200 N Bipropellant Thrusters for ESA's ATV

About the Automated Transfer Vehicle propulsion system and 200 N attitude control and braking thruster.

Automated Transfer Vehicle with
220 N thruster clusters shown fwd and aft

On 9 March 2008, Europe's first Automated Transfer Vehicle (ATV), was launched by an ES ATV version of Ariane 5. Its mission, to deliver its 45 m³ pressurised module containing up to 7.2 tonnes of equipment, fuel, food, water and air to the crew of the International Space Station (ISS). This, the maiden flight of ATV was named 'Jules Verne' and a further 7 ATV's have been planned for resupplying the ISS every 15 months.

Currently, about 1,500 people in different European countries are working on this €900-million ESA programme.

As its name implies, the ATV is a truly automated vehicle. It can navigate and safely dock to the station and accomplish its mission without any human intervention whatsoever. The ATV is therefore the first fully automatic resupply spacecraft of its kind. Such autonomy, together with fault tolerance requirements, imposes about one million lines of software code for the various onboard computers.

The ATV is launched on an ES ATV version of Ariane 5, which places the spaceship into a 260 km circular low Earth orbit inclined to 51.6°. From this orbit, the ATV will use its own propulsion system to automatically navigate to, and dock with, the Space Station.

ATV Propulsion System

The ATV propulsion system is contained in the unpressurised Service Module, located aft of the habitable Pressurised Module. The Service Module also contains electrical power, computers, communications and avionics.

The bipropellant propulsion system is pressure fed with the propellant combination monomethyl hydrazine fuel and nitrogen tetroxide oxidiser. The main elements of the ATV propulsion being:

4 x 490 N main navigation engines.
28 x 200 N attitude control and braking thrusters.
8 titanium propellant tanks of 7 tonnes capacity.
2 high pressure carbon fibre-wound helium pressurant vessels.

Unpressurised service module
containing ATV's propulsion system

The propulsion system is designed to perform:

Navigation to the Space Station, after separation from Ariane 5.
Automatic manoeuvres for rendezvous and docking to the ISS.
While docked, the ATV will perform ISS attitude control, debris avoidance manoeuvres and raising of the 183 tonne station's orbit to overcome the effects of atmospheric drag.
After 6 months - de-docking and automatic departure manoeuvres.
Navigation to the orbital deorbitation point.
Retroburn and de-orbitation manoeuvres.

From the 7 tonnes of available propellant, approximately 2.3 tonnes is available for free flight manoeuvres and approximately 4.7 tonnes is available for manoeuvring the space station at intervals of 10 to 45 days.

In the event of a thruster, or main engine failure, redundant branches and control electronics are used to switch propulsive functions to fulfil operational objectives and safety requirements.

The scale of ATV, together with the complexity of propulsive manoeuvres and proximity to man, results in a propulsion subsystem that is one of the largest and most sophisticated ever built. In fact, the internal volume of the complete ATV is sufficient to accommodate a double-decker London bus.

The entire propulsion system in nominal and failure mode can be simulated using ATVSim. Using this software, the simulation process can be accomplished significantly faster than in real-time.

200 N Attitude Control and Braking Thrusters

ATV's 200 N attitude control and braking thruster cluster.

200 N Thruster Cluster

Production of the 200 N Attitude Control and Braking Thruster has been entrusted to the propulsion specialists at EADS Astrium Lampoldshausen.

A total of 28 x 200 N thrusters are used on ATV, located thus:

Fwd: 4 clusters of 2 thrusters.
Aft: 4 clusters of 5 thrusters.

The thruster clusters deliver both steady state thrust and impulse bit and can also be used as back-up in the event of main engine failure.

Safety and redundancy are major design drivers and each thruster is equipped to measure and detect malfunctions and problems by continuously measuring chamber temperature and combustion pressure.

Astrium Lampoldshausen are also responsible for the production, integration and acceptance testing of:

ATV propulsion module pressure control assemblies (PCA).
Propellant Isolation Assembly (PIA).
Propulsion system qualification

200 N Thruster Requirements

The 200 N thruster was designed and developed in accordance with the special ATV requirements, as shown in the table below:

200 N Bipropellant Thruster Requirements for ATV

Propellants	Oxidiser: N2O4, MON1, MON3 Fuel: MMH
Thruster Operating Box	Ox 12.9 to 23.9 bar, Fu 12.8 to 23.2 bar
Propellant Temperatures	-2 to +48°C (Tox/Tfu)
Inlet Voltage	23.5 to 28.5 VDC (36 VDC for CAM sequence)
Performance at Nominal operation point	Fvac: 216 ± 10 N Isp: > 2650 m/s mr: 1.65 ± 0.035
Operating Box Performance	Fvac: 180 to 270 N ± 15 N Ivac: > 2350 m/s
Thrust Repeatability	± 1.5% firing to firing
Thrust Roughness	± 12% (f>100 Hz) ± 3% (f>100 Hz)
Bubble Ingestion	Oxidiser: 2500 scc He Fuel: 1500 scc He
Isp Repeatability	< 25 m/s firing to firing
Minimum Impulse Bit	< 8 Ns
Impulse Bit Repeatability	< ± 10% at 3 Sigma
Type of Pulse Sequence	Any combination at 1Hz pulse frequency
Thrust	Rise time: < 50 ms Decay time: < 50 ms Centroid delay < 100 ms
Mission Requirements	Total 'ON' time: 12.9 h Total number of pulses: 160,000

Injector Selection

200 N thruster hot fire test

A showerhead injector, derived from Lampoldshausen's 400 N apogee engine, was selected for the ATV since it has a highly reliable space proven heritage dating back to 1974. Since that time, the engine has achieved 100% mission success, having been used on numerous international GTO spacecraft and demonstrating a 14-year service life on NASA's interplanetary Galileo mission. This robust thruster has proven to yield the following advantages: