Ares I booster shortened

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Changes to crew launch vehicle rocket and fuel tanks made in first design analysis

This week NASA is to hold an industry day and interviews for potential Ares I upper-stage constructors.

The first design analysis cycle (DAC-1) was completed last month with key changes to the booster that will launch its Orion crew exploration vehicle, and which consists of a solid rocket booster (SRB) first stage and cryogenic upper stage.

NASA has switched from an intertank structure between the upper-stage liquid oxygen and liquid hydrogen tanks to a common bulkhead with a resulting mass saving, including residual propellant, of around 635kg (1,400lb). At 79.4m (260ft) long, the Ares I is now about 1.83m shorter with the common bulkhead.

"Anything we can do to increase controllability [by shortening the booster] is welcome. All of our cryogenic upper stages for manned spaceflight historically have used a common bulkhead," says Steve Cook, director of NASA Marshall Space Flight Center's exploration launch projects office.

NASA has also agreed on a composite interstage to connect the first and upper stages and a star-like, 12-lobe geometry has been selected for the solid propellant in the first stage, to make the burn rate faster than in the Space Shuttle SRB on which it is based.

The same polybutadiene acrylonitrile (PBAN) propellent used in the Shuttle SRB has also been selected, with reduced iron oxide content for burn rate amelioration.

Following a system requirements review in December, the DAC-2 design cycle for Ares I began on 3 January. This could decide the number of roll control system (RCS) thrusters that the first and upper stages will have.

Roll torque results from a calibrated Shuttle SRB ground-test firing will be available to NASA in February. It will be compared with Lockheed Martin-supplied roll data from the Athena rocket to help determine the size and number of RCS thrusters required.

Thrust requirements mean the baseline Ares I RCS is monopropellant for the upper stage and bipropellant for the first stage. Cooks says the first stage could be monopropellant after analysis of the SRB and Athena roll torque data.

DAC-2 could also tackle the Ares I staging system, which may employ forward-facing booster separation motors or a pneumatic system. The Ares I-1 first test flight, set for 2009, will only use pyrotechnics to separate the first stage and dummy upper stage.