Is this the simplest sequence of events and machinery they could come up with the achieve the landing? There are a lot of motors, sensors, and propulsion engines that could fail.
Why is the heat shield jettisoned 7km above the surface exposing the rover to the atmosphere while its travelling at over 500kph? Does anyone have any insight into the design decisions in this landing sequence?
is a paper written by the engineers who designed the system, explaining these decisions. I suspect that the reason is that - in Mars's thin atmosphere - 500kph isn't as much pressure as it would be on Earth. (The constraint appears to be that the heat shield needs to fall away.)
I found the way MSL detects landing to be clever. Instead of a switch or sensor, it records the throttle setting required to descend at a constant speed. When that setting drops (for a sufficient time), the rover must have touched down.
“We call it the Supersonic Transition Problem,” said Manning. “Unique to Mars, there is a velocity-altitude gap below Mach 5. The gap is between the delivery capability of large entry systems at Mars and the capability of super-and sub-sonic decelerator technologies to get below the speed of sound.”
The article is in the context of landing humans on Mars, but the principles are the same.
Thanks for sharing the link, it's a really interesting read.
I'm curious - wouldn't a large, variable geometry lifting body do the trick? Land like a shuttle would, but with a bigger wing, and deploy more wing and lift devices as you get lower.
The wingspan required for something like a manned spacecraft would put the Spruce Goose to shame. You need something huge to be able to slow you to a reasonable speed.
You can bet that there's no better simple solution. This problem has been analyzed extensively by groups with lots of resources (intellectual and monetary).
Besides the other commenter who mentioned the size of the lift surface, think of the mass requirements, the variable atmosphere density mentioned in the linked article, the need for an autonomous lander (because of light time), and the small amount of time you have to land.
Thanks for that. From wikipedia, surface atmospheric pressure is less than 1% of that on Earth so you're probably right. A wind pressure from 500 kph wind at the surface of Mars would be roughly equivalent to wind pressure from a 50kph (Force 7) wind on Earth. It will be less still higher up in the atmosphere. (Assuming I didn't just get the physics embarrassingly wrong, please correct me if I did)
At T-4 minutes they hold for 10 minutes and the commentary said this was planned beforehand. Does anyone know why they don't just plan to launch 10 minutes later without the 'hold'?
The pre-programmed holds (as opposed to unplanned holds) exist so that if there were any unexpected temporary hitches in the launch sequence, there is still time to make the launch window. Imagine for example that you have a 1 minute launch window, and a slower than expected fuel pump means that it takes 5 minutes longer to top off the fuel tanks. Well, you just missed your entire launch. Stand everyone down, and put the spacecraft back into standby before trying again tomorrow. If you have a planned hold, you simply reduce the planned hold time, and you can still hit the launch window.
The hold is also used as a sync point between all of the different activities that have to be completed at certain points in the launch preparation. For example, at the t-4 hold for the Curiousity launch, they needed to pass the go/no-go from the entire team. If one team had been running late up till that point it wasn't a problem, the other processes just entered the hold earlier. Once the last hold has been cleared, all of the remaining processes have to stay synchronised.
Basically, holds are used to give the personnel some breathing room to make checks and ensure everything is okay (and presumably to relax a bit, go to toilet, and other human necessities). If something does not check out, a hold could be extended to debug the problem - on the other hand, when the clock is running, the schedule cannot be changed because many things are preprogrammed in the launch vehicle and the spacecraft to occur at certain clock ticks.
There are very specific things that have to happen at certain points during the countdown. I expect that the plan to have the 10 minute hold was created after the software was configured for all the last minute tests. Reconfiguring all the software would be impossible right before the launch, so instead, they stop the clock. Stopping the clock is not unusual, and all the software that works off the clock is built to deal with a hold.
http://www.youtube.com/watch?v=BudlaGh1A0o