I actually think the plan is probably to prepare the ground for MCT using stuff landed on a Dragon. Or at least was at one point.
Quote from: Robotbeat on 06/10/2015 12:53 amI actually think the plan is probably to prepare the ground for MCT using stuff landed on a Dragon. Or at least was at one point.Hmmmm...I don't recall hearing that. Also be interested to know what they could land in a Dragon that could actually feasibly clear an area sufficiently.
Skipping entry is another option.
Zond did a skip. Zond is basically a Soyuz. So no, not much L/D is needed at all.
3.2 Direct Entry versus AerocaptureEvery Mars entry since Pathfinder has utilized a directentry, in which the entry vehicle performs EDLwithout first going into orbit around the planet. Thisresults in entry velocities around 6 km/s, depending onthe Earth-Mars trajectory. This large entry velocityresults in significantly higher heat rates and heat loadsthan entry from low Mars orbit; however, direct entryhas the benefit of not requiring extra heat shields, deployable decelerators, or propulsion for an insertionmaneuver, possibly resulting in lower mass andcomplexity.Direct entry with both a blunt body (L/D = 0.23) and aslender lifting body (L/D = 1) were considered. Entrymass was assumed to be 70 MT for both vehicles,which is based on initial weights and sizing estimates.The blunt body flew a trajectory that took it to Mach 5at 10 km altitude. This altitude helps determine howmuch time will be left for performing supersonicdeceleration. This can be thought of as a surrogate forlanded accuracy as with increased timeline moremaneuvers are possible to target the landing site.On the other hand, the slender body has a tendency toexit the atmosphere when the trajectory was flown liftup.When flown at a constant bank angle of 85°, theslender body was able to stay in the atmosphere, butdue to its low hypersonic drag coefficient, the Mach 5transition altitude is at 6 km, leaving little timelinemargin for the rest of deceleration. When bank angleis set back to 0° once entry is assured, this transitionaltitude can be increased; the downside to this class oftrajectory is that heat rates and heat loads are highcompared to other trajectories and vehicleconfigurations.Aerocapture involves entering the atmosphere from ahyperbolic trajectory and using drag to slow the vehicledown enough to exit the atmosphere in a closed orbitaround Mars. Previous studies have shown thataerocapture can lead to a mass and cost savings overother options including direct entry, aerobraking, andpropulsive insertion. Aerocapture reduces the kineticenergy of the entry vehicle by 20-40% at entryatmospheric interface. Entry from this slower velocityreduces the severity of the heating environmentexperienced by the vehicle, allowing for a thermalprotection system mass savings. This strategy alsoallows the entry vehicle to reach Mach 5 at a higheraltitude compared to direct entry.The performance advantages that aerocapture providesmust be weighed against the operational disadvantages.With two entry sequences, any errors in orbit after theaerocapture trajectory must be detected and correctedto ensure that the entry sequence begins as planned.The heat imparted to the heatshield on the aerocapturepass also must be dealt with. Nested dual heatshieldshave been proposed, in which the aerocaptureheatshield is jettisoned after the first pass through theatmosphere with a second heatshield is used on entry[15]. An alternative is the use of a hypersonic IADduring aerocapture. The larger drag area decreases theballistic coefficient sufficiently that the vehicledecelerates higher in the atmosphere, and sufficientlyreduces the heat rates seen on the vehicle. Overall,aerocapture is a more complex mission mode than direct entry. While these trade studies can be analyzedfor their performance at a conceptual level, moredetailed studies should be performed to identify themost cost and mass efficient option.
From what I've read a radiative cooling system (which is what the shuttle used and what the metallic systems would be) are limited in their heat FLUX capacity, but generally are able to endure for a long time.This would make them a poor match with an airo-capture maneuver as they would experience brief pulses of very high intensity heat flux. Also the whole system would need to be engineered to meet the high heat of the first pass which from my understanding is only mildly lower then the heat flux of direct entry at the same speed.Now Airo-braking (many very high altitude very gentle passes to lower periapsis would be more compatible but this requires that your initial entry velocity is only just above capture which means it needs to be proceeded by propulsion.Using propulsion to capture and circularize down to LMO looks to me to be the only way to truly have a light weight and fully reusable TPS. Interacting with the atmosphere while still at high speed just gives all kinds of trouble and when we have high ISP propulsion available in the form of SEP the cost in propellent mass should be far far less then the TPS needs which are converted into PURE PAYLOAD for the lander.
Could a large SEP tug, take a lander to Mars large enough to land a large metal pad that could unroll for the MCT to land on? Maybe several landers in the same area with several rolls that could cover an area large enough for the MCT. The landers could then be salvaged for building materials or habitats.
Arguably the Metalic TPS is the option to go with because of it's much higher TRL, my understanding is that it's basically already been done as part of the X-33 development, in fact it seems to be one of the few parts of the design that weren't giving anyone a headache. http://www.nasa.gov/centers/ames/news/releases/1999/99_09AR.html And remember this is a system considered capable of Earth entry velocity, we would be DOWN GRADING this material to accommodate a Mars entry which would be done by making the insulation thinner and or using a less 'super' alloy in the skin like mere titanium rather then inconel.