An Aerial Platform for Mars – a Technology Demonstrator for the Future

The name ARCHIMEDES is an acronym derived from the set of onboard instruments and means: Aerial Robot Carrying High Resolution Imaging, a Magnetometer Experiment and Direct Environment Sensors. Accordingly the ARCHIMEDES instrument pod will comprise three main devices:

• a high resolution camera able to achieve a resolution of up to 20 centimetres per pixel at a distance of seven kilometres from the surface

• a magnetometer designed to analyse the planet’s magnetic field as well as the interaction between the solar wind and the Martian atmosphere

• an atmospheric sensor package provided by the Finnish Meteorological Institute

In addition to these experiments, there are two more sets of instruments, which are intended to become operational prior to and during atmospheric entry. The heating and flow field experiment COMPARE will measure thermal loads and pressure during entry. Acceleration sensors will gauge the deceleration of ARCHIMEDES in the upper atmosphere. These instruments will be built into the nose cover assembly, which will protect the instrument pod and its sensors from the hypersonic environment. During entry and descent, the P5-A orbiter will use its radio ranging equipment to track the inflated balloon. This data will later serve to complement the deceleration data and thus provide a detailed profile of the atmospheric conditions at high altitudes.

The unique mission scenario highlights the significance of ARCHIMEDES as an advanced technology platform. As a technological first, this new and innovative concept will explore the use of a large and lightweight drag body to decelerate a probe far outside the denser parts of a planet’s atmosphere. This technology, commonly known as aerobraking, is key not only to the design of aerial vehicles on other planets, but will also make an important contribution to the development of fuel-efficient deep-space manoeuvring techniques.

Mission Profile

AMSAT-DL has firm plans to use the 2009 launch window to send its P5-A robotic probe to Mars. The German amateur radio satellite operator is offering ARCHIMEDES a ride as a passenger. On completion of orbit insertion, ARCHIMEDES will separate from the mother ship and autonomously perform a de-orbit manoeuvre. It will then decelerate to an approach trajectory. Once the correct approach is reached, the flight system will de-spin, deploy and inflate the balloon hull.

After a number of consecutive passes through the atmosphere, ARCHIMEDES will finally begin its descent to the surface. P5-A will remain in orbit and provide a radio link to ground stations. Vital mission data can therefore be transmitted during approach and descent and will thus be available even in the case of a major malfunction during entry.

After sound barrier transition, the protective nose cover assembly with the two entry experiments will be jettisoned, and the instrument pod will be exposed to the Martian atmosphere. If successful, these would be the first measurements and data ever taken from an altitude of 50 – 60 kilometres. The balloon will slowly descent to the ground while continuing to collect atmospheric and magnetometer data. This will allow researchers back on earth to establish the first directly measured altitude profile of the Martian atmosphere ranging from the ionosphere right down to the ground.

In-Space Inflation

Inflation of the balloon takes place during approach and prior to entry (In-Space Inflation Concept). The balloon will be turned into a large drag body, thus facilitating operations and rendering many of the otherwise critical manoeuvres unnecessary.

ARCHIMEDES follows a two-stage inflation concept. The first stage will release the balloon package together with the payload. At that point a valve will open to inject a small amount of helium into the pre-deployment section, which can then widen and gently open the rest of the package. Further on, the main stage of the inflation mechanism will kick in to pressurise the balloon. At last, the second stage of the release mechanism takes over to close the balloon and separate it from the inflation system. Upon entry, the large size and low weight characteristics of the balloon will provide enough drag to decelerate the system efficiently, while still in the upper portions of the atmosphere. The probe will therefore avoid the high thermal loads of a conventional hot entry into the much denser lower layers of the atmosphere.

Technical Hurdles

A number of engineering issues still require attention. The balloon needs to avoid local thermal peaks (hot spots) and must sustain loads encountered during off-nominal entry conditions. The shock of hypersonic particles can easily cause the destruction of the balloon’s surface, so careful material selection and trajectory design are vitally important.

Storage and packaging of the balloon material still present further challenges. The material is exposed to the vacuum of space during the six months cruise period. Prior to launch the balloon’s skin must therefore be carefully folded: Rapidly expanding pockets of trapped air could wreak havoc during launch. However, a residual amount of air must stay in the package, and a very thin helium atmosphere, bleed gas from the inflation system, will be maintained inside the transportation container to prevent cold welding.

A first Test in Space

Testing components on the ground is important, but sometimes testing in weightlessness under space conditions is required to ensure the mechanisms work as planned. The REGINA test flight in early April onboard the REXUS 3 sounding rocket offered the first critical shakeout for ARCHIMEDES hardware. Initially the rocket blasted off from its launch pad in Kiruna, Sweden as planned. However a collision in mid-flight caused loss of important imaging data. The campaign was therefore considered only a partial success. Yet REGINA returned some critical information that will help shape the future of the programme. A second set of parabolic flights as well as future spaceflight tests will certainly benefit from the experience gained.

Bildunterschrift: Rexus 3 and REGINA

Rocket Science

Project ARCHIMEDES has seen tremendous progress in recent months. We are currently in the midst of the exciting but painstaking process of component development.

While we spent the better part of 2003/04 to finalise the overall mission concept, we began last year with the actual design and testing of the various spacecraft subsystems. In order to better manage the minutiae of the development process, the team is currently pursuing a three-pronged approach involving the following semi-independent design programmes.

CLEOPATRA

CLEOPATRA will focus on the mechanics, electronics and operations of the spacecraft. One critical mechanical component is the ARCHIMEDES Release Mechanism (ARM). Its main task is the automatic deployment and separation of the space probe. After last year's successful parabolic flight test campaign onboard a specially equipped Airbus, there was a follow-up programme this year called REGINA. In April the ARM Stage-1 mechanism as well as the balloon package was tested under near-space conditions onboard a REXUS sounding rocket in Kiruna, Sweden.

Although the freefalling REXUS trajectory only provides for two or three minutes of weightlessness at best, project members were able to glean valuable information on how the flight hardware performs in a vacuum and microgravity environment. The REGINA flight system also carried an experimental electronics package for telemetry and flight control. If approved, a second parabolic flight campaign will wrap up this year's busy testing schedule. This time, the spacecraft separation mechanism, or ARM Stage-2 as it's called, will be the focus of the experiment, but also an improved version of the ARM-Stage-1.

ARCHIMATTER

The ARCHIMATTER programme will help us pick the right materials for the manufacture of the ARCHIMEDES balloon. The programme was only launched in January and is still in its initial phases. Finding materials that are fit to withstand the stresses of launch and exposure to the vacuum of space as well as the hypersonic entry environment isn't trivial. Furthermore the team is still looking for the best way to weld or sew together large pieces of material. Finally packaging and inflation techniques likewise require rigorous testing. There is currently some preliminary research into materials and manufacturing techniques. But this is only intended to eventually pave the way for larger scale trials at the University of the German Federal Armed Forces in Munich.

ARCHYFLOW

With ARCHYFLOW the team is tackling the intricacies of the aerodynamics and thermodynamics of the mission. ARCHIMEDES will hit the planet's atmosphere at hypersonic speed. Based on the vehicle's flight trajectory, ARCHYFLOW will establish a mathematical model in order to predict the mechanical stresses exerted on the balloon during this phase. The model will then be used to determine the best shape for the nose cap assembly and the resulting shape of the balloon itself. The results will likewise benefit the ARCHIMATTER programme in its quest for the best materials.

By the summer of 2007 all three programmes are expected to have finished their respective testing campaigns. At this point the three programmes will be reintegrated, and additional test campaigns are currently envisaged. One proposal for a more realistic test scenario involves a larger-than-life test balloon that is sent to an altitude upwards of 100,000 feet. This way we can simulate here on Earth the low-pressure atmospheric conditions on Mars. The programme is called MATHILDA (Mars-equivalent Altitude Test and Hardware In the Loop Demonstration for ARCHIMEDES). As another twist, MATHILDA could be programmed to communicate with the AMSAT P3-E satellite and thus simulate communication with an orbiting spacecraft. However, MATHILDA is currently suspended pending the further development of yet another, maybe even more crucial flight test: MIRIAM.

This next space flight mission test is a full-blown flight systems test, where the complete ARCHIMEDES mission scenario will be checked out close to planet Earth prior to the actual start of the mission. As a follow-up to REGINA, an ARCHIMEDES precursor equipped with ARM stages 1 and 2 as well as various transponders and instruments will be launched on a suborbital flight into space. The flight system will then descent back to the ground fully inflated. This mission is currently prepared under the acronym MIRIAM (Main Inflated Reentry Into the Atmosphere Mission test). MIRIAM will allow the payload to reach much higher altitudes and speeds than with REGINA and will thus add to the realism of the simulation. But it will also require a more expensive, two stage launcher. MIRIAM is foreseen to launch in April 2007.