dlp_79 asked about my earlier concerns towards Beagle-2's aeroshell. My biggest quarrel with Beagle-2's design was the half angle used. Beagle-2's aeroshell was simply the Huygen aeroshell rescaled to a smaller vehicle (same half angle along with the same nose radius to base radius ratio). There are two basic aeroshell types, there's the type intended to go all the way down to the surface, e.g. to carrry a nuclear weapon to a target or there's the second type intended to deliver a payload to as high an altitude as possible when reaching a subsonic velocity. Examples of aeroshells enabling delivery of nonmilitary payloads to planetary surfaces are DS-2 and Pioneer Venus. Both aeroshells had half angles of 45 deg. If the designer opts for high altitude, then he must make the aeroshell as blunt as possible. What limits maximum bluntness is "shock detachment". Shock detachment is very sensitive to the atmosphere's chemisty. The critical parameter is "isentropic exponent" or sometimes called "ratio of specific heats". The name "ratio of specific heats" is inappropriate for hypersonic entry because the perfect gas assumption brakes down due to the atmospheric gas undergoing chemical dissociation. If one does the math, the greatest allowed half angle (maximum bluntness) for entry into a carbon dioxide atmosphere is about 70 deg. For a nitrogen atmosphere the maximum half angle is about 60 deg. Titan has a nitrogen atmosphere. The Huygen probe was correctly designed for Titan entry (a nitrogen atmosphere). Mars has a carbon dioxide atmosphere. The Viking, Mars Pathfinder, and MER aeroshells have 70 deg. half angles and are correctly designed for Mars. Now there are additional complications concerning the design of a Mars aeroshell. A 70 deg. half angle aeroshell is stable in carbon dioxide at Mach numbers greater than 1.8 . When a 70 deg. aeroshell reaches Mach = 1.8, it crosses over to dynamic instability. The angle-of-attack behaves as an undamped sinusoid function of time. The aeroshell will tumble before it goes subsonic. For this reason, the parachute is always deployed at Mach numbers equal to or greater than 1.8 . Supersonic parachutes are a bit of a black art. American and British engineers have long known how to design supersonic aeroshells. The technology was developed for deploying thermonuclear weapons from supersonic bombers. Now there is a mystery attached to the Russian Mars landers (they were an unsucessful design). The Russians used a 60 deg. half angle. Why? Were they uncomfortable with their supersonic parachute designs? Perhaps it was simple stupidity, i.e. they had a pre-existing design for the Earth's atmosphere (a nitrogen atmosphere) and reused that for Mars. Who knows? Beagle-2's designers had neither excuse. They had access to supersonic parachutes, knew about American aeroshell design and had accurate models of the Martian atmosphere. The penalty for using a 60 deg. aeroshell on Mars was that Beagle-2 gave up at least 1 km. of atltitude before its parachute could deploy. The altitude margin on Mars is very small and giving up 1 km for no good reason is poor design.

The half angle issue sticks out like a sore thumb. Here are other issues:

1) Beagle 2 did not use a rocket decellerator and landed directly on a parachute. The British designers probably thought (incorrectly) that scaling laws permitted this.
2) Beagle 2 used only three air bags of unproven reliability.
3) Beagle 2 did not use a long bridle like Mars Pathfinder and MER. A long bridle enables a short angular swing while the payload hangs from the parachute. The British engineers might have reasoned that by not using a rocket decellerator, they didn't need to worry about parachute swing. However a wide swing means Beagle-2 would almost certainly have hit the surface on one airbag.

These are the technical nitty gritties against Beagle-2. However the biggest factor that killed Beagle-2 was they didn't spend enough money on their engineering. American Mars lander heritage is based upon Viking. About $3 billion in current day dollars was spent on Viking. Normally NASA engineers are very happy to cooperate with foreign space exploration efforts, e.g. Huygen. Foreign space exploration programs can usually get a fair amount of information passed quietly under the table for free. However the guy running Beagle-2 really went out of his way to be as obnoxious as possible. He made it very clear from the beginning that he thought American aerospace engineers were spending way too much money on their Mars spacecraft and that he was going to show the world the "right way" to get to Mars. Also his stated agenda with Beagle-2 was to search the surface of Mars for existing life. Current knowledge based upon American exploration of Mars has made it clear that life on the surface of Mars is not possible (there is still the slight possibility of subsurface life). Only the lunatic fringe still believes that there can be life on the Martian surface. Thus in one fell swoop, the guy running Beagle-2 pissed off both the American engineers and planetary scientists. Nobody was interested in cooperating with this guy which meant he had to reproduce $3 billion dollars worth of technology with a budget under $50 million. Beagle-2 was doomed from the very beginning. The flaws in its management and design were fundamental. The guy who ran Beagle-2 should never again be trusted with a major project.

Gary