Author Topic: MCT Speculation and Discussion Thread 3  (Read 249730 times)


Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 3
« Reply #1 on: 02/16/2015 12:21 PM »
Quote from:  RonM
What kind of trajectory can give a 100 day flight time to Mars? Can that be done with a single chemical rocket burn or would it be with a chemical rocket boost and SEP constant thrust?

I'd say that's a pretty pertinent question, although from the last thread I don't think RonM got an answer from anybody. I'm rather desperate to know the same, anybody got an idea?
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Offline jsgirald

Re: MCT Speculation and Discussion Thread 3
« Reply #2 on: 02/16/2015 01:47 PM »
Quote from:  RonM
What kind of trajectory can give a 100 day flight time to Mars? Can that be done with a single chemical rocket burn or would it be with a chemical rocket boost and SEP constant thrust?

I'd say that's a pretty pertinent question, although from the last thread I don't think RonM got an answer from anybody. I'm rather desperate to know the same, anybody got an idea?

Have a look at Nasa's Trajectory Browser.
http://trajbrowser.arc.nasa.gov/

I was surprised to find that there are 128 days trajectories for just a bit over 6 km/s delta v.
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Offline symbios

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Re: MCT Speculation and Discussion Thread 3
« Reply #3 on: 02/16/2015 02:16 PM »
Quote from:  RonM
What kind of trajectory can give a 100 day flight time to Mars? Can that be done with a single chemical rocket burn or would it be with a chemical rocket boost and SEP constant thrust?

I'd say that's a pretty pertinent question, although from the last thread I don't think RonM got an answer from anybody. I'm rather desperate to know the same, anybody got an idea?

Have a look at Nasa's Trajectory Browser.
http://trajbrowser.arc.nasa.gov/

I was surprised to find that there are 128 days trajectories for just a bit over 6 km/s delta v.

But 1,6 km/s of that is post Mars injection. This part will be greatly affected by what kind of post injection we are talking about.

2018 you can get a 80 day trajectory for a delta-v of 9,96 km/s but then 4,7 km/s of is post injection delta.

Will post injection delta-v be higher or lower for the planed landing profile then calculated in this tool?
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 3
« Reply #4 on: 02/16/2015 04:12 PM »
Right. You can certainly do near 100 day trajectories all-chemical, ESPECIALLY if you refuel in a high orbit (high elliptical Earth orbit, EML1/2, or distant lunar retrograde) before your final departure burn (on flyby of Earth to get the biggest Oberth boost).

But there's evidence that SpaceX is now considering SEP as part of their MCT architecture (based on comments by Shotwell), although you don't get the impression that they've decided on it, yet.

...But I definitely don't think we should assume that EVERY opportunity will have a 100 day trajectory. In some years, the delta-v is just too high to be worth doing such a short trip. Also, if you extend the time by just a couple weeks (110-120 days), the delta-v requirements decrease dramatically, depending on the transit. So 100 days should be treated as more of a guideline than an absolute rule.
« Last Edit: 02/16/2015 04:15 PM by Robotbeat »
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Offline JamesH

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Re: MCT Speculation and Discussion Thread 3
« Reply #5 on: 02/16/2015 04:43 PM »
From this video, MCT will be about 100x the size of an SUV.

https://www.youtube.com/watch?v=hwMIAKabRng&feature=youtu.be





Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 3
« Reply #6 on: 02/16/2015 05:33 PM »
From this video, MCT will be about 100x the size of an SUV.

We don't know whether this refers to internal volume of a SUV or external volume, if internal about 7m^3, if external maybe 15m^3 perhaps up to 22m^3.

We also don't know if it refers to total volume, pressurized volume or livable volume of the MCT.

700m^3 would be pretty small for the total MCT volume, while 2200m^3 would be very generous for the livable volume. Any of the other combinations seem possible to me, but if pressed I would go for 1500m^3 for the pressurized volume.

Offline SleeperService

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Re: MCT Speculation and Discussion Thread 3
« Reply #7 on: 02/16/2015 05:41 PM »
We also don't know if it is an African or European SUV...  ;)

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 3
« Reply #8 on: 02/16/2015 05:46 PM »
But there's evidence that SpaceX is now considering SEP as part of their MCT architecture (based on comments by Shotwell), although you don't get the impression that they've decided on it, yet.

The exact quote is in answer to the question "and to power something for 8 months, and then presumably to turn around at some point, to get these people back home, you must be looking at some sort of renewable fuel?"

Quote
So we are looking at solar-electric propulsion, I think we are go to look at some other interesting propulsion (in-space propulsion) technologies, but our lift of both from the surface of Mars as well as Earth will probably be liquid oxygen and methane

I take this to mean that SpaceX are investigating various technologies, but they are not baselined for MCT at present.

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 3
« Reply #9 on: 02/16/2015 05:51 PM »
Quote
So we are looking at solar-electric propulsion, I think we are go to look at some other interesting propulsion (in-space propulsion) technologies, but our lift of both from the surface of Mars as well as Earth will probably be liquid oxygen and methane

I'm more interested in what these might be.

Offline SleeperService

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Re: MCT Speculation and Discussion Thread 3
« Reply #10 on: 02/16/2015 05:54 PM »
Quote
So we are looking at solar-electric propulsion, I think we are go to look at some other interesting propulsion (in-space propulsion) technologies, but our lift of both from the surface of Mars as well as Earth will probably be liquid oxygen and methane

I'm more interested in what these might be.

http://forum.nasaspaceflight.com/index.php?topic=36313.0

Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 3
« Reply #11 on: 02/16/2015 06:11 PM »
http://forum.nasaspaceflight.com/index.php?topic=36313.0

Not likely to be an EM drive in my opinion - I am skeptical that they work, even if they do work the reported thrust levels are too low (by several orders of magnitude) to be useful for a Mars transit.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 3
« Reply #12 on: 02/16/2015 06:13 PM »
http://forum.nasaspaceflight.com/index.php?topic=36313.0

Not likely to be an EM drive in my opinion - I am skeptical that they work, even if they do work the reported thrust levels are too low (by several orders of magnitude) to be useful for a Mars transit.

It's got to be at a minimum SEP level thrust or higher, otherwise it doesn't seem like it would be worth it.

Offline SleeperService

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Re: MCT Speculation and Discussion Thread 3
« Reply #13 on: 02/16/2015 06:22 PM »
Agreed. However the Chinese researchers and Shawyer have shown much greater thrust/KW...
The jury is out at the mo, but I think it is this line of work that comes under the 'interesting propulsion' description.

Offline CuddlyRocket

Re: MCT Speculation and Discussion Thread 3
« Reply #14 on: 02/16/2015 07:15 PM »
Quote
So we are looking at solar-electric propulsion, I think we are go to look at some other interesting propulsion (in-space propulsion) technologies, but our lift of both from the surface of Mars as well as Earth will probably be liquid oxygen and methane

I'm more interested in what these might be.

Well, there's nuclear; but I'd be astonished if they were contemplating that! The only other ones I can think of are solar thermal and light sails, but neither seems to me to offer an advantage over chemical in this mission scenario. But then again, they're better engineers than I am! :)

Online RotoSequence

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Re: MCT Speculation and Discussion Thread 3
« Reply #15 on: 02/16/2015 09:18 PM »
Agreed. However the Chinese researchers and Shawyer have shown much greater thrust/KW...
The jury is out at the mo, but I think it is this line of work that comes under the 'interesting propulsion' description.

For now, we should dismiss anything more exotic than solar electric propulsion technologies, which have already been demonstrated and proven in space flight.

I don't think SpaceX is going to design anything around technologies that have not already been thoroughly researched. When you want to keep costs low, unproven technology is a liability. Technology that doesn't actually exist is unthinkable.
« Last Edit: 02/16/2015 09:21 PM by RotoSequence »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 3
« Reply #16 on: 02/16/2015 10:39 PM »
For a decent heliocentric transfer from EML1 using SEP you need about 1 mm/s acceleration, or 1 N per mt of vehicle mass.  That will get you to Mars in around 6 months and you will be at low velocity relative to Mars so you don't need to do any high-speed entry or airo-capture rather you can just spiral down which will take another month.  HALL thrusters and thin-film solar are adequate for this.

Faster transits require rapidly rising acceleration rates because your both going on a sharper more eccentric orbit around the sun and you have less total time in which to both accumulate and dissipate your speed.  If your willing to just accelerate and use Mars atmosphere for all your braking then you can reduce your needed acceleration for a given transit time as well as reducing your total propellent needs, but you get back the challenge of airo-capture maneuver which are still considered too dangerous for crew due to the highly variable nature of the Martian atmosphere.

I foresee several different ways SEP might be employed.  From minimal to maximum they are...

Limp-back-thrusters:  MCT has a DeltaV capability of ~4.5 km/s sized primarily for Mars Assent/Earth Escape and transits to Mars fully chemically, with or without assistance depending on desired speed.  On Mars it refuels and returns to LMO, at this point it deploys small solar arrays and returns to Earth via a very slow transfer using it's residual Methane/LOX or some Argon collected as a waste byproduct during the production of the latter propellents, any of these would yield high ISP and require very low propellent fractions.  Obviously this is applicable only for a cargo lander or a crew lander that's not taking anyone back to Earth. 

Primary Advantages:  large reduction in the demanded volume of Propellent production on Mars surface and reduction in chemical Delta-V capability needed on the MCT vehicle.  May allow early unmanned cargo vehicles that would otherwise be stranded on Mars to return and be reused.  Single vehicle design, works with any volume.

Primary Disadvantages: Electric thrusters capable of using low atomic mass propellents for long times not well established, can't return crew.  SEP system integration into lander inherently wasteful unless solar array is also used on surface which may make it heavy.


Cis-Lunar Tug:  MCT stages in LEO is refueled and then pushed to EML1 by a dedicated tug which immediately returns to LEO and never leaves cis-lunar space, tug is supplied with conventional Xenon/Krypton propellents launched from Earth for each pushing operation.  The MCT fleet is accumulated at EML1 until the transfer window opens at which point they all depart by chemical burns likely using lunar and Earth swing-by to maximize efficiency.  Landing on Mars and return to Earth are all chemical from their on out with Mars providing the propellents for Earth Return.  This will require crew to rendezvous with MCTs at EML1 via fast taxi craft such as Dragon, but any fleet-marshaling strategy requires the same rendezvous, this just puts the rendezvous at higher orbit which likely turns F9 launch into F9H launch.

Primary Advantages:  Significantly improves fraction of launch mass sent through TMI.  Alternatively faster transit may be had with the same TMI fraction.  Cargo containers outside of landers could be transported and only loaded into landers when in LMO to further increase efficiency of mass delivery.

Primary Disadvantages:  Two vehicle system, though SEP tug should be less development cost then lander.  Dock and rendezvous with autonomous vehicle necessary.  Requires a moderate volume and marshaling of vehicles to amortize Tug over.  Crew rendezvous slightly harder.  Dose not significantly effect MCT Delta-V capability need because it is driven by concurrent Mars-Assent and Earth-return needs.


Space-Freighter:  Combination of the above strategies in which SEP is used for ALL propulsion in-space and chemical thrusts in used only for Planetary assents/descents.  The marshaling of landers and cargo thus needs to be done in LEO with the spiral out to high orbit beginning months ahead of the Mars transfer window opening.  SEP vehicle would need to be larger then the tug concept if they pair 1:1 with landers, but could be massively larger if it carries multiple landers and cargo containers.  The landers descend with cargo and return with residual propellent multiple times which is transferred to the SEP vehicle and used to return to Earth keeping propellent tanks and mass to a minimum.  Both loading of cargo at Earth and unloading at Mars can be long-term operations taking the full time period between departure windows.

Primary Advantages:  Excellent mass transfer efficiency, on the order of 75% launch mass may be cargo containers once all vehicles are in place with 25% remainder being propellents.  Allows for large voluminous habitats to be attached to SEP vehicle which allow Landers to be relived of long-term habitation duty and function only as brief taxis with air-liner like densities.  Makes very efficient use of landers by using them to deposit multiple loads on Mars surface per synod, while putting lowest Delta-V needs and lowest habitation needs on lander, but at cost of higher re-usage between servicing needs.

Primary Disadvantages:   Requires large to huge volume to amortize over, including large and continual propellent production on Mars.  Requires on-orbit assembly of the largest concept SEP vehicle.  SEP vehicles of this size are beyond most current concepts and will require many MW of power pushing solar arrays well beyond current tech or even beyond near term goals of NASA.  Requires significant operations in LMO, loading containers into landers and transferring of propellent, likely autonomously.  Crew/Passengers need to rendezvous with vehicle in high Earth orbit as in tug scenario because of Van-Allen belt radiation.  Thrusters running off low atomic mass elements need more development.



« Last Edit: 02/16/2015 10:48 PM by Impaler »

Offline JamesH

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Re: MCT Speculation and Discussion Thread 3
« Reply #17 on: 02/17/2015 10:10 AM »
Agreed. However the Chinese researchers and Shawyer have shown much greater thrust/KW...
The jury is out at the mo, but I think it is this line of work that comes under the 'interesting propulsion' description.

For now, we should dismiss anything more exotic than solar electric propulsion technologies, which have already been demonstrated and proven in space flight.

I don't think SpaceX is going to design anything around technologies that have not already been thoroughly researched. When you want to keep costs low, unproven technology is a liability. Technology that doesn't actually exist is unthinkable.

Although EM does like unusual technologies, and isn't afraid to try stuff out (the forthcoming Hyperloop test track for example), so I wonder if they are looking in to some of the newer techs for future use.

Offline Cinder

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Re: MCT Speculation and Discussion Thread 3
« Reply #18 on: 02/17/2015 11:08 AM »
Yes, but especially for a group as keen to progress as quickly as possible as SpaceX, what concrete plans could they actually make if the technology in question is such an unknown?  We already know that as far as MCT goes, SpaceX is far ahead of the TRL of EM Drive.  You can only delay planning such a major part of a Mars Architecture as the MCT for the sake of such an unknown, for so long.
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Offline Ben the Space Brit

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Re: MCT Speculation and Discussion Thread 3
« Reply #19 on: 02/17/2015 11:43 AM »
This is me but I have this curious mental image of the first MCT landing beside the Mars One encampment and Elon Musk personally asking the camp commander's permission to disembark onto their planet.
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