Tier One was a Scaled Composites' 1990s–2004 program of suborbital human spaceflight using the reusable spacecraft SpaceShipOne and its launcher White Knight. The craft was designed by Burt Rutan, and the project was funded with 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 crewed spacecraft. The objective of the project was to develop technology for low-cost routine access to space. SpaceShipOne was not itself intended to carry paying passengers, but was envisioned that there would 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 in the late 2010s using a spacecraft based on Tier One technology. This led to Virgin Galactic's SpaceShipTwo program, which has concluded in June 2024 with only a few flights with tourists having taken place starting in 2023.
Design
Program components The design concept of Tier One was to air launch a three-person piloted spacecraft which climbs to slightly above 100 km (62 mi; 54 nmi) altitude using a hybrid rocket motor and then glides to the ground and lands horizontally. Scaled Composites lists the following components of the program:
Mission control 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:
telemetry monitoring and recording telecommunications auxiliary environment control for White Knight and SpaceShipOne 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 (170 mi). 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). The MONODS is built on an open trailer, which can be carried by road in conventional manner. It consists principally of a 6.5 m3 (230 cu ft) 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 (700 psi). The MONODS is refilled from a commercial supplier, which uses 50 m3 (1,800 cu ft) tankers and delivers the nitrous oxide at about −17 °C (1 °F) and 2 MPa (290 psi). The MONODS heats the nitrous oxide to room temperature, increasing its pressure.
Propulsion testing 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 the 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.
Flight simulator 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. 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.
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