Compact Air Separation Technology For In-Flight Oxygen Collection

In the context of an ESA-funded study on an air separation device intended to provide in-flight Oxygen Collection capability to future launchers, experimental and system level investigations are performed. The project consortium is run by the Université Libre de Bruxelles and includes the Belgian Defence, Techspace Aero (Safran Group) and Liège university among others.
In-flight Oxygen Collection has already been described in previous publications, mainly
as a conceptual approach. The improvement potentials have been studied and possible uses of the technology go beyond the concept presented here. That is in the aerospace field and also outside, in a much wider range of applications. Hardware work on compact separation are seldom described. As the setup was nearing completion, design know how improved. The last problems were solved and the setup was made fully operational and subject to testing.
The following review summarizes the work with latest information on the technological and experimental aspects.
In the vehicles using air separation and collection, the cooling capacity of the fuel, liquid hydrogen, is used during a first air breathing phase of the flight to enrich in oxygen and liquefy a fraction of the incoming air. Compact and light weight heat exchanger, coupled to the separator, perform that task. The main advantages of the concept, applied in our preferred concept to TSTO’s, are a much reduced take off mass, a reduced mass for the liquid rocket engines and a much increased operational flexibility in terms of reachable orbit and launch window. Moreover, unlike scramjet propulsion, separation does not rely on high technology material but on readily available material and hardware used with proper design know how and appropriate expertise.
The experimental setup development follows a somewhat different route. Liquid and saturated gaseous air are prepared in special heat exchangers before the separator can be fed, the cold source used being liquid nitrogen. The experimental separator itself is, in reality, a 'section' of a real separator. Disposing of the fluids in this machine deserves special care, since pressure has to be controlled while a liquid seal has to be kept stable in most of the operating conditions. Some workarounds had to be developed for that last problem and different solutions were kept available during the testing. Separating is an aspect, interactively measuring -required to tune to proper performance- is another. The operating fluids, that are supposed to be liquid or gaseous in the simple modeling, are in practice often two phase, which is difficult to asses during test and which can strongly impact flow and concentration measurements. High temperature gradients, low temperatures had to be considered for mechanical, sealing and bearing design, nevertheless, strong uncertainty remained before separation results are presented and demonstrate the concept. Most result went above expectations and much further compactification potentials are present. The know how gathered from testing experience allows to foresee improvement directions, both in the global concept and the detail design of a real unit. Those results will allow to extrapolate the real potentials of the separator design developed by our laboratory to the bigger units required for a real concept.