecraft fuselage. The tank is a short cylinder of diameter approximately 1.5 m, with domed ends, and is the forwardmost part of the engine. The fuel casing is a narrow cylinder cantilevered to the tank, pointing backwards. The cantilevered design means that a variety of motor sizes can be accommodated without changing the interface or other components. The nozzle is a simple extension of the fuel casing; the casing and nozzle are actually a single component, referred to as the CTN (case, throat, and nozzle). Burt Rutan has applied for a patent on this engine configuration.
There is considerable use of composite materials in the engine design. The oxidiser tank consists of a composite liner with graphite/epoxy over-wrap and titanium interface flanges. The CTN uses a high-temperature composite insulator with a graphite/epoxy structure. Incorporating the solid fuel (and hence the main part of the engine) and the ablative nozzle into this single bonded component minimizes the possible leak paths.
The oxidiser tank and CTN are bolted together at the main valve bulkhead, which is integrated into the tank. There are O-rings at the interface to prevent leakage; this is the main potential leak path in the engine. The ignition system, main control valve, and injector are mounted on the valve bulkhead, inside the tank. Slosh baffles are also mounted on this bulkhead. Because the oxidiser is stored under pressure, no pump is required.
The tank liner and the fuel casing are built in-house by Scaled Composites. The tank over-wrap is supplied by Thiokol. The ablative nozzle is supplied by AAE Aerospace. The oxidiser fill, vent, and dump system is supplied by Environmental Aeroscience Corporation. The remaining components — the ignition system, main control valve, injector, tank bulkheads, electronic controls, and solid fuel casting — are supplied by SpaceDev.
The CTN must be replaced between firings. This is the only part of the craft, other than the fuel and oxidiser themselves, that must be replaced.
The solid fuel is cast with four holes. This has the disadvantage that it is possible for chunks of fuel between the holes to become detached during a burn and obstruct the flow of oxidiser and exhaust. Such situations tend to rapidly self-correct.
The oxidiser tank is filled and vented through its forward bulkhead, on the opposite side of the tank from the fuel and the rest of the engine. This improves safety. It is filled to a pressure of 4.8 MPa at room temperature.
The nozzle has an expansion ratio of 25:1, which is optimised for the upper part of the atmosphere. A different nozzle, with an expansion ratio of 10:1, is used for test firing on the ground. The nozzles are black on the outside, but for aerodynamic testing, red dummy nozzles are used instead.
The rocket is not throttleable. Once lit, the burn can be aborted, but the power output cannot otherwise be controlled. The thrust in fact varies, for two reasons. Firstly, as the pressure in the oxidiser tank decreases, the flow rate reduces, reducing thrust. Secondly, in the late stages of a burn the oxidiser tank contains a mixture of liquid and gaseous oxidiser, and the power output of the engine varies greatly depending on whether it's using liquid or gaseous oxidiser at a particular moment. (The liquid, being far denser, allows a greater burn rate.)
Both the fuel and oxidiser can be stored without special precautions, and they do not burn when brought together without a significant source of heat. This makes the rocket far safer than conventional liquid or solid rockets. It is also relatively non-polluting: the combustion products are water vapor, carbon dioxide, hydrogen, nitrogen, and some carbon monoxide.
The engine was upgraded in September 2004, between flights 15P and 16P. The upgrade increased the oxidiser tank size, to provide greater thrust in the early part of the burn, allow a longer burn, and delay the onset of the variable thrust phase at the end of the burn. Prior to the upgrade the engine generated 76 kN thrust and could burn for 76 s. After the upgrade it was capable of 88 kN thrust and an 87 s burn.
SpaceShipOne takes off from the ground, attached to White Knight in a parasite configuration, and under White Knight's power. The combination of SpaceShipOne and White Knight can take off, land, and fly under jet power to high altitude. A captive carry flight is one where the two craft land together without launching SpaceShipOne; this is one of the main abort modes available.
For launch, the combined craft flies to an altitude of around 14 km, which takes about an hour. SpaceShipOne is then drop-released, and briefly glides unpowered. Rocket ignition may take place immediately, or may be delayed. If the rocket is never lit then SpaceShipOne can glide down to the ground. This is another major abort mode, in addition to being flown deliberately in glide tests.
The rocket engine is ignited while the spacecraft is gliding. Once under power, it is raised into a 65° climb, which is further steepened in the higher part of the trajectory. The maximum possible burn duration is about 80 s. The maximum possible acceleration is about 4 g.
By the end of the burn the craft is flying upwards at some multiple of the speed of sound, up to about 900 m/s and Mach 3.5, and it continues to coast upwards unpowered (ie ballistically). If the burn was long enough then it will exceed an altitude of 100 km, at which height the atmosphere presents no appreciable resistance, and the craft experiences free fall for a few minutes.
While at apogee the wings are reconfigured into high-drag mode. As the craft falls back it achieves high speeds comparable to those achieved on the way up; when it subsequently reenters the atmosphere it decelerates violently, up to about 5 g. At some altitude between 10 km and 20 km it reconfigures into low-drag glider mode, and glides down to a landing in about 20 minutes.
White Knight takes longer to descend, and typically lands a few minutes after SpaceShipOne.
In addition to an office-based mission control, Tier One has a mobile mission control center. This is relatively small, built into a large road-going truck. It bears the Scaled Composites logo, but no other overt indication of its link to Tier One. The vehicle performs a combination of support functions:
This control center is used to support both rocket motor ground tests and all flight tests of White Knight and SpaceShipOne. Its primary function is to monitor and record test data, and to this end it is equipped with computers and radio communication gear. SpaceShipOne's avionics displays are duplicated in mission control. Telemetry data is received on a Data Reduction System (DRS), which automatically directs radio antennas to point at the craft being monitored. The telemetry system has a range of about 280 km.
The control center is equipped to communicate with Scaled Composites' offices, as well as the aircraft and spacecraft.
The control center maintains a temperature-controlled atmosphere for its staff, and can be hooked up to provide temperature control for the White Knight and SpaceShipOne cabins. The physical structure of mission control also provides easier access to the White Knight cabin.
Nitrous oxide delivery
Unlike the solid fuel, the nitrous oxide oxidiser is handled as a bulk commodity and pumped into the spacecraft's oxidiser tank in the field. Tier One therefore has a mobile delivery system for nitrous oxide, which they call MONODS (mobile nitrous oxide delivery system).
MONODS is built on an open trailer, which can be carried by road in conventional manner. It consists principally of a 6.5 cubic metre tank, a temperature control unit, and a generator to power the temperature control unit. The nitrous oxide is stored at room temperature, at a pressure of 4.8 MPa.
MONODS is refilled from a commercial supplier, which uses 50 cubic metre tankers and delivers the nitrous oxide at about -17°C (0°F) and 2.0 MPa. MONODS heats the nitrous oxide to room temperature, increasing its pressure.
Tier One has a mobile thrust test stand, known as the Test Stand Trailer (TST). The advantage of making it mobile is that all the mounting and instrumentation work can be done in the hangar, so that at the test site all that needs to be done is to fill the oxidiser tank (from MONODS) and conduct the firing.
The test stand replicates the essential structural components of the spacecraft. It has an oxidiser tank and associated fittings identical to the one used in flight. This means that the motor test also automatically performs appropriate vibration, stress, and heat tests of the spacecraft structure. The crew cabin, however, is not replicated.
For ground-based thrust tests, a rocket nozzle with an expansion ratio of 10:1 is used, differing from the 25:1 nozzle used at altitude during actual flight.
The test stand is instrumented to record not only thrust but also side force and temperature and strain experienced by components. Data is recorded on a computer in a bunker at the test site. The data acquisition computer is remotely controlled from mission control.
The SpaceShipOne flight simulator consists of a simulator program and a cockpit.
The flight simulator program aims to accurately simulate SpaceShipOne's behaviour under any circumstances and in all phases of flight. Rather than having a model of SpaceShipOne's overall flight behaviour, it uses computational fluid dynamics to model the air around the craft. It calculates the aerodynamic and other forces operating on the craft, taking into account the positions of its control surfaces. This simulation is based on the computer modelling that was used during the design process and refined using data from flight tests. This yields a highly accurate image of craft behaviour, even in unanticipated modes of flight. (This is one of the first modern aircraft to be designed without wind tunnel testing.)
The cockpit replica is on a static base, and so cannot accurately reproduce the equilibrioceptive and accelerative aspects of flight. However, White Knight is equipped to operate as a high-fidelity moving-base simulator; see White Knight's section above. The simulator cockpit is an accurate copy of the SpaceShipOne cabin, including its avionics. It is the system of pilot plus avionics, not just the pilot, that is being simulated to. The flight simulator program drives the sensor inputs that are used by the avionics, and also drives twelve display computers which use commercial graphics software to generate high-resolution images of the outside view for the pilot. These views appear on eleven monitors and one projector screen. Stick force feedback is not simulated in real time.
Ground-based flight simulation is not only used for pilot training. It is also used to train ground crew, develop procedures, and test the avionics software and hardware.
History and status
According to Scaled Composites, the concept for the program originated in April 1996, preliminary development began in 1999, and full development began in April 2001. It was initially kept secret, even after White Knight first flew on August 1, 2002. The program was announced to the public on April 18, 2003, when the program was ready to flight-test SpaceShipOne. Its first flight test, SpaceShipOne flight 01C, took place on May 20, 2003.
After months of glide tests, the first powered flight, SpaceShipOne flight 11P, was made on December 17, 2003. Further powered tests followed, reaching increasing altitudes, culminating on June 21, 2004 with the first privately funded human spaceflight, SpaceShipOne flight 15P. Ansari X Prize competitive flights followed. SpaceShipOne flight 16P on September 29, 2004 and SpaceShipOne flight 17P on October 4, 2004 were successful competitive flights, winning the X Prize.
The program will continue making test flights, to develop the technology further, in support of the development of successor spacecraft such as the Virgin SpaceShip. The program has ruled out carrying scientific payloads, despite several requests.
The costs of development, construction, and operation of Tier One, although not publicly released, are estimated to be in the range of 20 million to 30 million US dollars, roughly two to three times the value of the Ansari X Prize award. The sole sponsor, initially secret, was revealed to be Paul Allen, a co-founder of Microsoft and the fifth richest person in the world. The revelation, on December 17, 2003, the same day as the program's first powered flight test, followed speculation that Allen was involved.
Some commentators have drawn comparisons between the relative inexpense of the Tier One program and the high cost of the Space Shuttle program, though the technological difficulties of the two programs are completely different. SpaceShipOne, because it flies suborbitally, does not need to reach the high speeds of the Space Shuttle (Mach 3 vs. Mach 25), nor the same altitude (100 km suborbital vs. 400 km orbit). SpaceShipOne also does not carry the same crew (3 members vs. 7) or payload (negligible vs. 25 tons), and makes much shorter flights (a few minutes vs. several days). The SpaceShipOne program is a technical achievement more on a par with the X-15 than the Shuttle.
Inflation adjusted comparisons of the SpaceShipOne program with that of the X-15 budget, indicate that the Tier One program cost 1/100th that of the X-15 program, although the three X-15 aircraft made almost 200 test flights in their entire test program, typically exploring hypersonic flight between mach 4-7. Only a few dozen X-15 flights specificially sought to reach peak altitudes rather than achieve top speeds, though only two flights ever reached altitudes near those achieved by SpaceShipOne. On the other hand, the Tier One project also paid for construction of the White Knight mothership within its budget, while NASA had nearly free use of a pre-existing USAF B-52 bomber modified to perform drop tests of experimental aircraft of many kinds (currently in use for PegasusXL launches).
Tier One was initially developed secretly, as is Scaled Composites' policy with new programs. On April 18, 2003 the program was publicly announced, and SpaceShipOne and White Knight were demonstrated to the media at a rollout attended by between 550 and 600 people. Media interest was so intense that what had been intended as a Family and Friends Day on April 24, 2003 was turned into a second media day.
Scaled Composites again courted publicity by announcing in advance the final test flight, SpaceShipOne flight 15P, intended to be the program's first spaceflight. About 11,000 people went to Mojave Spaceport to watch the flight, which was also televised. The flight was run as an airshow, with both the principal craft and the chase planes making takeoffs and landings in front of the crowd, and celebratory flybys when the test succeeded. The flight was not only a technical success but also an unqualified popular success, triggering intense public interest in spaceflight.
Scaled Composites unsurprisingly remains very media-friendly with respect to Tier One, and more public spectacles are to be expected.
The program name "Tier One" raises the question of what Tier Two will be.
In The Space Review on June 21, 2004 (the day of Tier One's first spaceflight), Rutan was quoted as stating "The spaceship is model number 316 and the White Knight is model number 318. I will be making a presentation very quick of a model number 346.". Whether this refers to a prototype orbital craft, a suborbital tour bus, or some other concept remains to be seen.
In 2004 Rutan suggested that a ten-passenger suborbital spacecraft would make a profitable tour bus. On September 27, 2004, entrepreneur Sir Richard Branson announced a deal with Mojave Aerospace Ventures to carry paying passengers on suborbital tourist flights, using a scaled-up version of SpaceShipOne, possibly beginning service in 2008. That same year, Burt Rutan also said "we're heading for orbit sooner than you think".
During an interview in the documentary Black Sky: The Race for Space, Rutan stated that Tier One will cover suborbital flights, Tier Two will cover orbital flights, and Tier Three will cover flights beyond Earth's orbit (including flights to the moon and other planets). In the same documentary he displayed designs for an orbital craft based on SpaceShipOne, which had a rocket roughly twice SpaceShipOne's length mounted to the ship's rear.
The stated objective of the Tier One program is to demonstrate suborbital human spaceflight operations at low cost. Before Burt Rutan began considering this project, there were three major barriers to the goal of affordable suborbital spaceflight:
Rutan's design appears to provide a cost-effective solution to all three issues.
Tier One itself is not intended to carry paying passengers, and US Government permits would be required if it did intend to do so. It is a technology testbed, and it is expressly intended that the technology developed in the program will later be used in commercial spaceflights. To that end, Paul Allen and Burt Rutan created a company, Mojave Aerospace Ventures, which owns the project's intellectual property and will manage all commercial exploitation of it.
Scaled Composites initially expressed a hope that by about 2013 it would be possible for members of the public to experience a suborbital flight for about the price of a luxury cruise. On September 25, 2004 a deal was struck with Virgin Galactic to develop the Virgin SpaceShip based on a scaled-up version of SpaceShipOne. These spacecraft will be built by The Spaceship Company.
Published in July 2009.
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