The MetNet Mars Lander – Probing the Red Planet for climatological dataAuthor Matti Palin
A new Mars lander concept has been developed in cooperation between the Finnish Meteorological Institute (FMI) and the Lavochkin Association (LA). Instead of retrorockets, which were used in previous Mars missions, the lander concept relies on the use of aerodynamic breaking. This approach will greatly reduce fuel costs and improve the delivered-payload-to-mass ratio.
The Red Planet, named after the god of war, can be as intimidating as the name suggests. The details about its weather and climatology are still very much a mystery to mankind. It is nevertheless our first choice when it comes to looking for another habitable planet. So far, there have been eight successful landings on Mars, and the Opportunity and Curiosity rovers are still operational today. All of these missions used retrorockets in the landing phase.
The genius of the MetNet concept is the use of pure aerodynamics to ensure a safe landing for the vehicle. The landing system consists of a pair of inflatable braking devices that produce the necessary deceleration. This technique not only increases the payloadto-mass ratio, but also is much more ecological since no extra fuel is needed in the landing phase. See Figure 1.
Figure 1. The Main Inflatable Braking Unit in flow simulation. CFD analysis is used to solve the flow field and to calculate the induced aerodynamic forces and moments.
Revolutionary rationale requires revolutionary research
The big question on everyone’s lips is of course: Will it work? The engineering research commenced already in the early 2000s and a body prototype was built between 2001 and 2004. Wind tunnel and heat flux tests have been conducted for the key components of the structure, but the problem with these tests is that it is difficult to replicate the actual descent and landing conditions on Mars. This is because the atmosphere in Mars differs greatly from that on Earth. Even though the speed of sound is of the same order of magnitude as on Earth, the Mach number at the beginning of the landing phase will still be large (exceeding Ma=20). Thus, it is out of the question to conduct one-to-one tests of the mission.
From 2015, the aerodynamic research has been carried out by Finflo Ltd. The technique used in the analysis is computational fluid mechanics analysis, or CFD for short. CFD is Finflo’s speciality and a self-titled code is used. In a CFD analysis, the flow field around an object is solved and the aerodynamic forces and moment can thus be calculated.
The most evident research question regarding aerodynamics is whether the aerodynamic braking is sufficient to slow down the vehicle to an admissible impact speed. The impact speed is rather easy to derive algebraically, and the condition for the admissible impact speed ultimately boils down to the vehicle having a sufficient drag coefficient. The drag coefficient is a measure of how much aerodynamic drag force, opposing the speed of movement, an object generates. The simulation results for the landing configuration are illustrated in Figure 2.
Figure 2. Comparison between the drag coefficients obtained with FINFLO simulations and from LA for the AIBD case. The results indicate that the drag coefficient is well above the required value to ensure the designed impact speed.
The second research question is a far more difficult one. In order to land safely, the vehicle must land upright. In other words, it must not start spinning wildly around its centre of mass. This phenomenon can be examined by analysing the aerodynamic stability of the vehicle.
In this context, stability is defined as the tendency of a state to return to its original position after a perturbation. State, in this case, is the angle of attack of the vehicle. This is a natural choice, because a steady angle of attack means that the vehicle will not tumble around.
In practise, most of the current research is focusing on determining the aerodynamic stability qualities of the MetNet lander. This is by no means a simple task and has already required years of computation time.
The time accurate simulations present a veritable challenge for modern computers. Fortunately, the results have been encouraging: it was quickly seen that the vehicle is statically stable. This means that it does generate moments that oppose a change. Another side of the question, however, is dynamic stability. Does the vehicle generate a sufficiently aerodynamic pitch damping moment?
Pushing the envelope
Although the simulations are still in progress, the results have indicated that in some cases the vehicle is also dynamically stable. However, with the design in its current form, it would not be safe to send the MetNet lander to Mars; full stability has not been ensured and it is possible that the vehicle would not land in an upright position.
In airplane design, longitudinal stability is ensured by making sure that the centre of pressure is aft from the centre of mass. This idea can also be used in the case of MetNet: the stabilising characteristics of the vehicle can be enhanced by pushing the centre of mass forward. This brings yet another level of complication to the simulations, since an optimum location for the point has to be found.
Due to the long simulation times, the aerodynamic design will still take years. In addition to shifting the centre of mass, altering the aspect ratio of the geometry has also been considered as a solution for the stability problem. Once the simulations provide enough evidence for an aerodynamically solid design, a precursor mission to Mars is planned for the mid-2020s.
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