Author Topic: MCT Speculation and Discussion Thread 4  (Read 503984 times)

Offline llanitedave

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Re: MCT Speculation and Discussion Thread 4
« Reply #1400 on: 01/23/2016 03:47 PM »
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.


Yeah, that's the kind of equipment that's going to require multiple launches, be assembled on Mars, and then be able to operate during periods when no one is there to maintain it.  That's one reason why I expect that the first several BFS's are not going to return to Earth.
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Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1401 on: 01/23/2016 04:07 PM »

Or 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.

Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1402 on: 01/23/2016 04:20 PM »
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.

I don't get why using a huge interplanetary spacecraft for a rescue vehicle makes sense. All you want to do is get the people back to the base. A special purpose hopper could be way more efficient if built for this purpose. Of course, parts off MCT's or even Red Dragons could be useful.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1403 on: 01/23/2016 05:11 PM »
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.
« Last Edit: 01/23/2016 05:12 PM by nadreck »
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1404 on: 01/23/2016 05:19 PM »
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.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1405 on: 01/23/2016 05:20 PM »
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.

Huh? what use is it if it can't return to where it started from?
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Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #1406 on: 01/23/2016 05:53 PM »
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.

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.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1407 on: 01/23/2016 06:12 PM »
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.

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.

If you want to use ballistic vehicles of any type for exploration you need an orbital depot and a craft designed to go from orbit to surface to orbit on a full propellant load since the requirements for that are less than the requirements for a hopper that can go more than 700km point to point and return on a single propellant load.
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Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1408 on: 01/23/2016 06:24 PM »
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.

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.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1409 on: 01/23/2016 06:27 PM »

Or 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.

Reasonable estimates for ISPP equipment necessary to refuel one vehicle over one synod once power sources and water collection equipment are included (which is where most of the mass is) is easily half of the 100 mt cargo capacity and may be several whole loads worth depending on how huge the vehicles propellant needs are.  It would be completely impractical to include this in every vehicle as it would cut deliverable payloads at a minimum in half.

In addition its simply inefficient because we want any ISPP equipment to stay on Mars FOREVER operating continuously and storing all that propellant for later use.  Integrated into the vehicle the equipment is dead-weight on every Earth return trip as well as a pure loss in potential propellant production by removing a functional system from the surface.

In the long run the fraction of cargo flights carrying ISPP equipment may be small because ideally the equipment will operate for decades and pay dividends of hundreds or thousands of times it's own mass in propellants.  But initially ISPP cargo will dominate and the ability to scale up propellant production will be a limiting factor in the ability to scale up the whole SpaceX transport infrastructure.

Offline dror

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Re: MCT Speculation and Discussion Thread 4
« Reply #1410 on: 01/23/2016 06:39 PM »
...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.

That's where the incentive for Phobos\Deimos\asteroid mining and atmospheric scooping lay.
If a mission like ARM is considered here, I bet it could be considered there, closer to the asteroid belt, as a viable solution for a LMO depot.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1411 on: 01/23/2016 06:43 PM »
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.

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.

Actually once you are more than 700km away it takes more ΔV than orbital velocity to get there and land propulsively and the absolute limit of what is needed is somewhere between 6 and 7.5 km/s for an antipode(furthest point away on the surface of Mars - 10,000km) flight depending on how much aerodynamics can be used.

Basically the ΔV requirements look something like this for one way hops:

2km/s ~ 150km (aerolift/braking would make no difference)
4km/s ~ 750km (aerolift/braking would make no difference)
6km/s ~ 2500km (no aerolift/braking)
7.5km/s - 10,000km (no aerolift/braking)

So lets say your mission is more than 600km and your hopper can do 7.5 km/s of ΔV (limit for a two way flight) the amount of propellant required to refuel it and bring it back (at ISP 380) varies by fairly large factor but still is at least 2.5 times the total dry mass of the hopper, at the full range it would mass 7 times the mass of the hopper. So lets say the dry mass of the hopper is 10t (a very light craft if it is expected to land enough supplies for stranded people 10,000 km from base) you would need at least 25t of propellant that was trucked to the closest location and 70t to the furthest. That is an awful lot of extra mass to drag around on a rescue mission that needs to power itself all the way there and back.
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1412 on: 01/23/2016 06:50 PM »
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.
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Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #1413 on: 01/23/2016 06:55 PM »
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.
I was using 4.1km/s for surface to LMO

yes I was calculating for Mars for the point to point distances
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1414 on: 01/23/2016 06:58 PM »
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.
I was using 4.1km/s for surface to LMO

yes I was calculating for Mars for the point to point distances
care to show your work?
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1415 on: 01/23/2016 07:00 PM »
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.

Can you clarify how your calculation is performed?  Particularly are you assuming a fully propulsive landing equal in DeltaV to the take-off?  If landings are done with mostly atmospheric deceleration then this would greatly improve the range of the vehicle.   Also what if any cargo capacity are you assuming during the hop and is their any offloading or on-loading at the destination?

In any case the 700km figure that you came up with sounds adequate to do a down-range rescue of personnel from an aborted launch to orbit.  And it provides an excellent rescue option for ground vehicles, NASA plans limit ground vehicles to a 100km radius for logistics and safety reasons.  With a sub-orbital hop as a rescue and resupply option the vehicles could safely explore an area almost 50 times larger.  So all in all very useful even if it is of limited range.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1416 on: 01/23/2016 07:16 PM »
It stands to reason that if a hopper over 700km exceeds orbital delta-V, then an aborted crew would not be more than ~700km away. That close, and a land-train of pressurized rovers would likely be able to meet them within a couple days of travel (or even 8-10 hours with a slightly prepared path, within EVA+contingency time). If the aborted crew aborted along with a rebreather system, they should be able to hang out in their spacesuits that long.

There would likely be only one orbital track for return from Earth, so you could possibly prepare a route before hand.
« Last Edit: 01/23/2016 07:20 PM by Robotbeat »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1417 on: 01/23/2016 07:45 PM »
I don't think their is that much of a rush with people having nothing but space suits on, I'm assuming the whole MCT/BFS aborted it's launch and simply landed down range, that means the personnel are in a fully functional habitat being carried inside the vehicle.  This might even be the 25 mt Earth-return habitat which would have sustained them for several months (in probably very Spartan conditions).  So life-support isn't as dire as Hollywood would like it to be. 

Now the rescue vehicle might have nothing in it but bucket seats that they strap themselves into while suited for the ~10 minute flight back to base but this is hardly much of a concern with an 8 hour EVA endurance and the personnel only need to leave the habitat on the aborted vehicle once the rescue vehicle has landed and gone through full diagnostics.

The main time pressure I see is getting the personnel back to base so they can be put into another vehicle for Earth return before the window closes so they are not stuck on Mars for another 2 years.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1418 on: 01/23/2016 07:51 PM »
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.
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Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1419 on: 01/23/2016 08:01 PM »
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.

So in this case your looking at a crash sight where getting to the survivors as fast as possible is imperative. Don't want to use rovers for that. Although a BFS might work, seems like a specially built rescue hopper would be more appropriate.

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