About a Megawatt of power is needed to produce enough propellant for a MCT in a year. 40 tons unless you hook up to base-side infrastructure. If you have nuclear, you need huge radiators (or ground infrastructure) and shielding of some kind. Solar of 1MW requires even larger deployment on the ground. Electrolysis and Sabatier are a little smaller but still substantial.It simply isn't a good idea to keep it on the MCTs.
Or ISRU methane production is built into the early BFS's, or maybe all of them for redundancy. How large would the device be?
It's been suggested before that a small number of the very first BFS's (formerly MCT) landers on Mars will not return to Earth. It shouldn't be hard to modify one or two into a point-to-point planetary hopper. It's a useful tool to have, and serves as a rescue vehicle.
A launch-on-need hopper can be a one-way trip. And if it has 7km/s of delta-V, it can go anywhere on Mars.
Quote from: Robotbeat on 01/23/2016 05:19 pmA launch-on-need hopper can be a one-way trip. And if it has 7km/s of delta-V, it can go anywhere on Mars.Huh? what use is it if it can't return to where it started from?
Quote from: nadreck on 01/23/2016 05:20 pmQuote from: Robotbeat on 01/23/2016 05:19 pmA launch-on-need hopper can be a one-way trip. And if it has 7km/s of delta-V, it can go anywhere on Mars.Huh? what use is it if it can't return to where it started from?It can carry enough supplies and solar power units to keep the stranded crew alive until a ground retrieve mission can reach them. If need be additional resupply flights.
If it can only get as far as the stranded people it can't make subsequent flights. You are using up hoppers on a one way trip. So whether it is a custom designed hopper or an MCT that was left behind it is an emergency solution as much as a left behind MCT (and eventually several) will be the evac solution for a settlement if something makes it non viable early on.
Quote from: docmordrid on 01/23/2016 01:43 pmOr ISRU methane production is built into the early BFS's, or maybe all of them for redundancy. How large would the device be?Bingo. There will be a lot of cargo MCTs to develop a colony, very few will likely carry ISRU equipment.
...however every kilo of propellant used in that endeavour would represent 3 - 4 kilos of ISRU propellant and at say 200t of propellant per tanker flight, you are adding engine cycles on your reusable MCT's fairly quickly.
Quote from: nadreck on 01/23/2016 06:12 pmIf it can only get as far as the stranded people it can't make subsequent flights. You are using up hoppers on a one way trip. So whether it is a custom designed hopper or an MCT that was left behind it is an emergency solution as much as a left behind MCT (and eventually several) will be the evac solution for a settlement if something makes it non viable early on.Isn't this highly dependent on how far the people are away from base and how the hopper is built? I would assume a custom hopper could weigh quit a bit less than an MCT. Even if it could only make a one-way trip, why couldn't a slow moving wheeled tanker just come along later and refuel it? Sure a orbital depot might make sense for exploration, but not IMO required for a rescue op.
So just go into very low Orbit and deorbit. That'd take less than ~5km/s delta-v.btw, are you relying on Earth figures, or are you recalculating them for Mars? The lower gravity makes a huge difference here....BTW, this is all off-topic. I just brought up the idea to counter the false (but oft-repeated) claim that abort would be useless for MCT because there'd be no way to get to them.
Quote from: Robotbeat on 01/23/2016 06:50 pmSo just go into very low Orbit and deorbit. That'd take less than ~5km/s delta-v.btw, are you relying on Earth figures, or are you recalculating them for Mars? The lower gravity makes a huge difference here....BTW, this is all off-topic. I just brought up the idea to counter the false (but oft-repeated) claim that abort would be useless for MCT because there'd be no way to get to them.I was using 4.1km/s for surface to LMOyes I was calculating for Mars for the point to point distances
The concept of "hoppers" for Mars is not really practical the way many people are throwing it around here. IF you expect the hopper to go point to point on Mars and return to the origin point without refuelling the range of a hopper that has enough ΔV to make it to low Mars orbit is less than 200km. The same hopper could go nearly 1000km if it can refuel at its destination, but that is not useful for exploration because you can't just go anywhere. For serious exploration by rocket powered craft what is needed is a craft that can descend from orbit fully fuelled to any location on Mars and, after landing have enough fuel to make it back to orbit. If that can be achieved then there is a point to using that sort of craft to travel on Mars, otherwise the limitations on such craft is such that wheeled vehicles will be far more practical. Even so to make it work you would need fuel depots in Mars orbit.A fully fuelled MCT (ΔV = 7.5km/s) on the surface of Mars could make a one way trip to any point on Mars most likely, but a two way trip would be limited to something around 700km. Note MCT left at Mars for their use at Mars would make excellent support craft for a Mars Orbital Station/Depot and if a few dedicated smaller craft optimized for orbit-surface-orbit on a single load of propellant existed that depot could support exploration of any point on Mars surface, however every kilo of propellant used in that endeavour would represent 3 - 4 kilos of ISRU propellant and at say 200t of propellant per tanker flight, you are adding engine cycles on your reusable MCT's fairly quickly.Still as pointed out above the real limit early on will be the ISRU capacity.
The BFS is a stage in and of itself. If it's intact enough to land, you'd likely be better off aborting to orbit.BTW, MCT is the whole system, BFR and BFS together. What you call the MCT is what Musk calls the BFS.