This first Artemis flight is an unmanned test flight which will be used to qualify the launcherlauncher Space Launch System (SLS) and the Orion vehicle with, therefore, several objectives. Unsurprisingly, the NasaNasa will want to ensure the correct sizing of the heat shield of the capsule which will be the sole protection of the astronautsastronauts when they return to Earth. The frequent trips back and forth between Earth and Space stationSpace station are not comparable with a return to Earth from a lunar mission, the speedspeed back to schoolatmosphereatmosphere of a return mission from the MoonMoon being far superior to a return missionorbitorbiteleven kilometers per second against “only” seven kilometers per second.
The in-flight behavior of the service module of the Orion vehicle, provided by the European Space Agency and under the supervision of Airbus, will also be closely monitored. NASA and the European Space Agency (ESA) want to push it to its limits and test it inside its flight envelope, so as to see under what conditions and how it can operate in various configurations. The propulsion system of theEuropean service module (ESM) will therefore be at the heart of the tests that NASA hopes to carry out, and particularly scrutinized.
As Philippe Deloo, in charge of the service module (ESM) project at ESA, points out, it is the ” first time the system will be tested in space “. True, the ESM module has already “ been tested on the ground but the representativeness of the ground tests is never 100% “. There, we will be able to refine our models. ” We will learn a lot with this flight “, he says. For Airbus, this test flight should remove several development uncertainties and qualify the module for subsequent missions, including Artemis III which will bring a crew of two astronauts to the Moon.
A multitude of engines inherited from past vehicles that have made history
As Antoine Alouani, Orion’s propulsion system engineer at Airbus, reminds us, this ” propulsion system has no less than… 33 engines “, including a main engine, ” which is one of those of the space shuttle which has already flown and which provides a thrust of 26 kilonewtons », 8 auxiliary engines, « derived from those of the automatic transfer vehicle (ATVATV) », and 24 attitude control engines. There is therefore an obvious interest in testing them in flight.
Although this module is based on the legacy of the ATV, the architecture of the ESM differs in many respects. ” We want to check if our design choices are the right ones because we considered several other configurations. »
“For Airbus, this test mission is much more important than it seems”
But if NASA wishes to ensure that the service module will be able to carry out all the planned maneuvers, ” remember that without this module the Orion vehicle cannot work », for Airbus this test mission is much more important than it seems. What concerns Airbus’ propulsion teams the most are, on the one hand, ” this multitude of different engines that provide Orion with unprecedented thrust performance and versatility “, but also the fact that” these engines will use a tank common to all, which represents an immense difficulty in development “. To this already strong constraint, are added others and not the least.
First, you should know that these three engines were developed at different times, by different manufacturers, in different countries, on different systems, for different missions. Certainly they use the same oxidizeroxidizer and fuel but their operation differs. ” Not all engine types have the same mixture ratio and each has its own areas of operation”. The challenge is to achieve deliver fuel to each of the three engine types with the levels of debitdebitof pressurepressureand temperature required “. In some flight configurations, it is possible for 13 engines of two or three different types to operate at the same time, ” which is a major source of operational complexity for the whole system !
This choice of three different engines was imposed by NASA, which wished ” use the Space Shuttle engine as the prime mover due to the unprecedented levels of thrust and reliability that Artemis missions will require, especially for lunar and trans-Earth maneuvers “.
The state of the art pushed to its limits
To connect and operate these three engines from this single tank, Airbus has ” produced one of the most complex propulsion subsystems ever built in Europe “. Result, a tangle of pipes that go in several directions, of different diameters and geometric shapes ! This sub-branch architecture, which can be likened to a mikado, is the solution found by Airbus to ” supply and maintain fuel to each of the engines at the required pressure levels and flow rates “.
” It was one of the hardest things to come up with. A unique architecture, severely constrained by draconian requirements, in particular of available volume, massmassand reliability. »
In addition, it must also be taken into account that the engines in operation ” drain the tank which must remain at the correct pressure “. As the fuel liquidliquid is consumed, it is necessary to replace it with gas and maintain it at an almost constant pressure level, throughout Orion’s propelled maneuvers “. For this, Airbus has chosen to use ” I‘helium injected at high pressure, from a pressurized tank at 400 bars ! A tank with such a level of pressure on board a manned vehicle may come as a surprise. To understand this choice, it is necessary to know that “ the footprint is so small inside the ESM ‘, and that to embark all theheliumhelium necessary for the mission in a tank with dimensions constrained to fit inside the module, “ we had no choice but these 400 bars !
This unique and unprecedented design was “ carried out in a very constrained programmatic environment “. As in all projects, the engineering did not have a free hand to develop and qualify certain elements of the ESM as desired. It was necessary to take into account the programmatic conditions set by ESA and NASA which ” led us to make certain decisions that were perhaps not necessarily our first choices as an engineer “. So there is a ” certain residual dose of uncertainty for some equipment which will be lifted with the flight, which is intrinsic to any space program, and in particular in these types of missions where the limits are pushed.
Did you know ?
To our knowledge, there is only one company capable of producing this kind of composite tanks in the world and it is French. This is the ArianeGroup Aquitaine site, in Saint-Médard-en-Jalles, near Bordeaux. For the record, they are derived from those of Ariane 5 to pressurize the cryogenic main stage (EPC) and the storable propulsion upper stage (EPS).
” But I remain optimistic and I am confident about the flight behavior of our module. As very often in space, the “ margins are planned from the design phase to deal with contingencies “. There is certainly a level of risk specific to any new vehicle flying for the first time, but “ if I stick to our studies, our risk analyzes and our tests, it is difficult to rationally predict that it will not work “.
A rather surprising take-off risk!
There is yet another hard point that we think is interesting to address because “ if it were to malfunction a few seconds after the launcher took off “, the mission could be compromised and ” would cause the emergency extraction of the vehicle from Orion and its rapid removal from the launcher without there being any risk of explosion of the launcher or the service module !
For obvious security reasons, the ” fuel is locked inside its tank prior to launch “. The three motors of the ESM are therefore not powered. ” This is to avoid any risk of leakage to the ground which could be fatal. ! You should know that a single drop of this fuel can kill a human and breathing its vapors is enough to cause cancercancer in very short time.
To prevent fuel once filled from leaking out, ” a series of three successive valves keep it in the tank and prevent leakage through the engines, especially on the ground “. This rule of three mechanical barriers between the tank and each engine is a fundamental safety constraint for the design of the propulsion system. For the proper functioning of the ESM module, it is necessary to bring and condition the fuel to the engine inlets. For this, these valves will all have to open in just 40 seconds in a precise sequence and release fuel at the required pressure levels “. If this maneuver is not performed correctly, or only partially, ” the mission will not be done because it will not be possible to turn on the engines “. In the case of a manned mission, the only solution will be ” to eject the Orion vehicle urgently because without an engine the vehicle will not be drivable and can go nowhere “. This sequence will be played in just 40 seconds, two minutes after the SLS launcher has taken off.
In conclusion, ” one is reasonably confident, I would say. I still have a little apprehension about the tank pressurization system. But Artemis I is the only way to know if we’ve done well, so there’s no time to procrastinate, we have to get started “. This first test flight will show the actual performance of the module, see its margins so that ” data analysis can allow us to test our different models, to check if they are sufficiently robust in terms of mattermatter of design architecture, and ultimately locate any design flaws that need to be corrected “.