Author Topic: MCT Speculation and Discussion Thread 4  (Read 502823 times)

Offline JamesH

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Re: MCT Speculation and Discussion Thread 4
« Reply #760 on: 10/14/2015 08:00 PM »
Whatever gets discussed here, I'd put money on it being a lot more radical that people are currently thinking! SEP, chemical, something else, Battlestar Galactica, Death Star, Dark Star. Going to be fun whatever comes out!

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #761 on: 10/14/2015 08:07 PM »
The fact that SpaceX were looking into SEP could mean they were looking at an all-in-one vehicle with both SEP and chemical propulsion.

As was already mentioned in the thread, that would require the large solar arrays not only somehow be retracted into the MCT itself but also be deployed again after re-launch from Mars, and thus be so reliably shielded from re-entry (and issues on the surface like dust) that you can guarantee they will redeploy for the return trip to Earth. There's simply no advantage to justify that complexity.

MCT will remain a "land the entire thing" architecture, no matter how disagreeable you find the prospect of it.

The original land-the-entire-thing comment was made in the context of the typical multi-stage launch/landing architectures. It doesn't mean that Musk has ruled out having a SEP stage that remains in orbit.

And given that he's happy to do EOR refuelling, to avoid the restrictions of monolithic launches that are typical of Mars architectures, I can't see why anyone would think he'd arbitrarily rule out such an option just to satisfy one interpretation of a throw away quote.

We might find out in a month or so. Until then, these options are on the table... "No matter how disagreeable you find the prospect."

His original understanding based on the interview he gave was that they would try to eliminate all complexity by doing a mission without any orbital rendezvous: Take off from Earth, go directly to Mars (in 3-4 months), come directly back.

The delta V requirements for that are enormous, however you slice it.  It's not a workable plan: The math does not permit a savvy businessman to do this.  He backpedalled in his next interview by saying something about LEO refueling, which is the first of a number of interventions they're going to have to do to cut the maximum stage delta V requirement (ISRU was the zeroeth).  The requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage. are still 7.6km/s in that one stage.
« Last Edit: 10/14/2015 09:29 PM by Burninate »

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #762 on: 10/14/2015 08:21 PM »

His original understanding based on the interview he gave was that they would try to eliminate all complexity by doing a mission without any orbital rendezvous: Take off from Earth, go directly to Mars (in 3-4 months), come directly back.

I disagree with that interpretation. He never said they go without refuelling in orbit. He just did not mention it, probably thinking it is obvious. So he did not backpedal from such a statement. He just clarified that there would be refuelling.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #763 on: 10/14/2015 08:23 PM »
The requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage.

Are you sure you are adding up the right numbers? Because with the most efficient (not fastest) Hohmann transfer, i get ~9.5 km/s from LEO to the Martian surface, but that is with NO aerobraking at Mars. With aerobraking that should come down to less than 8km/s, right?

Yes, a lot of the ideas that Musk have thrown out for "MCT" seem almost impossible (or actually impossible) if one assumes they are all true, at once. Many of them I interpret as "nice to have" for future versions, and one of them I see that way is the "fast transit". I don't expect that to happen until a propellant depot is established in LMO.
« Last Edit: 10/14/2015 08:23 PM by Lars-J »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #764 on: 10/14/2015 09:27 PM »
The requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage. are still 7.6km/s in that one stage.

Are you sure you are adding up the right numbers? Because with the most efficient (not fastest) Hohmann transfer, i get ~9.5 km/s from LEO to the Martian surface, but that is with NO aerobraking at Mars. With aerobraking that should come down to less than 8km/s, right?

Yes, a lot of the ideas that Musk have thrown out for "MCT" seem almost impossible (or actually impossible) if one assumes they are all true, at once. Many of them I interpret as "nice to have" for future versions, and one of them I see that way is the "fast transit". I don't expect that to happen until a propellant depot is established in LMO.

I'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.

How is this possible?  Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said).  NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.

The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn.  For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). 

Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s.  So we get to have both fast transit and easy low speed EDL.

The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver).  In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.

The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed.  At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT).  The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.

IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.

BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.
Do you have math to back this mission plan up?

Edit: For the case of the highly simplified circular Mars / circular Earth orbit rendezvous, based on:
http://forum.nasaspaceflight.com/index.php?topic=37536.msg1371984#msg1371984

If you start at high Earth orbit and descend to perigee, then you can burn for 2241m/s to raise aphelion to 3.31AU, which drops time of flight to 100.03 days.  Then you come in to the Mars approach at 12880m/s, and have to burn off 9275m/s in order to capture into a highly elliptical orbit.  This is fairly difficult (capture intensity goes up faster than linearly because you have a shorter chord of atmosphere to cut through, not just less time in that atmosphere), and exceeds the capabilities of any chemical propulsion capacity in this planning exercise by a large measure.

  -snip-  (see original post)  -snip-

The tradeoff is: you can keep the BFR as a 4.5km/s vehicle instead of making it a (3.2km/s earth escape) + (2.4km/s Impaler Short Transit injection) + (~2km/s or whatever you need for EDL) ~= a 7.6km/s vehicle. 

You're also going to need comparable efforts to get the thing home on a fast transit, though.
Yeah, I made an error there (I added escape velocity twice).  7.6km/s to get to Mars from LEO in 100 days assuming that perfect aerobraking accepts all excess velocity for free.  But you go too far on the other point: Without aerobraking you'rs looking at 9.3km/s just for capture, making the total stage capability more like 16.9km/s.  If you assume a direct descent to a Vacuum Mars Analog, the figure is 12.9km/s plus gravity losses, making for a total stage capability of 20.5km/s.

These numbers are so high because 100 day transfers require lots of extra energy that 200 day transfers do not. 

If you're content with 200 day transfers...

-numbers to come, WIP-
« Last Edit: 10/14/2015 09:36 PM by Burninate »

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #765 on: 10/14/2015 09:57 PM »
My question is, why not send the crew up in traditional capsules to dock and load into the MCT.  If the MCT has to refuel in LEO.  The refueling station could be where the crew transfers in. No need to put launch escape system on MCT.

Yes. Not a literal "capsule" necessarily, but if you are building an escape vehicle into the nose... as the nose... of the MCT, you have already developed a suitably sized "LEO taxi" which can instead launch your passengers separately. By only using it separately, you eliminate the cost and complexity of trying to integrate it into the already complex MCT.

It also provides SpaceX with another second income stream, ferrying commercial passengers in and out of LEO (and perhaps to cis-Lunar activities) in addition to the Mars colonists, like an upsized version of commercial crew.

It also provides them with a stepping stone for development. I mean the gap between Dragon and MCT is... substantial.

My question is, why not send the crew up in traditional capsules to dock and load into the MCT.
Because that'd be way expensive cumulatively than launching the MCT itself, and cost is a factor.

Whereas the cost of trying to build a LAS capsule into the the MCT which also somehow functions as part of the MCT during normal landing...

Complexity doesn't just have increased development costs, it also inevitably increases ongoing operational costs.

Now, you may argue that this is me reading too deeply into quotes

Way too deeply. Especially as we are talking about a single throw away line from someone who we know will change his plans in response to reality, rather than chase the nominal "goal" at the expense of the original reason for pursuing that goal. (As often happens in spaceflight. See the Shuttle development, X-33, Ares, Orion, Altair, SLS, JWST...)

Musk dropped the idea of a reusable F9 second stage as soon as re-entry for stages proved more difficult than expected. He dropped or delayed cross-feed on FH when uprating Merlin proved more effective at improving payload. He's changed fuel. He's changed engine size for Raptor. He's dropped the idea of multi-core from BFR in response to the issues that must have arisen during FH development.

If they are doing trades on SEP, then clearly everything is on the table. As Lars-J said, Musk clearly has a wishlist of desirable features, and IMO he'll pursue whichever turn out to be viable, abandon whichever don't, and spin on a dime when required.

I think there's been a tendency for a group-mind to form around specific interpretations: A small group interprets Musk's comments as having one meaning, and suddenly that interpretation is the only one allowed and anyone who dares to talk about an alternative path is "hijacking the thread" to "pursue their own personal agenda". It's not exactly conducive to an open discussion.

MCT "Speculation", it's right there in the thread name.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #766 on: 10/14/2015 09:58 PM »

But nowhere did I find any indication that they are stepping away from a monolithic MCT and looking into having dedicated in-space elements, which means the original statement by Elon that their design goal for the MCT is "land the whole thing" remains a valid criterion in ascertaining whether or not a marsbound architecture can be considered an MCT architecture or not.

Now, you may argue that this is me reading too deeply into quotes and that any design posted in this thread is, as it were, cut from whole cloth. While that is all true (Especially the former, as I love reading too deeply into things), the purpose of this thread is to design a vehicle and accompanying architecture based entirely on criteria extracted from the fragments of information we are able to scrounge up.

I like Impaler's proposed architecture. However, I disagree on it being a viable MCT.

I'm perplexed, you clearly quote Musk saying SEP is under consideration, yet it is not a viable as a speculative prediction of their design?

I've never said Musk statements rule out super-direct all chemical architectures, I've said physics, engineering and cost and other merit based evaluations rule them out.  And that Musk will arrive at that conclusion after trying his hardest to get it to work on paper because it looks like the simplest solution on the surface.

The Musk design space is/was huge, he clearly wanted to leave no stone unturned in the search for an efficient architecture and that space included everything from SEP to single massive integrated 2nd stages like others are championing.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #767 on: 10/14/2015 10:12 PM »

It is 'grossly improportional' if it also cripples your architecture. And I still don't think you are understanding how capable your abort capsule would have to be to avoid black zones on Mars. And parachutes won't help much for something this size. (they don't scale up well, and Mars atmosphere is thin to begin with) Consider this... Delta-V to reach low Mars orbit is over 3 km/s. So if you have a problem when you are *almost* in orbit, you now have ~2(?) km/s of propulsive capability that this capsule needs to have in order to land softly - and you will land far over the horizon, on the other side of the planet. Any capsule capable of 2+ km/s that is large enough to hold 100(!) people is going to be massive. MASSIVE. Your simple capsule just won't cut it. 

Sometimes abort capability is impractical, and you mitigate risks other ways. You make your vehicle capable of abort, by having engine out capabilities and redundant systems. But you cannot remove all risks. Airliners still sometimes fall out of the sky, yet we don't have escape pods or parachutes.

But I'm tired of arguing these points over and over. For some, launch abort seems to be an essential that they just cannot comprehend going without.

While we may disagree on propulsion technology, I strongly agree with this sentiment on abort capsules for use during mars assent.  They are simply pointless because they needs to be very nearly as functional as the assent vehicle they are aborting from, they need to carry away the majority of the assent cargo mass 25 mT which we should be assuming is all passenger compartments, need very nearly the full DeltaV of a lander, the same or even greater thermal protection, it needs terminal thrust, landing gear radars etc etc.

The impossibility of any rescue on the martian surface is almost moot when you can't cross even separate and land this hypothetical abort vehicle.

The only place where abort makes any sense is on Earth where I favor launching the whole MCT off the top of the launch rocket via a pusher abort system comprised mainly of the MCT's own integral engines.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #768 on: 10/14/2015 10:38 PM »
The requirements for an MCT vehicle which does perfect massless aerocapture and refuels only in LEO and on the Martian surface with ~100d transfers are still over 10km/s in that one stage. are still 7.6km/s in that one stage.

Are you sure you are adding up the right numbers? Because with the most efficient (not fastest) Hohmann transfer, i get ~9.5 km/s from LEO to the Martian surface, but that is with NO aerobraking at Mars. With aerobraking that should come down to less than 8km/s, right?

Yes, a lot of the ideas that Musk have thrown out for "MCT" seem almost impossible (or actually impossible) if one assumes they are all true, at once. Many of them I interpret as "nice to have" for future versions, and one of them I see that way is the "fast transit". I don't expect that to happen until a propellant depot is established in LMO.

I'm strongly in favor of SEP and see considerable use for it making VERY FAST transit possible.

How is this possible?  Wouldn't the system need to be huge and have magical power sources and such (insert anything Zubrin has ever said).  NO, you can get a Fast transit on the order of 100 days to Mars with a slow, low power SEP system.

The trick is you use your SEP to move your Mars bound vehicle with propellants up to high Earth orbit and then then drop by the Earth for a huge Oberth assisted burn.  For 2 km/s you should leave Earth with huge escape velocity and reach mars in 100 days (average). 

Now the problem is capturing at mars, the answer is Magneto-Plasma Aerocapture, this lets us avoid expensive propulsive capture and is then followed by about a week of Plasma assisted aerobraking which lets the eventual EDL be from a gentle 4 km/s.  So we get to have both fast transit and easy low speed EDL.

The SEP system has not even left Earth yet in this scenario, so you can do either one of two things, bring it back down to LEO for refueling and do it again (basically making it a Cis-lunar tug), or send it to mars by the conventional slow method of spiraling out from the Earths SOI (the SEP is too delicate to take the high thrust of the Oberth maneuver).  In the latter case your going to arrive much later then the manned capsule but if this is a conjunction mission the crew will be spending around 600 days on mars so their is plenty of time for the SEP to arrive before it is needed for departure which is what I favor.

The MCT would only need to reach low Mars orbit and would then rendezvous with the SEP and head for Earth, this return transit is made reasonably short by the fact the MCT is a completely dry shell now of only 100 mT (75 vehicle mass + 25 return cargo) and the SEP is nearly dry too so power to weight ratios are increased, also were not aiming to match Earth's orbit and capture gently, were going to simply intersect it on an elliptical orbit around the sun, that cuts the DeltaV needed.  At Earth we used the Magneto to capture again and bring both SEP and the MCT down to LEO (they probably need to separate to do this as the SEP is more delicate and would slow the process down for the MCT).  The crew can be retrieved via a Dragon capsule now, and we need to send another tanker to LEO to put landing propellants into the MCT, if we use enough the MCT can do a lot of retro-propulsion on entry and bring it's entry speed down from the 7.7 km/s of orbit down to the range of 4 km/s which matches it's mars entry speed, so all the thermal protection systems can be designed for this low performance point.

IMLEO is estimated at 570 mT of which 100 mT is the cargo load, 75 mT is the MCT dry mass, 200 mT is chemical propellant in the MCT (2 tanker loads of 100 mT each), 155 is SEP propellant, 15 mT is the SEP tank and 22 mT is the SEP hardware which has a power output of 4.5 MW which corresponds to an alpha value of 5 kg/kw.

BTW Using a braking system like Magneto Plasma is the only way I can see an Integrated Bi-conic and direct Earth return being viable, without it the entry conditions are too extreme to meet the low dry mass fractions that it's advocates are proposing.
Do you have math to back this mission plan up?

Edit: For the case of the highly simplified circular Mars / circular Earth orbit rendezvous, based on:
http://forum.nasaspaceflight.com/index.php?topic=37536.msg1371984#msg1371984

If you start at high Earth orbit and descend to perigee, then you can burn for 2241m/s to raise aphelion to 3.31AU, which drops time of flight to 100.03 days.  Then you come in to the Mars approach at 12880m/s, and have to burn off 9275m/s in order to capture into a highly elliptical orbit.  This is fairly difficult (capture intensity goes up faster than linearly because you have a shorter chord of atmosphere to cut through, not just less time in that atmosphere), and exceeds the capabilities of any chemical propulsion capacity in this planning exercise by a large measure.

  -snip-  (see original post)  -snip-

The tradeoff is: you can keep the BFR as a 4.5km/s vehicle instead of making it a (3.2km/s earth escape) + (2.4km/s Impaler Short Transit injection) + (~2km/s or whatever you need for EDL) ~= a 7.6km/s vehicle. 

You're also going to need comparable efforts to get the thing home on a fast transit, though.
Yeah, I made an error there (I added escape velocity twice).  7.6km/s to get to Mars from LEO in 100 days assuming that perfect aerobraking accepts all excess velocity for free.  But you go too far on the other point: Without aerobraking you'rs looking at 9.3km/s just for capture, making the total stage capability more like 16.9km/s.  If you assume a direct descent to a Vacuum Mars Analog, the figure is 12.9km/s plus gravity losses, making for a total stage capability of 20.5km/s.

These numbers are so high because 100 day transfers require lots of extra energy that 200 day transfers do not. 

If you're content with 200 day transfers...

-numbers to come, WIP-

From Hop_David's spreadsheet, I offer five cases:

Straight Hohmann with propulsive capture - 259 days
A straight-up Hohmann burn from a {1AUx1AU heliocentric, 300km x 300km altitude geocentric} orbit to a {1.524AUx1.524AU heliocentric, 3697x23459km semimajor axis aereocentric} coplanar orbit using a {1AUx1.5240001AU heliocentric} transfer costs 510m/s plus change on Earth departure, over and above the 3220m/s escape velocity burn.  Then to propulsively capture into a highly elliptical Mars orbit it costs 1021m/s.  Conservative aerobraking can reduce that ellipse down to LMO for another 668m/s that we don't need to pay, where standard EDL penalties apply (I'll assume them to be 2km/s for purposes of this discussion).

Total: 3220 + 510 + 1021 + 2000 = 6751m/s
Wet to drymass ratio at 380s Isp: 6.12 to 1

The spreadsheet author's suggested non-Hohmann trajectory with propulsive capture - 102 days
The author would reduce perihelion in order to balance the perigee and periaerion burn into something sensible, assuming that propulsive capture is necessary.  He uses non-prograde burns or a suboptimal burn time in Earth orbit, which is highly inefficient, to balance out the Earth and Mars sides for minimal total dV.  From a {1AUx1AU heliocentric, 300km x 300km altitude geocentric} orbit to a {1.524AUx1.524AU heliocentric, 3697x23459km semimajor axis aereocentric} coplanar orbit using a {0.87AUx1.8AU heliocentric} transfer orbit costs 3146m/s plus change on Earth departure, over and above the 3220m/s escape velocity burn.  Then to propulsively capture into a highly elliptical Mars orbit it costs 4668m/s.  Conservative aerobraking can reduce that ellipse down to LMO for another 668m/s that we don't need to pay, where standard EDL penalties apply (I'll assume them to be 2km/s for purposes of this discussion).

Total: 3220 + 3146 + 4668 + 2000 = 13034m/s
Wet to drymass ratio at 380s Isp: 33.1 to 1

A 100 day semi-Hohmann transit given perfect free aerocapture - 100 days
From a {1AUx1AU heliocentric, 300km x 300km altitude geocentric} orbit to a {1.524AUx1.524AU heliocentric, 3697x23459km semimajor axis aereocentric} coplanar orbit using a {1AUx3.31AU heliocentric} transfer orbit costs 2241m/s plus change on Earth departure, over and above the 3220m/s escape velocity burn.  Then to propulsively capture into a highly elliptical Mars orbit it costs 9276m/s, but we're not going to do that: Instead, magnetoshell or some other aerocapture technology is going to do that all in one go;  Then it's going to, in the same step, further reduce the elliptical orbit down to LMO  for another 668m/s that we don't need to pay, then go directly into EDL, where standard EDL penalties apply (I'll assume them to be 2km/s for purposes of this discussion).

Total: 3220 + 2241 + 2000 = 7461m/s
Wet to drymass ratio at 380s Isp: 7.41 to 1

A 100 day semi-Hohmann transit given propulsive capture - 100 days
Examining the previous proposition without the non-prograde burns that the spreadsheet author makes.  From a {1AUx1AU heliocentric, 300km x 300km altitude geocentric} orbit to a {1.524AUx1.524AU heliocentric, 3697x23459km semimajor axis aereocentric} coplanar orbit using a {1AUx3.31AU heliocentric} transfer orbit costs 2241m/s plus change on Earth departure, over and above the 3220m/s escape velocity burn.  Then to propulsively capture into a highly elliptical Mars orbit it costs 9276m/s.    Conservative aerobraking can reduce that ellipse down to LMO for another 668m/s that we don't need to pay, where standard EDL penalties apply (I'll assume them to be 2km/s for purposes of this discussion).

Total: 3220 + 2241 + 9276 + 2000 = 16737m/s
Wet to drymass ratio at 380s Isp: 89.3 to 1

A reasonable 180 day near-Hohmann transfer with mild aerocapture - 180 days
From a {1AUx1AU heliocentric, 300km x 300km altitude geocentric} orbit to a {1.524AUx1.524AU heliocentric, 3697x23459km semimajor axis aereocentric} coplanar orbit using a {1AUx1.652AU heliocentric} transfer orbit costs 655m/s plus change on Earth departure, over and above the 3220m/s escape velocity burn.  Then to propulsively capture into a highly elliptical Mars orbit it costs 2443m/s, but we're not going to do that: Instead, magnetoshell or some other aerocapture technology is going to do that.  Conservative aerobraking can reduce that ellipse down to LMO for another 668m/s that we don't need to pay, where standard EDL penalties apply (I'll assume them to be 2km/s for purposes of this discussion).

Total: 3220 + 655 + 2000 = 5875m/s
Wet to drymass ratio at 380s Isp: 4.84 to 1
« Last Edit: 10/14/2015 10:53 PM by Burninate »

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #769 on: 10/14/2015 10:58 PM »
Burninate,
Thanks for those numbers. The wet to dry mass numbers are fascinating as well.

I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #770 on: 10/14/2015 11:04 PM »
Burninate,
Thanks for those numbers. The wet to dry mass numbers are fascinating as well.

I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.

Oh, I think there's still room for an LMO prop depot even in the most conservative case.  There's still a question of whether slow-transit, LEO depot, plus LMO depot is *enough* for a vehicle with integrated ISRU capacity on Mars to do a first mission to a new site reusably.  A lot of it falls on what the mass return ratio for the ISRU system ends up being.  ISRU gear mass is some fraction of landed mass, and as ISRU gets less and less productive that fraction goes up, perhaps even eclipsing useful cargo.  That uncertainty is part of why I end up with bigger MCT landed mass figures than most of you.  The ISRU productivity is extremely important because when it goes down

If it's not high enough, you either *have* to resort to a nested MAV in a 1&2 synod split mission (and return the big'un after 26 more months), plan on a 2-synod human mission, or you just have to throw away a whole MCT for every site you land at, as a permanently landed asset.

Also keep in mind that the above figures are based on the simplifying assumption of circular coplanar orbits for Mars and Earth.  The real numbers are probably a bit worse.
« Last Edit: 10/14/2015 11:18 PM by Burninate »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #771 on: 10/14/2015 11:14 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.
« Last Edit: 10/14/2015 11:15 PM by Burninate »

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #772 on: 10/14/2015 11:15 PM »
Burninate,
Thanks for those numbers. The wet to dry mass numbers are fascinating as well.

I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.

Oh, I think there's still room for an LMO prop depot even in the most conservative case.  There's still a question of whether slow-transit, LEO depot, plus LMO depot is *enough* for a vehicle with integrated ISRU capacity on Mars to do a first mission to a new site reusably.  A lot of it falls on what the mass return ratio for the ISRU system ends up being.

If it's not high enough, you either *have* to resort to a nested MAV (and return the big'un next synod), or you just have to throw away a whole MCT for every site you land at, as a permanently landed asset.

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. ;)

This is not flags and footprints that are planned here. The goal is to create a permanent manned outpost. But the first crews would not be launched until the next launch window after ISRU production is well underway.
« Last Edit: 10/14/2015 11:19 PM by Lars-J »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #773 on: 10/14/2015 11:21 PM »
Burninate,
Thanks for those numbers. The wet to dry mass numbers are fascinating as well.

I certainly favor the slower, more delta-V conservative approaches. And I think it is the best hope of making an "MCT" work, without a LMO propellant depot.

Oh, I think there's still room for an LMO prop depot even in the most conservative case.  There's still a question of whether slow-transit, LEO depot, plus LMO depot is *enough* for a vehicle with integrated ISRU capacity on Mars to do a first mission to a new site reusably.  A lot of it falls on what the mass return ratio for the ISRU system ends up being.

If it's not high enough, you either *have* to resort to a nested MAV (and return the big'un next synod), or you just have to throw away a whole MCT for every site you land at, as a permanently landed asset.

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. ;)

This is not flags and footprints that are planned here. The goal is to create a permanent manned outpost. But the first crews would not be launched until the next launch window after ISRU production is well underway.

The threshold you have to pass, then, is being able to set up and run an ISRU water-mining workflow without any human assistance, perhaps without any realtime telerobotics, if we don't go for a landing.

You also need it to last a decade.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #774 on: 10/14/2015 11:29 PM »
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. ;)

This is not flags and footprints that are planned here. The goal is to create a permanent manned outpost. But the first crews would not be launched until the next launch window after ISRU production is well underway.

The threshold you have to pass, then, is being able to set up and run an ISRU water-mining workflow without any human assistance, perhaps without any realtime telerobotics, if we don't go for a landing.

Yes, certainly. How else would you return crews? They would be stuck until they got it going. The MCT model, as I interpret it, is dependent on being able to "fill up" on Mars surface. Otherwise you have to add another ~4 km/s capability to the lander, so it can reach MLO again. 

Sending unmanned MCTs with ISRU in advance also allows you to reduce risk by testing/verifying the most risky part of the mission. (EDL)

You also need it to last a decade.

Yes. But that is less of a problem, as I would expect a permanent population after that, so they can maintain it and replace it. Every synod/launch window would bring additional crew, supplies, and spare parts.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #775 on: 10/14/2015 11:31 PM »
Burninate, I still see a potential for SEP instead of the magneto hydrodynamic breaking in what you describe.

Imagine going with say a 120 day transfer to a point where Mars would have been about 20 days before the craft gets there, but for the full length of the 150 day flight SEP is running at 0.4 mm/s2 to alter the orbit so that it actually arrives at Mars later, a little further along on Mars orbit, but at almost the same speed for a relatively low energy capture (say around 1km/s for capture).

and yes that is just roughed out, I don't have Andy Weir's continuous thrust orbit model software. So really it is a matter of optimizing for the mass of the SEP system including solar arrays against how much faster than a standard half ellipse Hohmann you want to fly.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #776 on: 10/15/2015 12:09 AM »
Burninate, I still see a potential for SEP instead of the magneto hydrodynamic breaking in what you describe.

Imagine going with say a 120 day transfer to a point where Mars would have been about 20 days before the craft gets there, but for the full length of the 150 day flight SEP is running at 0.4 mm/s2 to alter the orbit so that it actually arrives at Mars later, a little further along on Mars orbit, but at almost the same speed for a relatively low energy capture (say around 1km/s for capture).

and yes that is just roughed out, I don't have Andy Weir's continuous thrust orbit model software. So really it is a matter of optimizing for the mass of the SEP system including solar arrays against how much faster than a standard half ellipse Hohmann you want to fly.

This is my preference as well.  Absent a good characterization of MAC, let's keep its use at a manageably small level.  We have a good characterization of SEP, on the other hand, and SEP can be used for much of capture.  The simplified case that's most conservative on MAC would be to go with the 180-day version I calculated above, but plan on burning for about 2 months while approaching aphelion.  There are other benefits to using modest amounts of SEP;  It makes things like Mars-orbit habs and propellant much easier to deal with.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #777 on: 10/15/2015 01:06 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.

Offline meekGee

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Re: MCT Speculation and Discussion Thread 4
« Reply #778 on: 10/15/2015 01:26 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.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #779 on: 10/15/2015 01:37 AM »

This is my preference as well.  Absent a good characterization of MAC, let's keep its use at a manageably small level.  We have a good characterization of SEP, on the other hand, and SEP can be used for much of capture.  The simplified case that's most conservative on MAC would be to go with the 180-day version I calculated above, but plan on burning for about 2 months while approaching aphelion.  There are other benefits to using modest amounts of SEP;  It makes things like Mars-orbit habs and propellant much easier to deal with.

From what I have read their is really no practical upper limit to how large of a effective drag radius that can be generated and the total drag force generated on the vehicle.  The limits are all how much g-force the vehicle and crew can tolerate.  As the ratio of ring diameter to drag radius looks to be nearly 20:1, aka papers describing 2.5 meter diameter magneto rings massing under a ton would generate an effective drag radius of 40 meter diameter.

Given the bi-conic configurations that are popular it may be possible to put a ring nearly as large as the vehicle tucked under the outer edge of vehicles bottom frustum.  If dragged by a number of cables along it's perimeter a ring this size would generate an enormous drag area.  By adjusting the tension on these cables in flight the ring could be tilted relative to the main craft, this may allow the bi-conic to generate more lift and further control the process as my current understanding is that the lack of lift generation from the Magneto will cap the amount of deceleration for a given g-limit.  With high lift you can 'skim' the planets atmosphere in a lift-down configuration and do a long low g arc through the atmosphere.

So to be conservative I'm limiting the Capture DeltaV to around 3 km/s.  That would mean having a velocity at mars periapsis of <8 km/s initially to bring one down to just under escape at 5 km/s or less.

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