Author Topic: MCT Speculation and Discussion Thread 4  (Read 626141 times)

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #720 on: 10/12/2015 02:35 PM »
SEP tugs themselves could be refueled in LEO. 

A refueling assembly station at L1.  This would be a fuel depot and cargo storage facility. 

SEP tugs would refuel and pick up cargo and fuel and transfer this to L1 on a continuous basis. 

MCT would launch, refuel at a LEO station, go to L1, refuel and load cargo containers, then go to Mars.

Or

Another set of SEP tugs would take cargo to Mars orbit.

A reusable lander would be stationed at Mars.  RL for short

RL's would make their fuel on Mars and bring up from Mars Argon of the SEP tugs.

RL's would refuel the SEP tugs and load cargo container and land this container on Mars. 

RL would refuel itself and refuel argon tank for re-launch later.  Cargo could be as little as 20 ton units. 

So you would have a continuous running of SEP tugs between L1 and Mars.  A continuous running of SEP tugs between LEO and L1.  And a continuous running of small reusable landers taking smaller cargo loads between Mars orbit and Mars and back. 

All of this can be done with existing rockets.  This would keep everyone happy, including foreign suppliers who want to get in on the Mars colonization.

MCT might not have to be as large if refueling is done in LEO, L1 and at Mars.  With SEP just carrying fuel to L1 from LEO, then they might not even be necessary between L1 and Mars, or build a huge SEP tug to carry 100 tons of cargo to Mars with enough argon to fly from L1 to Mars and Back. 

SEP allows for lots of flexibility. 

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #721 on: 10/12/2015 07:14 PM »
Using FH to launch SEP tugs.  At the very least, they could install a Martian communication satellite system around Mars and the moon.  We need communication before we go to Mars.  Also when Mars is on the far side of the sun away from Earth, some type of system would need to be in place to communicate around the sun.  Also, if we ever get an L2 station, communication on the back side of the moon will be necessary.  SEP tugs could put all these satellites and backup satellites in place.  GPS systems for the moon and Mars are eventually going to be needed also.  So in the near future, Falcon 9, and Falcon heavy will be busy installing not only Wi-Fi systems around earth, but systems around the moon and Mars.  LEO MCT refueling stations could be built with FH or they could wait for BFR.  50 ton SEP tugs can probably carry a lot of satellites or cargo payloads before BFR and MCT are built. 

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #722 on: 10/12/2015 09:29 PM »
SEP tugs themselves could be refueled in LEO. 

A refueling assembly station at L1.  This would be a fuel depot and cargo storage facility. 

SEP tugs would refuel and pick up cargo and fuel and transfer this to L1 on a continuous basis. 

MCT would launch, refuel at a LEO station, go to L1, refuel and load cargo containers, then go to Mars.

I like the picture you paint. It does have complexity but regularity to it. Including the low-speed cargo runs to Mars.

But when thinking SpaceX I try to judge by scalability and price. A few years down the track, if this is the plan, how many launches does it take to send 20 MCTs to Mars and back? Does it scale down significantly in price as the launch numbers increase to this and beyond?

I'm trying to envision this by that standard and I feel like there are too many launches and rendezvous. How do you feel it scales?

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #723 on: 10/12/2015 10:00 PM »
Yep. SEP is a non-starter for a Mars transit. At a minimum it needs to be augmented by significant chemical propulsion for most of the effective delta-V. So why bother?

And if you treat the ship as essentially a cycler (in high orbits on both ends), then you are dealing with the same problem that cycler-advocates keep forgetting... In order to reach your SEP "battlestar galactica", you have to use chemical propulsion to accelerate all the crew, supplies, cargo, AND lander propellant to that high (almost escape) orbit for rendezvous. This last element is what SEP and cycler advocates seem to always forget about.
I think you're misunderstanding the concept, which seems sound to me, other than the notion that a technology as immature as MAC could be in the critical path of the plan without SpaceX having already purchased MSNW.  I think you guys are talking past each other because you may not understand the concept.

I understand why the crew will require chemical propulsion to reach high orbit rendezvous, but what about everything else?  Why can't they use SEP tugs to climb in and out of the gravity well?

What's your alternative that you are implying exists?

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #724 on: 10/12/2015 10:34 PM »
Yep. SEP is a non-starter for a Mars transit. At a minimum it needs to be augmented by significant chemical propulsion for most of the effective delta-V. So why bother?

And if you treat the ship as essentially a cycler (in high orbits on both ends), then you are dealing with the same problem that cycler-advocates keep forgetting... In order to reach your SEP "battlestar galactica", you have to use chemical propulsion to accelerate all the crew, supplies, cargo, AND lander propellant to that high (almost escape) orbit for rendezvous. This last element is what SEP and cycler advocates seem to always forget about.
I think you're misunderstanding the concept, which seems sound to me, other than the notion that a technology as immature as MAC could be in the critical path of the plan without SpaceX having already purchased MSNW.  I think you guys are talking past each other because you may not understand the concept.

I understand why the crew will require chemical propulsion to reach high orbit rendezvous, but what about everything else?  Why can't they use SEP tugs to climb in and out of the gravity well?

What's your alternative that you are implying exists?

Why are you quoting a days old post? Some of my misunderstandings have been corrected, but not my conviction that this scheme solves some problems but introduces/ignores others. (complexity, cost, unproven tech.)
« Last Edit: 10/12/2015 10:34 PM by Lars-J »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #725 on: 10/12/2015 11:36 PM »
You seem to be taking a Zubrinist position, that no new technology is needed and simple 'brute force' can achieve all of SpaceX's mars goals.  Cost was no object to Zubrin he didn't bat an eyelash at 100 Billion to perform a single mission to mars which would have been flags and footprints.  The price points Musk wants to achieve are so radically low in comparison that they are in almost another universe.  And we know that Zubrin was dead wrong about even being able to archive his Direct missions without new technology or at his estimated costs. 

So it stands to reason that we need a LOT of new tech to do what Musk wants.  With any transportation system the higher the upfront development cost the lower the per trip cost.  The trick is to leave plenty of margin and look at technologies that offer big payoffs.


That's quite fascinating... Because from my point of view, "DeltaV-only thinking" seems to be what *you* are doing, making you turn to SEP, interesting trajectories, and Magneto Plasma Aerocapture. Again, pursuing performance at any cost is not what SpaceX is known for doing. They prefer a more brute force but cost effective approach.

No, you might accuse a SEP advocate of being ISP focused or propellant fraction focused but it is not a focus on ONLY DeltaV.  I'm looking to save propulsive DeltaV where possible but I am more then willing to incur greater DeltaV if it lowers thermal and g-force problems.  DeltaV only thinking is when people just look at the longest 'pole' of DeltaV in the mission, declare the vehicle has that capability and is X many tons of propellant at Y propellant fraction and think they are done without even examining other constraints.

I'm looking at all constraints, even trying to see what kind of limits Magneto Plasma Aerocapture has as at some point the necessary braking force is too high in g-forces for the vehicle to survive no matter how large of an effective diameter the plasma has.

Offline meekGee

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Re: MCT Speculation and Discussion Thread 4
« Reply #726 on: 10/13/2015 01:00 AM »
Yep. SEP is a non-starter for a Mars transit. At a minimum it needs to be augmented by significant chemical propulsion for most of the effective delta-V. So why bother?...
"Non-starter"?? That clearly doesn't mean what you think it means. SEP is baselined for Mars transit in NASA's current Mars exploration plan, and SpaceX has mentioned it's being traded for MCT.

That makes SEP pretty much the opposite of "non-starter."

I don't know if "non-starter" is the right term, but the numbers don't look very good for SEP on a 100 day trip.

Why did NASA baseline it?  I didn't read said plan (I try to stick to non-fiction) but NASA makes a lot technological decisions that are not ideal.

Why is NASA still sticking with H2 and solids?  Why parachutes? 

NASA is a) not a singular body but rather made up of many "selfish" bodies, b) is influence by many irrelevant and political factors, and ) sometimes makes honest to goodness mistakes.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #727 on: 10/13/2015 02:07 AM »
Yep. SEP is a non-starter for a Mars transit. At a minimum it needs to be augmented by significant chemical propulsion for most of the effective delta-V. So why bother?...
"Non-starter"?? That clearly doesn't mean what you think it means. SEP is baselined for Mars transit in NASA's current Mars exploration plan, and SpaceX has mentioned it's being traded for MCT.

That makes SEP pretty much the opposite of "non-starter."

I don't know if "non-starter" is the right term, but the numbers don't look very good for SEP on a 100 day trip.

Why did NASA baseline it?  I didn't read said plan (I try to stick to non-fiction) but NASA makes a lot technological decisions that are not ideal.

Why is NASA still sticking with H2 and solids?  Why parachutes? 

NASA is a) not a singular body but rather made up of many "selfish" bodies, b) is influence by many irrelevant and political factors, and ) sometimes makes honest to goodness mistakes.

NASA isn't looking at 100 day transits much. SpaceX is. And SEP does reduce IMLEO dramatically for a typical 6-7 month trajectory that NASA likes to look at. And NASA is planning surface rendezvous, so most flights to Mars will carry cargo (which doesn't benefit much from fast transit).

SEP actually trades quite well, and technology improvements allow Isp improvements, not being limited in Isp like chemical and NTR by needing to keep your combustion chamber from melting.

SEP allows you to reduce transit time (versus an equivalent-IMLEO chemical-only system) if you have good enough solar array and thruster technology, but its chief benefit is usually a dramatic reduction in IMLEO. And SpaceX could leverage that just for hauling propellant from LEO to HEO if they so desired. Remember, SpaceX's LEO Constellation will use SEP tech, so SpaceX will be quite capable technically of building SEP tugs (and will be producing Megawatts' worth of SEP arrays/thrusters anyway just for the constellation).
« Last Edit: 10/13/2015 02:31 AM by Robotbeat »
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Offline meekGee

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Re: MCT Speculation and Discussion Thread 4
« Reply #728 on: 10/13/2015 05:07 AM »
Yep. SEP is a non-starter for a Mars transit. At a minimum it needs to be augmented by significant chemical propulsion for most of the effective delta-V. So why bother?...
"Non-starter"?? That clearly doesn't mean what you think it means. SEP is baselined for Mars transit in NASA's current Mars exploration plan, and SpaceX has mentioned it's being traded for MCT.

That makes SEP pretty much the opposite of "non-starter."

I don't know if "non-starter" is the right term, but the numbers don't look very good for SEP on a 100 day trip.

Why did NASA baseline it?  I didn't read said plan (I try to stick to non-fiction) but NASA makes a lot technological decisions that are not ideal.

Why is NASA still sticking with H2 and solids?  Why parachutes? 

NASA is a) not a singular body but rather made up of many "selfish" bodies, b) is influence by many irrelevant and political factors, and ) sometimes makes honest to goodness mistakes.

NASA isn't looking at 100 day transits much. SpaceX is. And SEP does reduce IMLEO dramatically for a typical 6-7 month trajectory that NASA likes to look at. And NASA is planning surface rendezvous, so most flights to Mars will carry cargo (which doesn't benefit much from fast transit).

SEP actually trades quite well, and technology improvements allow Isp improvements, not being limited in Isp like chemical and NTR by needing to keep your combustion chamber from melting.

SEP allows you to reduce transit time (versus an equivalent-IMLEO chemical-only system) if you have good enough solar array and thruster technology, but its chief benefit is usually a dramatic reduction in IMLEO. And SpaceX could leverage that just for hauling propellant from LEO to HEO if they so desired. Remember, SpaceX's LEO Constellation will use SEP tech, so SpaceX will be quite capable technically of building SEP tugs (and will be producing Megawatts' worth of SEP arrays/thrusters anyway just for the constellation).

Clearly, the longer the transit, the more time you have to benefit from it, but we're talking about MCT here.
You asked "so why does NASA use it" when someone challenged SEP in this context...  So here's your answer.

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.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #729 on: 10/13/2015 05:25 AM »
SEP has exponentially larger benefits the faster you want to get to Mars.

To do a slow Hohmann transfer you need about 3.5 km/s in LEO.  But to try to do 100 days from LEO takes even more on the order of 5.5 km/s which is around 80% propellant fraction and very likely multiple booster stages as the weight of a tank has some minimum fraction leaving something like 10% payload.  Worse your arrival velocity on Mars is high and must be shed via either propulsion or aerocapture which is still in it's infancy using an aeroshell.

With SEP you can basically get the first ~3 km/s of your LEO departure speed done outside of the vehicle actually being sent TMI which is achieving the remaining ~2 km/s vastly easier if your doing it via chemical propulsion because your propellant is now less massive then the payload.

While doing SEP directly to Mars is estimated to reduce the IMLEO by half, using it to set up a periapsis burn is going to reduce your required mass by something like a factor of 4 because the move from LEO to high orbit is going to only require a propellant fraction of 13% to 19% depending on ISP range of 5k to 3k.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #730 on: 10/13/2015 05:35 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.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #731 on: 10/13/2015 06:09 AM »
With SEP you can basically get the first ~3 km/s of your LEO departure speed done outside of the vehicle actually being sent TMI which is achieving the remaining ~2 km/s vastly easier if your doing it via chemical propulsion because your propellant is now less massive then the payload.

Again you completely hand-wave away complexity of a massive SEP booster stage. The fully loaded and fueled Mars transit vehicle/lander would mass what? 200t? 400t? More?  What delta-V of departure burn from the assembly point (EML1) is necessary? How large would your super-duper-mega SEP need to be to accelerate the assembled Mars craft to a 3 km/s boost at perigee? How long would it take to build that up the 3 km/s? Please... Some specifics. I dare you. (as a bonus, estimate the cost of establishing such an infrastructure of smaller SEP tugs and massive SEP boosters - one needed for every MCT!)

Congratulations on the thread hijack, BTW. Mission accomplished. Can we rename this thread to "MCT using SEP", so another thread can be created without this religious fervor?

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.

This is the scene I picture when Impaler reads something vaguely connecting SpaceX and SEP: (original movie scene transcript here:  http://www.imdb.com/title/tt0109686/quotes?item=qt0383410 )

SEP: What do you think the chances are of a guy like you and a girl like me... ending up together?
SpaceX: Well, SEP, that's difficult to say. I mean, we don't really...
SEP: Hit me with it! Just give it to me straight! I came a long way just to see you, SpaceX. The least you can do is level with me. What are my chances?
SpaceX: Not good.
SEP: You mean, not good like one out of a hundred?
SpaceX: I'd say more like one out of a million.
[pause]
SEP: So you're telling me there's a chance... YEAH!

 :)
« Last Edit: 10/13/2015 06:23 AM by Lars-J »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #732 on: 10/13/2015 06:24 AM »
Please list some new technologies/systems you actually support developing other then huge boosters.

I'm still shocked that anyone can consider HALL thrusters EXOTIC propulsion in this day and age, they have existed for decades, are on hundreds of com-sats.  The bloody Ruskies have had them as baseline propulsion systems for their Mars mission plans for decades (first nuclear powered now solar powered), and they are kind of know for using 'brute force' technologies.  Just because NASA and American Aerospace only 'discovered' them 10 years ago dosn't mean they are some finniky bleeding edge tech.

The only remotely cutting edge tech I'm advocating is Magneto-Plasma Aerocapture and it has excellent effects on the IMLEO, even more then in NASA DRM plans because the MCT needs to soft land on Earth which means we can use it twice on each mission.  Frankly all-chem advocates like yourself SHOULD be jumping on this as a means to catch up with Electric propulsion architectures as the savings for chem of not having to do propulsive braking are far higher then for electric propulsion which can brake for a few scant additional percentages of propellant fraction.
« Last Edit: 10/13/2015 06:30 AM by Impaler »

Offline Rocket Surgeon

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Re: MCT Speculation and Discussion Thread 4
« Reply #733 on: 10/13/2015 06:44 AM »


No. If you are truly scaling this to be able to deliver 100t of cargo, you really need to have it in the middle. Not up front with the hab volume. When you do atmospheric entry, the propellant tanks will be mostly empty, so then by placing the cargo up top you are now forcing yourself to have to have a substantial minimum cargo load or the thing won't fly right.

Think about it. 100t. And it could be there, or it could be empty. If you do a sideways re-entry, that DOES constrain you to a center placement of cargo. OR you need to split the cargo into two balanced areas, one below and one above propellant tanks.

It's 100mt of payload.   That may be 100mt of pure cargo down the road at some point, but for quite some time it will be mixed cargo and crew/hab. 

So you have your IBMCT.  It has it's fairly heavy MPS on the bottom.  Above that you have two stacked cylindrical tanks that go up to about the center of the overall vehicle (when measured from tip to tail)...up a little over half way up the cylindrical portion.  Above that you'll have a cargo deck for mixed flights that is maybe 3 meters tall.  As the cargo will have to be lowered to the surface, none of it can be -too- large in one piece, but it will be heavy overall.  Above that you will have a pressurized hab volume filling the rest of the cylinder, and then another tapered pressurized hab volume in the nose.  If an LAS is required, the nose will have LAS/landing engines it it as well, along with small pressurized tanks to fuel them.   Overall, it will have a fair amount of mass in it.
So again, you'll have a mass area in the tail, amidships, and in the nose. 

With 100mt of pure cargo, and no hab area at all, that may be a little more tricky if all that 100mt is between the nose and the tanks.  They'd probably stow the heaviest pieces just above the tanks, with lighter and lighter pieces above that to help with weight distribution.

But, it's obviously something that'd have to be looked at in more detail by actual SpaceX engineers during the actual design process to see how the real weight distribution will interact with the EDL profile.

This divided concept would have the problem of being nose-light.  The mass will be concentrated in the middle, and at the aft in the MPS.  But given it's tapered overall OML, maybe that still makes for a feasible distribution for EDL?  It's a little above my area.  :-)

With the SDMCT, obviously that's not a problem because it's always vertical.

To go back to balancing the Biconic for a bit, would it be possible to have a configuration similar to the Phoenix (see attached), except applied to a Biconic and with some sort of escape system in the nose (sorry, really like the idea of the nose section being a dedicated 'lifeboat'. I just see it as a way to solve so many problems) It would still have the central column, as a way to move down to the main pressurized area.
Not sure what this would do to the mass in total, but wouldn't it allow for a more evenly distributed mass during EDL?

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #734 on: 10/13/2015 07:00 AM »
To go back to balancing the Biconic for a bit, would it be possible to have a configuration similar to the Phoenix (see attached), except applied to a Biconic and with some sort of escape system in the nose (sorry, really like the idea of the nose section being a dedicated 'lifeboat'. I just see it as a way to solve so many problems) It would still have the central column, as a way to move down to the main pressurized area.
Not sure what this would do to the mass in total, but wouldn't it allow for a more evenly distributed mass during EDL?

I do like that image, it is close to what I have in mind for an MCT. But I think a lengthy tunnel through a cryogenic tank might have issues.

If you insist on have a nose-mounted LAS, another way around it might be to have just a giant central tank (split between oxidizer and fuel), and then have a lower cargo bay (just above engines), and an upper cargo bay in the nose. It would force some level of cargo balance, but that would allow you to put all the pressurized volume up top. And as a bonus the unpressurized/heavy cargo would be easily offloaded near the surface.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #735 on: 10/13/2015 08:19 AM »
To go back to balancing the Biconic for a bit, would it be possible to have a configuration similar to the Phoenix (see attached), except applied to a Biconic and with some sort of escape system in the nose (sorry, really like the idea of the nose section being a dedicated 'lifeboat'. I just see it as a way to solve so many problems) It would still have the central column, as a way to move down to the main pressurized area.
Not sure what this would do to the mass in total, but wouldn't it allow for a more evenly distributed mass during EDL?

I do like that image, it is close to what I have in mind for an MCT. But I think a lengthy tunnel through a cryogenic tank might have issues.

If you insist on have a nose-mounted LAS, another way around it might be to have just a giant central tank (split between oxidizer and fuel), and then have a lower cargo bay (just above engines), and an upper cargo bay in the nose. It would force some level of cargo balance, but that would allow you to put all the pressurized volume up top. And as a bonus the unpressurized/heavy cargo would be easily offloaded near the surface.
One additional benefit to a tunnel through a fuel tank: It makes the best solar storm shelter you can buy on the mass required for the tunnel segment and some electric blankets.

Online hkultala

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Re: MCT Speculation and Discussion Thread 4
« Reply #736 on: 10/13/2015 09:06 AM »
Please list some new technologies/systems you actually support developing other then huge boosters.

I'm still shocked that anyone can consider HALL thrusters EXOTIC propulsion in this day and age, they have existed for decades, are on hundreds of com-sats.  The bloody Ruskies have had them as baseline propulsion systems for their Mars mission plans for decades (first nuclear powered now solar powered), and they are kind of know for using 'brute force' technologies.  Just because NASA and American Aerospace only 'discovered' them 10 years ago dosn't mean they are some finniky bleeding edge tech.

station-keeping thruster with millinewtons thrust is totally different thing than electric engine that can give multiple km/s delta-v in reasonable short time for a big spacecraft.

The energy source for big SEV spacecraft is the exotic part, not the thruster itself.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #737 on: 10/13/2015 09:53 AM »
With SEP you can basically get the first ~3 km/s of your LEO departure speed done outside of the vehicle actually being sent TMI which is achieving the remaining ~2 km/s vastly easier if your doing it via chemical propulsion because your propellant is now less massive then the payload.

Again you completely hand-wave away complexity of a massive SEP booster stage. The fully loaded and fueled Mars transit vehicle/lander would mass what? 200t? 400t? More?  What delta-V of departure burn from the assembly point (EML1) is necessary? How large would your super-duper-mega SEP need to be to accelerate the assembled Mars craft to a 3 km/s boost at perigee? How long would it take to build that up the 3 km/s? Please... Some specifics. I dare you. (as a bonus, estimate the cost of establishing such an infrastructure of smaller SEP tugs and massive SEP boosters - one needed for every MCT!)

Small-thruster SEP is now routine on commsats for stationkeeping.  Hall Effect Thrusters have recently solved the erosion problems that limited their lifetime.  Large arrays of small thrusters are trivially easy.  The design of large arrays of solar panels require only moderate scaleup efforts relative to the monetary outlays discussed in Mars mission planning;  The only reason we don't have them yet seems to be that we're not pushing much money into those efforts.  The current generation of commsat builders are all selling SEP orbit-raising solutions for GTO -> GSO, the "all-electric propulsion satellite";  The business model seems to be "Two satellites for the launch price of one" rather than "A satellite twice as big" for now.

With SEP you can basically get the first ~3 km/s of your LEO departure speed done outside of the vehicle actually being sent TMI which is achieving the remaining ~2 km/s vastly easier if your doing it via chemical propulsion because your propellant is now less massive then the payload.

Again you completely hand-wave away complexity of a massive SEP booster stage. The fully loaded and fueled Mars transit vehicle/lander would mass what? 200t? 400t? More?  What delta-V of departure burn from the assembly point (EML1) is necessary? How large would your super-duper-mega SEP need to be to accelerate the assembled Mars craft to a 3 km/s boost at perigee? How long would it take to build that up the 3 km/s? Please... Some specifics. I dare you. (as a bonus, estimate the cost of establishing such an infrastructure of smaller SEP tugs and massive SEP boosters - one needed for every MCT!)
Let's work with this.  Let's say that after the BFR+MCT unknown is answered, we can get 100 tons of large-diameter freight to the high end of LEO in a completely reusable manner, with nothing left in orbit.

Our cargo staging point is some undecided orbit in the general vicinity of the Moon (DRO, EML1, EML2), with geocentric semimajor axis about 384,400km, orbital velocity about 1018m/s.

Our cargo will launch into a 1500x1500km Low Earth Orbit, high enough that it doesn't contribute to the debris problem and (not coincidentally) right in the middle of a high radiation zone.  Orbital velocity at this altitude is about 7117m/s.

Delta V for a low-thrust transfer between circular orbits on the same plane, seems to approximate the difference in orbital velocities.  Delta V required to reach cargo staging point on SEP: 6099m/s.

Let's arbitrarily add another 901m/s for plane change and whatever costs are associated with Lunar capture.  That gives us 7000m/s dV.  We're going to add another 1000m/s dV for the terminal staging, which I'll get to later.

The NEXT thruster has been tested to 5.5 years of burn;  Earth-Mars synodic cycles are more like 2.135 years.  We'll give the tug 4 years after launch to get on target, and give it NEXT thrusters for propulsion, with 4190s Isp.  At 4190s Isp, a vehicle that weighs 100 tons wet reaches 8km/s after depleting enough propellant that it weighs 82.3 tons dry.

The tug requires an average acceleration over this time of 8000m/s / 4 years, or 5.55*10^-5 m/s.  The vehicle mass averages about 91.15 tons during its burn, so average acceleration needs to be about 5.78N.  This is a lot relative to current designs.  A  NEXT thruster gives us only about 236mN of thrust.  We would need 25 of them per tug to do the mission in 4 years.  Let's arbitrarily bring it up to 40 - there's going to be some time when the vehicle is shadowed by Earth, after all.  40 NEXT thrusters consume 6.9kW apiece for 276kW total;  Let's bring that up to 300kW since there will be some degradation over time.  I'm going to assume with enough work we can achieve a 300kW array that weighs 2000kg (150W/kg).  40 NEXT thrusters weighs around 2000kg as well.  Give 1000kg to structural support for these things.  Assume that the chemical tank and solar shield the SEP tug is carrying as cargo can be done for 7300kg.

That leaves 70 tons of cargo chemical propellant in our Lunar staging orbit with 1000m/s left, for every BFR launch.

Next stage: We define the _Deep Elliptical Orbit_, a name chosen because of how much ambiguity "H" and "E" have, as an orbit with one leg on the Moon and one leg on Earth.  The propellant tugs reach Lunar staging orbit and start to burn retrograde, lowering periapsis back to Earth.  This is wasteful relative to the chemical manner of achieving elliptical orbits, but they can afford it, because 4190s Isp.  They burn for about 1000m/s and drop perigee down to 300km.

You now have 70 tons of chemical propellant where you need it for the Impaler Short Transit.

---

This proposal is external to the issue of how you would send the vehicle up, whether the lander would reach Lunar staging orbit or whether it would go with the crew capsule or what.  Use my math to deduce the answers to those questions.

If you can indeed MAC away all your excess velocity at Mars, and you still need chemical propulsion for descent, then you can send the lander up to DEO, running out of propellant at ~3.1km/s, then refuel from the propellant depot already in DEO, and you'll have the 2241m/s (plus some small number, that was based on "descent from high orbit") that you need to go from DEO to the Impaler Short Transit.  Then, assuming 4.5km/s total vehicle capability (you need that to get from the Mars surface to LMO), you'll have another 2259m/s to work with for descent and course corrections.  For a 400 ton loaded-but-dry-weight lander, that 4.5km/s wet mass is 1338 tons, or 938 tons of propellant.  The vehicle in DEO won't be truly empty, though - it will still have another ((4.5 - 3.1 = 1.4km/s) run through the delta V equation = 183 tons) in it, useful for abort-to-Earth-landing contingencies.  In the face of that propellant delivery need, (938 - 183) = 755 tons, you'll want to have 11 BFR launches of SEP propellant tugs to DEO, on top of the prop required in LEO to get the BFR itself to DEO (or however you wanna do that, with SEP perhaps if you have a separate nested manned capsule).

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.
« Last Edit: 10/13/2015 10:09 AM by Burninate »

Offline RotoSequence

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Re: MCT Speculation and Discussion Thread 4
« Reply #738 on: 10/13/2015 09:56 AM »
Has anyone suggested the possibility of SpaceX's announcement including "a reusable, nuclear (fission, fusion, either or) upper stage for the BFR" yet?
« Last Edit: 10/13/2015 03:15 PM by RotoSequence »

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #739 on: 10/13/2015 09:14 PM »
Re: "Non-starter"

That really is one of the most obnoxious terms commonly used on this site. I wish the mods would add it to the unacceptable-language list.

You seem to be taking a Zubrinist position, that no new technology is needed and simple 'brute force' can achieve all of SpaceX's mars goals.  Cost was no object to Zubrin he didn't bat an eyelash at 100 Billion to perform a single mission to mars which would have been flags and footprints.

While Zubrin is prone to exaggerate problems in rival proposals, and underplay issues with his own, we have to be fair and remember he came into the game trying to suggest a realistic alternative to a late '80s/early '90s NASA proposal that was priced somewhere around $500 billion.

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