Author Topic: MCT Speculation and Discussion Thread 4  (Read 621442 times)

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #780 on: 10/15/2015 02:12 AM »

I'll also add that in addition to the trip duration, there's the "land the whole thing" mentality.   If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.

If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.

But again, this is about MCT.

We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle.  And that such a vehicle trades very well against a single massive direct vehicle.
It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!

RB.  You just used the word "in fact" along with stating you can build an engine with a T/W of 0.25, and an ISP in the thousands.

Forget Mars...  With your engine you can roam the solar system at will!   

You need a sanity check sometimes.   You can't just add brochure numbers for this and that (solar panels, thruster) and arrive at meaningful numbers.

...Um, what the heck are you talking about? You don't land using the electric thrusters, you land with chemical rockets. The only time you wouldn't use chemical rockets is on very low-gravity bodies. I said "land on SEP on Mars," not "land using electric thrusters."

But it is possible to fold in a solar array and protect it during reentry. X-37B does this, and I believe the original crewed Dragon concept did this (folded into the nose). It's awkward and heavy (less so with extremely high performance solar arrays), but it is possible. (And if MCT is supposed to have a big payload bay for cargo, then they could fold the solar array into that with plenty of room to spare.) If there's a problem retracting it, just blow some explosive bolts and separate it before reentry. If there's problem deploying it, you can either take a slower route or you can dock with another MCT. Either way, it's something that can be engineered around, and is an easier problem to solve than, say, their first stage recovery and reuse for Falcon 9.

I doubt this is what SpaceX is planning, so if they are considering SEP, they are most likely considering it as a separate craft which does not land along with the MCT. Either it is a propulsion stage that stays attached to the MCT to throw it to Mars (and then back to Earth) or perhaps it's to be used to shuttle propellant around. The latter would be a fairly easy "add-on" that need not be there for initial trips.
« Last Edit: 10/15/2015 02:31 AM by Robotbeat »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #781 on: 10/15/2015 02:17 AM »
Would anyone like to do the work of reversing that spreadsheet so we could look at more refined estimates of return delta V?  I might try it at some point, but not today.

Orbits are time reversible so don't we just need to look at the 'Vinf at mars' and calculate the DeltaV needed to archive that escape velocity from mars surface, then we would (if we pointed ourselves in the right direction) be headed back down the equivalent half of the outbound orbit and we should reach Earth in the specified time and with the specified Vinf so we know what we need to do to aerocapture at Earth as well.

It would be nice to have this done for us on the spreadsheet though.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #782 on: 10/15/2015 02:25 AM »
There is an example of a reusable spacecraft that contains an unfolding solar array that can be stowed for thruster firings and reentry:

https://www.newscientist.com/blogs/shortsharpscience/2011/03/solar-stowaways-give-us-spacep.html

In fact, it's in orbit right now, and it's the fourth flight (two flights each for two vehicles).

Here is the patent:
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=6,581,883.PN.&OS=PN/6,581,883&RS=PN/6,581,883
« Last Edit: 10/15/2015 02:35 AM by Robotbeat »
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #783 on: 10/15/2015 02:45 AM »
Stowable solar arrays of around that size will certainly need to be part of MCT even it is an all chemical vehicle (you don't survived months in transit on batteries). 

My own design for solar arrays on a biconic is to have body flaps along the bottom of the vehicle (for aerodynamic control and braking), these can hinge out to almost perpendicular in orbit and the solar arrays which are stowed in boxes just behind the flaps and extend out on scissor type extensions much like ISS radiators past the flaps to point at the sun which is over the nose of the vehicle.  Giwires an retract the arrays and on the surface of mars they can extend again with the wires taking the load and the angle just a bit below horizontal so they touch the surface and rest on a boot which protects them when folded by being a cap.

But the difference between this and the scale needed to power meaningful SEP is huge.  Were talking about MW solar arrays here and while we have the technology to do that none of the current solar arrays of the size needed to do the job can fold up and making them do so would very likely cripple their energy density.
« Last Edit: 10/15/2015 05:44 AM by Impaler »

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #784 on: 10/15/2015 04:03 AM »

That has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots.

Hmmm, in theory, only the first site or two would need that. Once you have the initial 3-6 MCTs left at those 1-2 sites, they could be used as suborbital hoppers for ferrying equipment (even people) to secondary sites to prep them for the incoming (fully reusable) MCTs.

Yes, good point.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #785 on: 10/15/2015 05:37 AM »
If these MCT are in good enough shape to do suborbital hops they are probably in good enough shape to go back to earth. That's assuming the heatshield is not the limiting factor.

I expect two scenarios:

1) The new site is close enough that equipment can be sent on the ground to prepare for MCT landing.

2) They do it with new MCT. Hopefully there will be enough experience that they have to expend only one or maybe none.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #786 on: 10/15/2015 07:12 PM »
[images of Apollo CM+SM and Dragon+trunk]

Apollo didn't and Dragon doesn't do EDL with those modules attached. The level of integration your require is an order of magnitude more complex.


That is correct, but note that Dragon 2 will abort with it's trunk still attached (which presumably would have at least some cargo in it.).  If the joint can survive being aborted off an exploding booster in tension, then there's no reason it couldn't propulsively land that way too, as the tensile loads would be much less for landing than abort.

So you have a joint there that can handle compression, and rapid tension as well.  It is 2/3 of the way to a lifeboat on a biconic MCT.  Designing that joint to additionally handle lateral loading during atmospheric entry would be an engineering hurdle to overcome, certainly.

It's mainly a question of if you think SpaceX could design that in or not, as to if this would be feasible or not.


Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #787 on: 10/15/2015 07:43 PM »
Just on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight,

Don't make the mistake of thinking that all an abort capsule needs to do is taking passengers away from an imminent explosion, to just survive the imminent danger. That works on Earth, because rescue is nearby, and the environment is survivable.

But that is NOT enough for Mars. An abort capsule is USELESS if it just dooms you to death a couple of hours later. To be a proper abort/rescue vehicle it basically needs to be a fully independent spacecraft/lander, built into another spacecraft/lander. By adding enough rescue/survival capability, you get into a dark spiral that eventually leads you to conclude that your escape capsule needs another escape capsule.

But there is another way. Build safety margins and redundancy into your MCT/lander and forego an abort system that is effectively useless.


Lars,

You make some good points here.

My initial thought on the IBMCT is just that, to not have an LAS at all.  To use Dragon 2 as a LEO taxi for exploration crews for the first couple of decades until MCT builds up a track record of reliability.  Then go with the Space Shuttle or Airliner model when it's time to put a full 100 passengers on board.  MCT should have dozens of launches under it's belt by then.
It'd have full booster engine out capability, as well as full IBMCT engine out capability.  So an engine out during ascent on either stage would still result in a nominal LEO insertion.
It doesn't protect against one failure mode, an exploding booster.  It's popular to look at the booster explosions on the N-1 tests and take that to mean you must protect against that, but what really needs to be looked at is how often to American liquid boosters actually explode once they are out of test phase and in to actual production?  I don't know the answer to that exactly, but I don't think it's very often.  Especially in modern times.  The Challenger flight is one of the most famous examples, but that really wasn't a liquid booster failure.  It was an o-ring joint failure from a segmented SRB than then caused a failure of the liquid booster.  Had the O-ring failed on the outboard side so it didn't burn into the side of the ET, the stack likely wouldn't have exploded.  Once detected they probably would have done an emergency orbiter abort and glided to a contingency landing site.  Or the booster may have lasted to SRB staging.

There was a Delta II that failed spectacularly not so long ago, but I think that was an SRB failure as well, not of the liquid core.  I don't think there was a booster failure of the a production Saturn 1B or Saturn V, or Atlas II, III, or V, or Delta IV, III, or II outside of the SRB failure.
And I think most of the Titan IV failures were SRB issues, and not the core (which wasn't really a booster, but a 2nd stage that ignited after SRB sep.)

So, how often have liquid boosters went "boom" in the US?
How important is it to protect against that one failure mode?

Those are the questiona to ask and debate.

As you say Lars, aborting on Mars has different issues than on Earth.  If you are dooming the crew to a slow death marooned on the surface, that means an LAS system is -really- only good for Earth ascent...and won't really be a benefit for the 99% of the mission after Earth ascent.  And is arguably a lot of extra cost and complexity and performance hit for just protection in that first even of a 2+ year long mission.

Later, once there is a colony, an LAS system would be more plausible as there's a plausible chance of rescue coming from the colony.   Until then, it can be "problematic".

Now, that said, there are still LAS lifeboat scenarios on  Mars even early on.
Exploration crews of 6-7 could have sufficient hab volume, life support, and provisions on the lifeboat to survive for some time marooned on the surface.  For at least the projected cruise back of say 6 months.  They'll need that amount of provisions to get home, so they can life on the surface in that same hab with those same provisions for the same amount of time.
But...they will die after that if there's no rescue.  Can a rescue come from Earth in time?
Can there be another unmanned MCT on the suface somewhere accessible by the crew?  Can there be a pressurized rover that can drive autonomously from the landing location they just took off from that could drive to their emergency landing location, that could pick them up and take them to an unmanned backup MCT?
Can there be a surface hab with a cache of provisions the rover could take them too which could sustain them until a rescue can come from Earth?

So it's not automatically a death sentence to Abort a lifeboat on Mars Ascent.  But contingencies would NEED to be in place before it's not a death sentence.


Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #788 on: 10/15/2015 08:05 PM »
[Dragon's trunk] It is 2/3 of the way to a lifeboat on a biconic MCT.

It really, really isn't.

Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #789 on: 10/15/2015 08:12 PM »
So, how often have liquid boosters went "boom" in the US?
How important is it to protect against that one failure mode?

Antares, October 28, 2014.

Falcon 9, June 28, 2015.

That's two in the past 12 months.

At least launching from Earth, it's pretty important. As you mentioned, for early MCT missions with a small crew, send the crew up on a Dragon 2.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #790 on: 10/15/2015 10:37 PM »
Would anyone like to do the work of reversing that spreadsheet so we could look at more refined estimates of return delta V?  I might try it at some point, but not today.

Orbits are time reversible so don't we just need to look at the 'Vinf at mars' and calculate the DeltaV needed to archive that escape velocity from mars surface, then we would (if we pointed ourselves in the right direction) be headed back down the equivalent half of the outbound orbit and we should reach Earth in the specified time and with the specified Vinf so we know what we need to do to aerocapture at Earth as well.

It would be nice to have this done for us on the spreadsheet though.
They're reversible, but you're forgetting the Oberth effect: because on the way from Earth to Mars, you can dump your exhaust in a deeper gravity well than Mars to Earth, it takes less delta-v.
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Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #791 on: 10/15/2015 10:43 PM »
So, how often have liquid boosters went "boom" in the US?
How important is it to protect against that one failure mode?

Antares, October 28, 2014.

Falcon 9, June 28, 2015.

That's two in the past 12 months.

At least launching from Earth, it's pretty important. As you mentioned, for early MCT missions with a small crew, send the crew up on a Dragon 2.

Sort of.  I was referring to exploding boosters.
As I understand, F9's was the 2nd stage which blew up causing the booster to then fail, which is different than the booster itself exploding, and trying to abort away form it.  Which is usually what your LAS is trying to do.  If that were an Integrated MCT, then the LAS lifeboat would be within a few meters of the exploding tank.  Could an LAS could save it with such close proximity...especially if it exploded...rather than more "ruptured" as it appeared the F9US did.  (Which was one of the problems with putting Orion on a side mounted SDHLV if I recall correctly.  Orion would be right next to the ET, so even with an LAS system, it's unlikely the crew could get away from a sudden ET explosion). Maybe...
Again, I said that would be the question that would have to be asked and answered.   If answered "no", then that IBMCT with no LAS is a fairly slick option overall for simplicity and economics.  If answered "yes"...then either a lifeboat top to the IBMCT, or go with a non integrated biconic MCT launching unfueled on top of a dedicated 2nd stage, so that the whole vehicle aborts and you don't have to break apart the spacecraft.  Or a separate LEO Taxi with LAS to take large crews up to LEO for transfer to MCT.  (All are plausible solutions IMO, if an LAS is deemed absolutely necessary for Earth ascent.)

The F9 v1.1 booster failure from a few years ago was an engine out, which as I said, would be accounted for.

Antares was also an engine out.  But with just 2 engines, there was no engine out redundancy so they terminated the LV, as I understand.  Had Antares had engine out redundancy, there's not reason to think it wouldn't have made staging nominally.

But, yes, for that first several dozen flights of MCT, the crew can just go up on a simple F9/D2.  There will be 4 active Falcon pads in operation after one, 3 on the East Coast.  One of those launched per Mars mission seems pretty reasonable.
By the time you'd be looking at putting 100 people on it, there'll be many exploration missions over decades.  You'll probably have a pretty good idea of that point if it's reasonable to put people on MCT for launch, or go another route like a "Big Dragon" on a FH or something that can hold 100 people, and have a Earth LAS system.  Or some other route.
« Last Edit: 10/15/2015 11:03 PM by Lobo »

Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #792 on: 10/15/2015 11:03 PM »
So, how often have liquid boosters went "boom" in the US?
How important is it to protect against that one failure mode?

Antares, October 28, 2014.

Falcon 9, June 28, 2015.

That's two in the past 12 months.

At least launching from Earth, it's pretty important. As you mentioned, for early MCT missions with a small crew, send the crew up on a Dragon 2.

Not quite.  I was referring to exploding boosters.
As I understand, F9's was the 2nd stage which blew up causing the booster to then fail, which is different than the booster itself exploding, and trying to abort away form it.   If that were an Integrated MCT, then the LAS lifeboat would be within a few meters of the exploding tank.  Could an LAS could save it with such close proximity? (Which was one of the problems with putting Orion on a side mounted SDHLV if I recall correctly.  Orion would be right next to the ET, so even with an LAS system, it's unlikely the crew could get away from a sudden ET explosion).
The F9 v1.1 booster failure from a few years ago was an engine out, which as I said, would be accounted for.

Antares was also an engine out.  But with just 2 engines, there was no engine out redundancy so they terminated the LV, as I understand.  Had Antares had engine out redundancy, there's not reason to think it wouldn't have made staging nominally.

It doesn't matter whether it is an engine out or a tank rupture or first stage or second stage. In both cases, if there was a manned capsule, a LAS would have saved the crew. Without a LAS the crew would have been lost.

You can't design the perfect launcher to survive any possible contingency. No one imagined a F9 second stage coming apart while the first stage was still burning.

But, yes, for that first several dozen flights of MCT, the crew can just go up on a simple F9/D2.  There will be 4 active Falcon pads in operation after one, 3 on the East Coast.  One of those launched per Mars mission seems pretty reasonable.
By the time you'd be looking at putting 100 people on it, there'll be many exploration missions over decades.  You'll probably have a pretty good idea of that point if it's reasonable to put people on MCT for launch, or go another route like a "Big Dragon" on a FH or something that can hold 100 people, and have a Earth LAS system.  Or some other route.

That's reasonable. As I've posted before, I expect insurance and liability concerns will require a LAS of some sort on all Earth to orbit launches.

Offline sheltonjr

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Re: MCT Speculation and Discussion Thread 4
« Reply #793 on: 10/15/2015 11:12 PM »
So, how often have liquid boosters went "boom" in the US?
How important is it to protect against that one failure mode?

Antares, October 28, 2014.

Falcon 9, June 28, 2015.

That's two in the past 12 months.

At least launching from Earth, it's pretty important. As you mentioned, for early MCT missions with a small crew, send the crew up on a Dragon 2.

Falcon 9 turned out to be a mechanical failure.

Antares does not have, engine out capability,  so no real advantage to shutting down,

With advanced current and near future engine monitoring, a engine can be shut down before a RUD can occur.

I really like the choice configuration that they chose for Raptor. Great performance, but can still have a lower pressure pump and chamber for long life and reliability and no worry about a fuel/air interseal in the turbopump.

Offline Rocket Surgeon

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Re: MCT Speculation and Discussion Thread 4
« Reply #794 on: 10/16/2015 01:46 AM »
Just on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight,

Don't make the mistake of thinking that all an abort capsule needs to do is taking passengers away from an imminent explosion, to just survive the imminent danger. That works on Earth, because rescue is nearby, and the environment is survivable.

But that is NOT enough for Mars. An abort capsule is USELESS if it just dooms you to death a couple of hours later. To be a proper abort/rescue vehicle it basically needs to be a fully independent spacecraft/lander, built into another spacecraft/lander. By adding enough rescue/survival capability, you get into a dark spiral that eventually leads you to conclude that your escape capsule needs another escape capsule.

But there is another way. Build safety margins and redundancy into your MCT/lander and forego an abort system that is effectively useless.


Lars,

You make some good points here.

My initial thought on the IBMCT is just that, to not have an LAS at all.  To use Dragon 2 as a LEO taxi for exploration crews for the first couple of decades until MCT builds up a track record of reliability.  Then go with the Space Shuttle or Airliner model when it's time to put a full 100 passengers on board.  MCT should have dozens of launches under it's belt by then.
It'd have full booster engine out capability, as well as full IBMCT engine out capability.  So an engine out during ascent on either stage would still result in a nominal LEO insertion.
It doesn't protect against one failure mode, an exploding booster.  It's popular to look at the booster explosions on the N-1 tests and take that to mean you must protect against that, but what really needs to be looked at is how often to American liquid boosters actually explode once they are out of test phase and in to actual production?  I don't know the answer to that exactly, but I don't think it's very often.  Especially in modern times.  The Challenger flight is one of the most famous examples, but that really wasn't a liquid booster failure.  It was an o-ring joint failure from a segmented SRB than then caused a failure of the liquid booster.  Had the O-ring failed on the outboard side so it didn't burn into the side of the ET, the stack likely wouldn't have exploded.  Once detected they probably would have done an emergency orbiter abort and glided to a contingency landing site.  Or the booster may have lasted to SRB staging.

There was a Delta II that failed spectacularly not so long ago, but I think that was an SRB failure as well, not of the liquid core.  I don't think there was a booster failure of the a production Saturn 1B or Saturn V, or Atlas II, III, or V, or Delta IV, III, or II outside of the SRB failure.
And I think most of the Titan IV failures were SRB issues, and not the core (which wasn't really a booster, but a 2nd stage that ignited after SRB sep.)

So, how often have liquid boosters went "boom" in the US?
How important is it to protect against that one failure mode?

There is a potentially good compromise here. If there is still a lack of confidence in the BFR/MCT system to LEO, then could you launch the MCT empty, then man it and refuel it at the same time? If the MCT is being refueled in LEO, then you could make the tankers hybrid tankers/LEO transports, ferrying up, say XX people at a time and that could have an abort capability. It would take more flights, but if you're lowing the mass that is need to get to LEO but upping the flights, it may make things more operationally attractive (no need for an ASDS).
I'll admit I'm being convinced that a LAS is not necessary, but I'll point out that a Plane doesn't drop out of the sky if it loses all it's engines, and doesn't carry the liquid chemical equivalent of a nuclear bomb. The potential energy stored there cannot be ignored.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #795 on: 10/16/2015 01:54 AM »
Would anyone like to do the work of reversing that spreadsheet so we could look at more refined estimates of return delta V?  I might try it at some point, but not today.

Orbits are time reversible so don't we just need to look at the 'Vinf at mars' and calculate the DeltaV needed to archive that escape velocity from mars surface, then we would (if we pointed ourselves in the right direction) be headed back down the equivalent half of the outbound orbit and we should reach Earth in the specified time and with the specified Vinf so we know what we need to do to aerocapture at Earth as well.

It would be nice to have this done for us on the spreadsheet though.
They're reversible, but you're forgetting the Oberth effect: because on the way from Earth to Mars, you can dump your exhaust in a deeper gravity well than Mars to Earth, it takes less delta-v.

I was just referring to the heliocentric portion of the transit being reversible, we would obviously need to take mars's smaller gravity well into account, that's why I said the Vinf at mars is needs to be looked at.  Here let me take some of Burnate's example and reverse them into mars-Earth transits and show how I would do it.

Straight Hohmann with propulsive capture - 259 days
Vinf at mars is 2.652818827 km/s that is our velocity at this point in the heliocentric orbit before we enter the mars gravity well, naturally we accelerate when falling into that gravity well but we would just come out again with the same speed aka were hyperbolic.  To get back to Earth we need that same heliocentric speed (pointed back in now) when leaving mars to return to Earth in 259 days.  To determine this we just take the escape velocity at mars surface 5 km/s, square it along with the needed Vinf add them together and get the square root which is 5.660 km/s.

Likewise we can use the previously 'escape' burn from Earth to know how much deceleration we need to shed to go into that high orbit at Earth after which we will aerobrake down to LEO, I'll assume that we can be refueled by tanker at this point in order to do our Earth landing as that will avoid having to blast off from mars with propellant we wont need until now in LEO where we know their is a continuous refueling process in place.

Earth return reversal
Total: 5660 + 510 = 6170m/s
Wet to drymass ratio at 380s Isp:  5.24 to 1


The spreadsheet author's suggested non-Hohmann trajectory with propulsive capture - 102 days

Earth return reversal
Total 9241 + 3145 = 12386 m/s
Wet to drymass ratio at 380s Isp:  27.82 to 1


A 100 day semi-Hohmann transit given perfect free aerocapture - 100 days

Earth return reversal
Total 13816 m/s
Wet to drymass ratio at 380s Isp:  40.85 to 1

This can be improved over simply reversing the outbound orbit which was {1AUx3.31AU heliocentric}, I've found  {.76AUx1.67AU heliocentric} with the same travel time which can decrease the mars escape burn substantially but at the cost of raising Earth side capture needs.

Earth return reversal
Total 8792 m/s
Wet to drymass ratio at 380s Isp: 10.6  to 1


A 100 day semi-Hohmann transit given propulsive capture - 100 days

Earth return reversal
Total 13816 + 2241 = 16057 m/s
Wet to drymass ratio at 380s Isp:  74.57 to 1

Using my alternative trajectory
Total 8792 + 4597 m/s = 13389
Wet to drymass ratio at 380s Isp: 36.42  to 1


A reasonable 180 day near-Hohmann transfer with mild aerocapture - 180 days

Earth return reversal
Total 7047 m/s
Wet to drymass ratio at 380s Isp: 6.63 to 1


Conclusions:  There seems to be no viable fast (100 day) returns from mars, unlike on the outbound even with perfect free aerocapture doesn't allow it at any kind of propellant fraction and DeltaV which would be believable.  The problem is that were starting from mars surface unlike LEO in the the outbound assumptions, that's putting us around 3600 m/s behind where we would be.  If we had propellant depots in mars orbit fast return would be a reasonable number if we also had free aerocapture as the wet/dry ratio would be 4:1, which would require something around 300 mT of propellant in mars orbit a huge amount.
« Last Edit: 10/16/2015 02:15 AM by Impaler »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #796 on: 10/16/2015 02:38 AM »
Great work, but could you please break out your math a bit more rather than listing totals?  For example, what dV do you assume for Mars Ascent leg?
« Last Edit: 10/16/2015 03:12 AM by Burninate »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #797 on: 10/16/2015 02:42 AM »
Also: 300 tons of propellant in Mars orbit is not a very high amount, given that you can use SEP all the way and the gear ratio is excellent on that;  If you want a short transit you have to send more launches worth of propellant to deep elliptical earth orbit for the outbound leg than to LMO for the inbound leg (using your SEP propellant tug idea), which in turn is far fewer than if you avoided DEO rendezvous altogether and stuck to LEO rendezvous only.
« Last Edit: 10/16/2015 02:45 AM by Burninate »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #798 on: 10/16/2015 03:31 AM »
Impaler: I don't think Musk mentioned 100 day trajectory on the return trip, just on the way there. Also, exactly 100 is a little bit of, um, spurious precision. 102 days is essentially the same thing. (I know you know this, just want to point it out.)

Not sure SpaceX would start at LEO. A high orbit seems more realistic, as it allows you to leverage SEP without actually including SEP on MCT directly. SEP would be just used to haul up propellant from LEO. This helps reduce IMLEO a LOT.

But yeah, I still think MCT will start on the surface of Mars and go straight to Earth, with a much-longer-than-100-day trajectory.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #799 on: 10/16/2015 04:14 AM »
Impaler: I don't think Musk mentioned 100 day trajectory on the return trip, just on the way there. Also, exactly 100 is a little bit of, um, spurious precision. 102 days is essentially the same thing. (I know you know this, just want to point it out.)

Not sure SpaceX would start at LEO. A high orbit seems more realistic, as it allows you to leverage SEP without actually including SEP on MCT directly. SEP would be just used to haul up propellant from LEO. This helps reduce IMLEO a LOT.

But yeah, I still think MCT will start on the surface of Mars and go straight to Earth, with a much-longer-than-100-day trajectory.
The 102 day transfer above is very different from the 100 day transfer.  I included it because it's the author's default values on the spreadsheet;  The decision is made to go with a perihelion below 1AU in an attempt to balance propulsive capture with propulsive transfer injection, unlike in the 100 day transfer.

-

I don't see the point of selling the benefits of a 100 day transfer so hard unless you can have it in both directions. 

You say that a six month transfer to Mars is intolerable for passengers, so you guys are proposing that we go to these great lengths to avoid it.  And the identical six month transfer back from Mars is...  more tolerable?
« Last Edit: 10/16/2015 04:19 AM by Burninate »

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