Wednesday, November 5, 2008

Stampede Lander

Most of this was lifted from post #119 on the marsdrivemission Yahoo group. The actual concept work was done on or about 23 June 2007.

The Stampede Lander details: Entry Mass: 12000kg
Aeroshell Diameter: 6.5m
Aeroshell Drag Coefficient: 1.40
Aeroshell Lift Coefficient: 0.42
Aeroshell L/D: 0.30
(this is all achievable using the Apollo Command Module as a basis)
Ballistic Coefficient: 303 kg/m2

With roll control, inertial guidance and a navigational fix less thanan hour prior to entry interface, it should be possible to achieve anaccuracy to a 20km diameter circle (3-sigma limits), or a circular error probable (CEP) of 5km.

Parachute Mass: 530kg
Parachute Diameter: 30m Disk-Gap-Band
Parachute Drag Coefficient: 0.67
Parachute Opening Mach: 3.0
Parachute Opening Speed: 705m/s
Parachute Opening Dynamic Pressure: 2000Pa
(This parachute is therefore quite heavy for its size.)

Heatshield Mass: 1800kg (estimated at 15% of total, probably overkill)
The propulsion system is a 3000m/s (305.8sec) specific impulse hypergolic bipropellant system giving the lander a terminal crossrange of 4000m just in case the cameras show that the poor thing is headed towards a patch of unfriendly terrain that was accepted as a risk within the entry targeting zone. If the targeting zone is a point in flat terrain, Stampede should be able to put you down right on the spot about a third of the time, and is guaranteed (i.e.: 99.7% if everything works fine) to get you within 6000m, as long as there's a beacon on the target.

After the parachute opens, your beta will be 25.3kg/m2, giving you a terminal speed of 132.2m/s in atmosphere with a density of 0.011kg/m2 (Earth's atmospheric density at the equivalent Martian surface pressure; the actual air density is higher at the surface, but will be in this ballpark at parachute altitudes.) Your heatshield has a beta of 45.8kg/m2, which means it will come off just like it does in robotic missions (most larger landers can't do this.) After jettisoning the heatshield, your beta will be 21.5kg/m2 allowing you to slow to 121.9m/s before having to light the rockets. By this point you'll be hanging under the backshell, which contains the rocket motors. Assuming the worst case crossrange+margin maneuver will be performed, the backshell will begin firing at a tilt angle of 29.4deg from the vertical, inducing an accelleration of 12m/s2 (which will probably feel like about 3g after the interplanetary coast.) After 22.1sec, you'll be travelling horizontally at 132.7m/s. The parachute is jettisoned with a rocket when its riser slackens or exceeds a certain tilt angle. You will be 1500m from the original impact point. The craft then tilts the other way and fires its motors, inducing 4.2m/s2 of accelleration to maintain altitude and slow down. You'll come to a stop after 60.3 seconds, 5500m from the initial impact point...then run out of propellant and crash because this scenario used up the touchdown margin. The system's total delta-v is 536.5m/s, the system is assumed to have a maximum induced accelleration capability of 12m/s2 and throttles back to maintain that maximum. The tank mass is given 6% of thepropellant mass, and the motor mass is calculated on a basis of 350N/kg, producing a total touchdown propulsion system mass of 2011kg. There are 7659kg remaining in the vehicle, 1659kg (13.8% of the entrymass) is left to the rest of the landing system (backshell, hovercrane lowering system), and 6000kg is the payload.

Update 18 July 2008: I switched from a hover crane touchdown system (which loses the backshell), to a reusable overhead platform. In this type of lander, the legs are mounted on the backshell and extend astride the payload (typically a rover.) The payload has its own suspension, and may be lowered on flexible struts or a really short set of tethers. Once down, the rover can roll out from underneath the platform, or the platform can relight its motors and land somewhere else to clear the payload...assuming it has enough propellant to do so.

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