Author Topic: MCT Speculation and Discussion Thread 4  (Read 606218 times)

Offline DaveH62

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Re: MCT Speculation and Discussion Thread 4
« Reply #2800 on: 09/20/2016 05:32 AM »
Wouldn't it make more sense to refuel in lunar orbit and use earth gravity for a kick on your way out?

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2801 on: 09/20/2016 06:42 AM »
How about this?

Fully fuel MCT and a tanker in LEO. Launch them in tandem to a highly elliptic orbit, chosing the orbit so that the tanker has enough fuel left to fully fuel the MCT again. The tanker returns to earth on that orbit. The MCT does its earth departure burn at perigee with max. efficiency. How much delta-v would that gain over starting in LEO? Close to 3km/s?

It is operationally less complex than a depot. A depot may become more efficient when many flights go beyond Mars.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2802 on: 09/20/2016 07:18 AM »
It looks like MCT (ITS, BFS) can reach  433 Eros

and here are the one way best trajectories to Vesta, Ceres, Hygiea and Juno which maybe possible for it with a reduced payload.

Offline hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #2803 on: 09/20/2016 07:36 AM »
Wouldn't it make more sense to refuel in lunar orbit

Both HEO(at about GEO height) and lunar orbit are problematic, it would take about  4 km/s from LEO to get to either. If the craft starts from suborbital 2-3km/s velocity after staging(cannot stage much later if first stage is recovered, or needs enourmous first stage), this means about 9-10km/s total delta-v. This is just way too much, payload fraction gets very bad. You would need to refuel at both LEO and lunar orbit if you want to refuel at HEO or lunar orbit.

From delta-v point of view some highly elliptical earth orbit would make much more sense, but then there may be the problem of crossing the van allen belts multiple times.


Offline hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #2804 on: 09/20/2016 07:47 AM »
How about this?

Fully fuel MCT and a tanker in LEO. Launch them in tandem to a highly elliptic orbit, chosing the orbit so that the tanker has enough fuel left to fully fuel the MCT again. The tanker returns to earth on that orbit. The MCT does its earth departure burn at perigee with max. efficiency. How much delta-v would that gain over starting in LEO? Close to 3km/s?

It is operationally less complex than a depot. A depot may become more efficient when many flights go beyond Mars.

This seems like it could work and 3km/s is in the correct ballpark.

Though, the question is that could a single tanker refuel the MCT/ITS after it has already spent 3km/s impulse for acccelerating itself(and the fuel), or would multiple tankers be needed/would the tankers need to be much bigger than the MCT/ITS?

When the tanker is returning to LEO it does not need much fuel because it can do aerobraking. Or it could return directly to earth instead of returning to LEO.
« Last Edit: 09/20/2016 07:50 AM by hkultala »

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2805 on: 09/20/2016 08:40 AM »

Though, the question is that could a single tanker refuel the MCT/ITS after it has already spent 3km/s impulse for acccelerating itself(and the fuel), or would multiple tankers be needed/would the tankers need to be much bigger than the MCT/ITS?

When the tanker is returning to LEO it does not need much fuel because it can do aerobraking. Or it could return directly to earth instead of returning to LEO.

Without calculation it seems to me that one tanker could be enough or nearly enough to give the max advantage. The tanker is much lighter than MCT, at least by the 100t payload, probably a lot more. With a delta-v of much more than 12km/s it should have more than half its propellant load left to transfer after spending 3km/s delta-v.

My assumption, the tanker is purely the propulsion unit of an MCT and should have a dry weight at ~40t 60t, compared to  160t or more of MCT including payload.
« Last Edit: 09/20/2016 08:43 AM by guckyfan »

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2806 on: 09/20/2016 09:33 AM »
Without calculation it seems to me that one tanker could be enough or nearly enough to give the max advantage.

A simulation and analysis of this concept has been in L2 for some time.


Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2807 on: 09/20/2016 04:17 PM »
It's OK to post an L2 link here as non-members simply can't access it
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2808 on: 09/20/2016 05:19 PM »
It's OK to post an L2 link here as non-members simply can't access it

The header of the quote is the link.

Offline rakaydos

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Re: MCT Speculation and Discussion Thread 4
« Reply #2809 on: 09/20/2016 05:28 PM »
Would a BFR first stage be able to single-stage-to-orbit without the BFS?

I'm wondering how hard it would be to get a BFR into orbit, fuel it, send it to Titan, propulsively land, and use ISRU to use it repeatedly as a Titan launch vehical.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2810 on: 09/20/2016 05:44 PM »
BFR is never going beyond suborbital Earth trajectories.

If you want a vehicle to launch of other solar system bodies, why not use BFS, which will be designed to do exactly that?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2811 on: 09/20/2016 05:54 PM »
Would a BFR first stage be able to single-stage-to-orbit without the BFS?

I'm wondering how hard it would be to get a BFR into orbit, fuel it, send it to Titan, propulsively land, and use ISRU to use it repeatedly as a Titan launch vehical.
Without any payload? Quite possibly.

Your idea is impractical (BFR is not designed itself for interplanetary travel) but still very interesting. Should be capable of putting hundreds of tons into Titan orbit IF (big if!!!) fueled up on the surface.
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Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #2812 on: 09/20/2016 07:13 PM »
BFR as a system comprises the booster, the pad and the "refurb street".  Having only the booster landed on Titan gives you nothing, usability wise.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2813 on: 09/21/2016 05:31 PM »
Would a BFR first stage be able to single-stage-to-orbit without the BFS?

I'm wondering how hard it would be to get a BFR into orbit, fuel it, send it to Titan, propulsively land, and use ISRU to use it repeatedly as a Titan launch vehical.
Without any payload? Quite possibly.

Your idea is impractical (BFR is not designed itself for interplanetary travel) but still very interesting. Should be capable of putting hundreds of tons into Titan orbit IF (big if!!!) fueled up on the surface.
That should be thousands of tons into Titan orbit, not just hundreds. Could also land thousands of tons.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2814 on: 09/21/2016 05:39 PM »
BFR as a system comprises the booster, the pad and the "refurb street".  Having only the booster landed on Titan gives you nothing, usability wise.
Well, sure. Kind of is assumed you're landing a bunch of ISRU equipment first.

Anyway, not sure it's worth it this century. Titan is VERY far away. Ceres, on the other hand, is very near. And due to the very low gravity, a rocket like BFR or BFS should be able to launch ~5 its own mass in payload to orbit. It'd look weird doing that. Heck, you could make a space elevator really easily on Ceres, and it'd even be worth it, too, since it wouldn't have to be very long (so you could make like a round-trip every day on it) and the cable would be able to lift like a dozen times its own mass, and it should be able to sling stuff back to Earth fairly easily.

But anyway, yeah, BFS would be interesting for all kinds of destinations, not just Mars.
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Offline CuddlyRocket

Re: MCT Speculation and Discussion Thread 4
« Reply #2815 on: 09/21/2016 07:22 PM »
Heck, you could make a space elevator really easily on Ceres...

I make the Clarke orbit for Ceres to be ~1,200 km from the centre, or about 740 km above the surface. So, a lot easier to build a space elevator for Ceres than Earth!

Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #2816 on: 09/21/2016 07:50 PM »
It occurs to me to wonder if the whole MCT concept actually comprises two segments that are effectively considered one craft.

     The top section, the actual lander, would be based upon an enlarged version of the Dragon 2 space craft, while the transfer vehicle, (Essentially, the habitat section and Mars transfer section and fuel) are designed in much the same manner as the old Pilgrim Observer concept.

     On launch and landing, all the passengers and crew would be strapped into seats in the lander, but upon orbit, the Lander would detach, turn about and dock with the actual transfer craft.  They would fully fuel the transfer craft in orbit, as well as the lander.  Once fueling is complete, and after the initial TMI burn, the three arms would be extended, and crew and passengers would transfer to the habitats.  Rotation would then be started kept to approximately that of Mars.

     As the craft approached Mars, the spin would be discontinued, and arms retracted.  Either some form of aerobraking or direct thrust deceleration would be used to achieve orbit.  The crew and passengers would then transfer to the lander and descend to Mars, leaving the Mars Transfer Vehicle in orbit.

     As the lander would be largely empty of passengers for the return flight, much of the room, both passenger and cargo space, would be given over to fuel tankage to refuel the MTV and lander for the Earth return trip.

     Point of note, SpaceX has repeatedly stated that what they have in mind for the MCT was a Mars Colonial Transport SYSTEM not simply a stage and lander.

      I could be off base about this, but it does seem to make sense, and would provide a substantial infrastructure for continuous use as a colonizing system.
« Last Edit: 09/21/2016 07:58 PM by JasonAW3 »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2817 on: 09/21/2016 09:35 PM »
Reaching main belt asteroids via chemical propulsion is a no go.  It would take around 5 km/s burn at LEO to start the transfer and ANOTHER 5 km/s to land.  That mean SSTO propellant fractions.  Plus the transit time is a full year and their is no atmosphere to make propellant from for return.

Reaching Titan is actually easier then that, it would be 7.3 km/s burn at LEO but you can use friction to do all the deceleration at Titan.  Taking off again on Titan would be a nightmare because of the atmospheric thickness, per unit of surface area a column of Titans atmosphere is 7.3 times more massive then Earths and has huge scale height so the launch vehicle must spend a lot of time at low speed plowing through this atmosphere which adds to gravity losses too, finally the density of the atmosphere means rocket engines would yield less then sea-level ISP due to under-expansion in nozzles.

So don't expect to get a vehicle back from Titan, the transit times alone means their is little point anyway as the vehicle can't get amortized over enough missions to make reuse attractive.
« Last Edit: 09/21/2016 10:56 PM by Impaler »

Offline rakaydos

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Re: MCT Speculation and Discussion Thread 4
« Reply #2818 on: 09/21/2016 11:46 PM »
Reaching Titan is actually easier then that, it would be 7.3 km/s burn at LEO but you can use friction to do all the deceleration at Titan.  Taking off again on Titan would be a nightmare because of the atmospheric thickness, per unit of surface area a column of Titans atmosphere is 7.3 times more massive then Earths and has huge scale height so the launch vehicle must spend a lot of time at low speed plowing through this atmosphere which adds to gravity losses too, finally the density of the atmosphere means rocket engines would yield less then sea-level ISP due to under-expansion in nozzles.

So don't expect to get a vehicle back from Titan, the transit times alone means their is little point anyway as the vehicle can't get amortized over enough missions to make reuse attractive.
That's actually why I proposed the reusable Titan BFR-varient idea- because even if it requires some performace-penalizing modifications to make the trip, the core problems of lift through a thick atmosphere are solved for Earth, and many of the same solutions would be equally viable for Titan.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2819 on: 09/21/2016 11:52 PM »
Point of note, SpaceX has repeatedly stated that what they have in mind for the MCT was a Mars Colonial Transport SYSTEM not simply a stage and lander.

However, Musk has explicitly described the system as BFR + BFS. With BFR being a giant booster, and BFS being the upper-stage cum Mars transport cum Mars lander cum Earth return vehicle.

He may have changed his mind, and we'll hopefully find out soon, but there's nothing announced yet by SpaceX that implies he has.

Heck, you could make a space elevator really easily on Ceres...
I make the Clarke orbit for Ceres to be ~1,200 km from the centre, or about 740 km above the surface. So, a lot easier to build a space elevator for Ceres than Earth!

A ground-based, horizontally rotating, launch-sling tower would be even easier. (But that may be somewhat off-topic for an MCT thread.)

But since I'm going off-topic...

Taking off again on Titan would be a nightmare because of the atmospheric thickness

Titan seems like the only place where supersonic/hypersonic air-breathing hybrid SSTO makes sense. You only need small wings/lifting-body-effect to get off a runway (or lose very little from VTOL), making an optimised supersonic (or even hypersonic) airframe more compatible with the slow sub-sonic flight regime, reducing the need to trade.

I doubt there's enough hydrocarbon in the atmosphere to actually fuel the turbines/ramjets (via on-board LOx), I'm just assuming the atmosphere serves as bulk inert mass for a turbofan powered by a stored-fuel/LOx jet engine. Hell, even electric ducted-fans for subsonic flight (and landing) might be useful to get to and from high altitudes.

(Titan might also be the only place other novel/barely-workable/downright-silly ideas make sense. Like balloon launch. Air-launch. VTHL flyback boosters. Etc.)

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