Liquid apogee engine

In 2011 the REACH framework legislation added N2H4 to its candidate list of substances of very high concern.

[14][15] Exemptions are being sought to allow N2H4 to be used for space applications, however to mitigate this risk, companies are investigating alternative propellants and engine designs.

[16] A change over to these alternative propellants is not straightforward, and issues such as performance, reliability and compatibility (e.g. satellite propulsion system and launch-site infrastructure) require investigation.

However, there are many other details which influence performance: A typical 500 N-class hypergolic liquid apogee engine has a vacuum specific impulse in the region of 320 s,[17][18][19][20] with the practical limit estimated to be near 335 s.[7] Though marketed to deliver a particular nominal thrust and nominal specific impulse at nominal propellant feed conditions, these engines actually undergo rigorous testing where performance is mapped over a range of operating conditions before being deemed flight-qualified.

This means that a flight-qualified production engine can be tuned (within reason) by the manufacturer to meet particular mission requirements, such as higher thrust.

More practical considerations such as cost, lead time and export restrictions (e.g. ITAR) also play a part in the decision.

A 400 N hypergolic liquid apogee engine, including heat shield and mounting structure, on display at DLR visitors center, Lampoldshausen, Germany. The engine was designed for use on Symphonie satellites. These were the first three-axis stabilised communication satellites in geostationary orbit to use a liquid bipropellant apogee engine for orbit insertion. [ 1 ]