Inertial Upper Stage


First flight: 30-Oct-1982
Number flown: 19, through Mar-1996 (Boeing acknowledges only 17)
Principal uses: Shuttle, Titan 34D, Titan 4, Titan 3
Vehicle Success rate: 94.4% through Mar-1996
Performance: (motors can be offloaded 0-50%, vehicle sized for up to 3,600 kg payloads, but 7,200 kg possible with modifications)
GEO: 2,268 kg (Shuttle), 1,817 kg (Titan 34D), 2,364 kg (Titan 4 with SRM strap-ons), 2,860 kg (Titan 4B with SRMU strap-ons)
GTO: 4,944 kg (using payload for apogee kick into GEO)
Polar (96o, 24 h): 1,860 kg from 57o Cape Canaveral orbit
Molniya (925 x 39,450 km, 63o, 12 h): 3,645 kg (T4 SRM via 185 km, 55o), 3,866 kg (T4 SRM via 185 km, 57o), 4,189 kg (T4 SRM via 185 km, 63.5o), 4,933 kg (T4 SRMU via 185 km, 55o), 5,253 kg (T4B SRMU via 185 km, 57o)
Escape: 820 kg for C3 of 60 km2/s2, 3,550 kg for 7.0 km2/s2
Number of stages: 2 solid
Overall length: 5.17 m
Principal diameter: 2.9 m
Mass (excluding payload): 14,760 kg (Shuttle version), 13,100 kg (Titan 34D)

IUS incorporates two CSD solid propellant motors. On a typical GEO mission, the larger SRM-1 performs GTO injection, followed by SRM-2 insertion into GEO. Thrust vector pitch/yaw control is provided by the nozzles moving on a Techroll joint pressurization by silicon oil under the action of electro-mechanical actuators; SRM-1 can gimbal 4o, SRM-2 7o. Variations in mission requirements are accommodated by propellant offloading of up to 50%. SRM-2 can be flown without its Extendible Exit Cone (EEC), resulting in a reduced specific impulse.

Length: 3.52 m
Diameter: 2.34 m
Propellant mass: 4,853-9,709 kg
Thrust: 185.1 kN vac average
Specific impulse: 295.5 s
Burn time: 152 s, 100% loading

Length: 2.08 m without nozzle extension
Diameter: 1.61 m
Propellant mass: 1,361-2,749 kg
Thrust: 78.41 kN vac average
Specific impulse: 289.1 s vac (without EEC), 303.5 s vac (with EEC)
Burn time: 103.35 s, 100% loading

An aluminum skin stringer interstage connects the SRM-1 to the Equipment Support System (ESS), which houses SRM-2, most of the avionics and the payload interfaces (3.05 m diameter payload mounting ring and electrical connectors). The eight longerons on the aluminum skin stringer ESS assembly provide primary load paths and the spacecraft attach points.

Six modules each with two Kaiser Marquardt 133 N hydrazine thrusters provide roll control during main motor burns, 3-axis control during coast periods, vernier orbit corrections after SRM-2 burnout, and collision avoidance maneuvers after payload separation. The configuration provides dual-nozzle redundancy and there are no forward-facing thrusters to avoid payload contamination. Two tanks hold a total of 109 kg of hydrazine but one and three tank versions are possible. RCS burns to depletion after payload release in order to remove IUS as far as possible from GEO.

On Shuttle missions, the IUS/Payload is spring released from the ASE titled at 58o. The RCS does not activate until 10 min later at a separation of about 60 m. SRM-1 ignition does not occur for a further 50 min until the Orbiter is some 18 km distant.


United Technologies Chemical Systems Divison's (CSD) Orbus upper stage family includes Orbus 6/6E/21 (IUS), Orbus 7S (JCSat), Orbus 21S (Intelsat 6), Orbus 21 (TOS), Orbus 21D (LMLV launcher) and Orbus 1 (STARS and Starbird upper stage). The numerical designator corresponds to propellant mass to the nearest 1,000 lb, S denotes spin stabilisation and E the use of an extending exit nozzle (EEC) system.

Applications: IUS stage 1 motor, TOS motor, Intelsat 6 perigee boost motor, LMLV stage 2
First flown: 30-Oct-1982
Number flown: 20 (some classified) on IUS, 2 TOS, 6 Intelsat, 1 LMLV, through 1995
Length: 3.15 m (3.17 m 21D)
Maximum diameter: 2.34 m
Mass: 10,398 kg fully loaded (10,619 kg 21D); 639 kg burnout (780 kg 21D)
    type: 86% solids HTPB UTP-19360A
    shape: based on 52 cm diameter bore
    mass fraction: 0.935 fully loaded (0.941 for 21S without TVC system), 0.47 max 50% offloading (0.921 fully loaded, 0.457 max offloading for 21D)
Propellant mass: 4,853-9,709 kg (4,583-9,779 kg 21D)
Burn time: 154 s (146 s 21D)
Thrust: 195.7 kN vac average, 267.77 kN vac peak (192.4 kN vac average 21D)
Specific impulse: 295.5 s vac (293.5 s vac 21D)
Total impulse: 28.38 MNs vac (28.14 MNs vac 21D)
Expansion ratio: 63.9:1
Pressure: 57.9 atm max
Thrust vector control: +/-4o TVC (up to 20o/s) provided by battery-powered orthogonal electromechanical actuators and Techroll movable nozzle joint pressurized by silicon oil; system mass 22.4 kg

Applications: IUS stage 2 motor
First flown: 30-Oct-1982
Number flown: 20 (some classified) on IUS to end-1995
Length: 1.98 m with EEC stowed, 3.20 m EEC deployed
Maximum diameter: 1.60 m
Mass: 3,018/2,749 kg fully loaded with/without EEC
    type: 86% solids HTPB UTP-19360A
    shape: cylindrical
    mass fraction: 0.902/0.914 with/without EEC fully loaded (0.921 without TVC system), 0.45 with max 50% offloading
Burn time: 103 s fully loaded
Thrust: 80.95 kN vac average, 113.87 kN vac peak
Specific impulse: 303.5/289.1 s vac with/without EEC
Total impulse: 8.14 MNs vac
Expansion ratio: 181.1/47.3 with/without EEC
Pressure: 57.1 atm max
Thrust vector control: as Orbus 21 but 7o

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