Raoul said:

>About the airbags, the engineers were acutely aware of the problems and
>claimed to have solved them.
>
>About the nosecome shape: the same was used for the ARD which was
>successful (ok, the Earth isn't Mars).

Both ARD and Huygen are designed for nitrogen atmospheres. It has been documented that the Beagle 2 aeroshell was patterned after the Huygen aeroshell. Aeroshell half angle is determined by shockwave detachment. An ideal aeroshell design should be as blunt as possible with the shockwave still attached. Shockwave attachment is dependent upon isentropic exponent (ratio of specific heats if the gas is perfect). The isentropic exponent is specific to the atmospheric gas. This is why it makes no sense to use an aeoshell designed for a nitrogen atmosphere for entry into a carbon dioxide atmosphere.

>The nosecone angle is especially important (according to a friend who works
>at ESTEC) for the aerodynamic stability during entry.

That is correct. The above comments about shockwave attachment are driven by stability considerations.

>The lander is spinning and also the center of gravity matters.

An aeroshell is staticly stable when the center-of-gravity is between the shockwave and the center-of-pressure. However the center-of-pressure's location is a function of Mach number. The center-of-pressure tends to move towards the shockwave as Mach number reduces. For a 70 deg. sphere-cone used with Mars Pathfinder and MER, the center-of-pressure will coincide with the center-of-mass at a Mach number between 2.0 and 1.7 .

>About the height of the parachute opening: There is an unstable regime for
>mach 1.8-2. Beagle 2 has a more stable areodynamic shape allowing for a
>lower altitude parachute opening.

This is why Beagle 2's design is unsatisfactory. A parachute may not be deployed at Mach numbers greater than 2 otherwise it'll be shredded by the dynamic pressure (I find it amazing that a parachute will deploy at supersonic velocity). The 60-70 sphere cones become staticly unstable at around Mach 1.8. Consequently the parachute must be deployed before the aeroshell tumbles. However because the half angle of Beagle 2's aeroshell is smaller than it should be, the drag coefficient is smaller than it should be. Consequently Beagle 2 will reach Mach 1.8 at a lower altitude (they traded off drag coefficient for free stream density). Therefore Beagle 2 will deploy its parachute at an altitude lower than it should be. After the parachute deploys, the vehicle will have a wide swing underneath it. The vehicle needs lots of time for the swing to damp down. Pathfinder and MER tried to minimize this swing by hanging from a long bridle. There is a very strong luck element with Beagle 2. If the vehicle has swung up such that it hits the Martian surface sideways then it will be lost. Also Beagle 2 does not use decelerator rockets. I believe(?) the Soviets used rocket decelerators for their airbag EDL system and know the Americans use them. Beagle 2 is going straight in on parachutes alone. Beagle 2's design is exploiting the R^2 parachute area versus R^3 mass scaling laws by going with a very small vehicle. This is the only reason why not using a rocket decelerator could be an acceptable design (it still isn't acceptable). However by going with this cheaper approach, Beagle 2 is totaly at the mercy of parachute and wind dynamics. If there is any sort of wind blowing then Beagle 2 will fail. All practical designs trade some success probability for cost but I suspect Beagle 2 has gone too far. Fortunately the nice thing about probability is that a 90% probability of failure also means a 10% probability for success. Beagle 2 might get lucky.

Gary