Author Topic: MCT Speculation and Discussion Thread 4  (Read 625802 times)

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #700 on: 10/10/2015 05:24 PM »
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.
You probably want some chemical propulsion on your SEP stage, at least for the crew transfer, but not anything on the scale of MCT.
the thrust level of chemical propulsion would require the SEP to have far higher rigidity they it otherwise would and it's mass would essentially be parasitic when doing the chemical burn as we intend to leave Earth with all the escape velocity needed to reach mars.

Doing a drop'n'go Oberth manoeuvre means that your SEP is a high thrust type. (You can't spend days doing a HEO-LEO-MTO Oberth burn.)

That implies that your SEP-ship is already quite rigid. (Doing aerobraking into Mars orbit also implies that your ship is pretty rigid.)

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #701 on: 10/10/2015 05:43 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.
« Last Edit: 10/10/2015 05:44 PM by Lars-J »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #702 on: 10/10/2015 07:09 PM »
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.
You probably want some chemical propulsion on your SEP stage, at least for the crew transfer, but not anything on the scale of MCT.
the thrust level of chemical propulsion would require the SEP to have far higher rigidity they it otherwise would and it's mass would essentially be parasitic when doing the chemical burn as we intend to leave Earth with all the escape velocity needed to reach mars.

Doing a drop'n'go Oberth manoeuvre means that your SEP is a high thrust type. (You can't spend days doing a HEO-LEO-MTO Oberth burn.)

That implies that your SEP-ship is already quite rigid. (Doing aerobraking into Mars orbit also implies that your ship is pretty rigid.)

No you misunderstand, the SEP pushes the lander up to HEO first, then the Lander and SEP SEPARATE at HEO, only the lander with it's chemical propulsion system and inherent ability to tolerate high acceleration goes through the Oberth maneuver.

The SEP spirals away from the Earth on low thrust without any Oberth benefits, but it isn't carrying ANY cargo and isn't needed at mars for nearly 2 years so it can travel slowly and at high efficiency.



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.


No most of the effective acceleration is still coming from the SEP, to simply have the Lander perform depart form LEO with this much Escape velocity would require nearly 5.8 km/s DeltaV which would mean an IMLEO of 1000 mT for the same TMI mass.

A SEP transit vehicle is not at all like a cycler, it is making a controlled propulsive orbital insertion or braking maneuver at each planet, that means your not blasting off at escape velocity to catch the thing as it flies by on a hyperbolic trajectory each time.

No one who favors SEP for heliocentric HEO->Mars transfer would neglect using it for going between LEO->HEO as each of these is about equal in DeltaV for a low thrust system.  Only the crew in a small capsule needs to be sent via chemical propulsion for radiation avoidance, but all cargo, all consumables, all propellants and the vehicle hardware would obviously be a positioned in HEO by SEP leaving <1% of the mass to be moved by Chem. 



Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #703 on: 10/10/2015 08:08 PM »
No you misunderstand, the SEP pushes the lander up to HEO first, then the Lander and SEP SEPARATE at HEO, only the lander with it's chemical propulsion system and inherent ability to tolerate high acceleration goes through the Oberth maneuver.
The SEP spirals away from the Earth on low thrust without any Oberth benefits, but it isn't carrying ANY cargo and isn't needed at mars for nearly 2 years so it can travel slowly and at high efficiency.

Ah, missed that in your original post. I take it the SEP brings them back home?

No one who favors SEP for heliocentric HEO->Mars transfer would neglect using it for going between LEO->HEO

They might. The radiation during a slow LEO-HEO spiral is apparently hell on solar panels. That means your efficiency is already degraded by the time you do the long Mars flight.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #704 on: 10/10/2015 08:21 PM »

No one who favors SEP for heliocentric HEO->Mars transfer would neglect using it for going between LEO->HEO

They might. The radiation during a slow LEO-HEO spiral is apparently hell on solar panels. That means your efficiency is already degraded by the time you do the long Mars flight.

Actually I favour using high acceleration chemical or NTR for LEO to high energy TMI and using SEP for the remainder of the trip matching to Mars velocity allowing for a faster than Hohmann transit and a low energy capture at Mars.
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Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #705 on: 10/10/2015 11:17 PM »
Musk has had time of flight goals that are very aggressive as he seems to want to basically outrun the radiation danger and have a transit time that most people would find acceptable when crammed in small spaces.

Well if they're going to Mars I suppose they must get used to it anyway. And 100 days or so is still a lot of time so you're not getting away with less habitable space.

I don't believe it is necessary to make a hybrid drive SEP stage, the thrust level of chemical propulsion would require the SEP to have far higher rigidity they it otherwise would...

Not really, a thrust of 1/10 of the stage's mass is sufficient, and ISP can be low. Just look at NASA's designs.

It is low TRL now but it is in active development, a demonstration cube sat will be launched soon by NASA.

I haven't seen it being used in any of NASA's designs, not even mentioned.

But if it works, I agree it would be a big deal for any architecture. I think it could drastically reduce the transfer time for SEP only, for starters. Or capture habitats without aeroshell. Or, in the case of MCT, drastically reduce heat shield requirements.

And it is needed here to carry the return habitat from mars surface to orbit and it carries the primary Magneto system to brake with upon return to Earth.  The SEP would have a brake too but much smaller as it's more delicate and doesn't have the urgency because it doesn't carry the crew.

I thought the "Magneto system" needs a lot of power...?

I imagine the SEP would have to create a bigger field in order to reduce max Q.

It's a conservative value consistent with Hall thrusters with power conversion units and solar at 300 W/kg.  Perhaps you misread it as kw/kg, it is the other way.

Values I've come across are for example:

Power system alpha: 20kg/kw
Tank mass: 4% of propellant.
Structure mass: 27% of (tank mass + power system mass).

I think 5kg/kw is more of a long term goal.

I quote from a paper called "Fast Transits to Mars Using Electric Propulsion" (2010).

Quote
The potential does exist to use solar
power for exploration within the inner solar
system. Solar array technology continues
to improve in cell efficiencies and in
system alpha. The expanded use of electric
prolusion, and the synergistic benefit of
electric propulsion and increasing power
availability is driving commercial and
government towards higher power systems.
The SOA solar arrays are currently 15 - 20
kg/kW. The DARPA Fast Access
Spacecraft Testbed (FAST) program has
progressed the SOA has with near-term
projected goals of 8 kg/kW at 1 AU.19
NASA studies have far-term predictions
approaching 3.3 kg/kW including the array,
based on Stretched Lens Array Square
Rigger (SLASR) technology with advanced
cells, gimbals, booms, power cabling, etc.20
A point design for a 232kW system had a current best estimate (CBE) system mass of 781 kg.21 Using an advanced
cell, a 100 kW End-of-Life, after 10 years at GEO, stretched lens array design has a predicted mass performance of
1.85 kg/kW


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?

Actually the chemical propulsion would only provide little delta-v, around 600m/s. That allows for pressure-fed storable propulsion.
« Last Edit: 10/10/2015 11:28 PM by Oli »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #706 on: 10/10/2015 11:34 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?...
"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."
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Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #707 on: 10/11/2015 04:24 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."

Let me clarify. Anything that relies *primarily* on SEP for Mars transit propulsion is IMO a non-starter.

I've seen reasonable arguments for SEP during the long transit, but for Mars I don't think this helps a whole lot. (Going to the outer planets? Yes, essential, but not for the inner planets)

Especially in an MCT type architecture where "you land the whole thing" being a baseline assumption. With SEP you fall back into just another "Battlestar Galactica" mission mold (massive mother ships). If that is your thing... Sure, but there are more suitable threads for reviving those ideas.

« Last Edit: 10/11/2015 04:28 AM by Lars-J »

Offline Oli

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Re: MCT Speculation and Discussion Thread 4
« Reply #708 on: 10/11/2015 04:28 AM »
With SEP you fall back into just another "Battlestar Galactica" mission mold (massive mother ships).

Which is evidently wrong, because SEP vehicles tend to be a lot more lightweight. What makes them appear massive are the big solar arrays.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #709 on: 10/11/2015 04:40 AM »
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.

No most of the effective acceleration is still coming from the SEP, to simply have the Lander perform depart form LEO with this much Escape velocity would require nearly 5.8 km/s DeltaV which would mean an IMLEO of 1000 mT for the same TMI mass.

A SEP transit vehicle is not at all like a cycler, it is making a controlled propulsive orbital insertion or braking maneuver at each planet, that means your not blasting off at escape velocity to catch the thing as it flies by on a hyperbolic trajectory each time.

No one who favors SEP for heliocentric HEO->Mars transfer would neglect using it for going between LEO->HEO as each of these is about equal in DeltaV for a low thrust system.  Only the crew in a small capsule needs to be sent via chemical propulsion for radiation avoidance, but all cargo, all consumables, all propellants and the vehicle hardware would obviously be a positioned in HEO by SEP leaving <1% of the mass to be moved by Chem.

HEO (or even worse, a highly elliptical LEO-HEO transfer orbit) is a terrible assembly/rendezvous point, for a variety of reasons. Some that immediately come to mind:
 - Time to get there for SEP cargo runs - you need a LOT of SEP transfer vehicles to preposition cargo for a flotilla of your Mars transfer vehicles.
 - The number of runs you need to take through the Van Allen belts with a crew is significant, even if you dock and depart ASAP. It's not great for all the cargo spirals either.

So Im curious, what exactly do you mean by "HEO"?

Again what you propose does not have much left in common with what we know of the MCT architecture, so the discussion should probably move.
« Last Edit: 10/11/2015 04:50 AM by Lars-J »

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #710 on: 10/11/2015 04:43 AM »
With SEP you fall back into just another "Battlestar Galactica" mission mold (massive mother ships).

Which is evidently wrong, because SEP vehicles tend to be a lot more lightweight. What makes them appear massive are the big solar arrays.

Wrong in the strict sense of mass (not size)... But nothing that is designed to push around 100s of tons of mass is going to be flyweight. A SEP system trades mass for many other problematic tradeoffs.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #711 on: 10/11/2015 06:58 AM »

HEO (or even worse, a highly elliptical LEO-HEO transfer orbit) is a terrible assembly/rendezvous point, for a variety of reasons. Some that immediately come to mind:
 - Time to get there for SEP cargo runs - you need a LOT of SEP transfer vehicles to preposition cargo for a flotilla of your Mars transfer vehicles.
 - The number of runs you need to take through the Van Allen belts with a crew is significant, even if you dock and depart ASAP. It's not great for all the cargo spirals either.

So Im curious, what exactly do you mean by "HEO"?

Again what you propose does not have much left in common with what we know of the MCT architecture, so the discussion should probably move.



Our SEP tug is reusable and we expect to be launching continually from Earth to build up a

High  Earth Orbit, aka something near escape velocity, could be Lagrange point, Lunar Distant Retrograde, highly elliptical orbits etc etc, basically something out at the very edge of Earth's Sphere of influence, the DeltaV is similar for them all.

-  High Earth orbit is FAR better then LEO in every way, it is free of dangerous orbital junk, it is colder meaning propellant boil-off is minimized, the SEP won't be shaded as it would be in LEO meaning it can depart at full power, their is no appreciable drag which would require constantly re-boosting stuff.

- Time to conduct the raising of cargo is inconsequential because we expect a continual non-stop launch campaign on Earth putting cargo into a stream going to the assembly point, the SEP tugs and their propellants will be less massive then the equivalent propellants needed to move cargo to HEO and do TMI from their then doing it from LEO on chem even at the slowest transfer.

- Why do I have to keep shooting down this straw-man, I have told you on several occasions that crew are sent up to the assembly point by a capsule on chemical propulsion, they do not make multiple passes through the Belt and I have never heard any evidence that the belt will be harmful to cargo.  Amorphous silicon solar collectors are basically immune to radiation degradation and can make plenty of passes through the belt as well.

Your INTERPRETATION of Musk's comments is different then my own, your interpretation is common, even dominant I grant that but it basically amounts to a single stage from LEO being then going all the way to mars surface and then back to Earth surface with refueling at LEO and mars surface.  I've rejected this as physically impossible as it makes a SSTO vehicle on Earth look easy in comparison.  I interpret his landing comment to refer to just the lander, aka the lander dose not shed any parts during EDL, a transit vehicle in mars orbit will be necessary to return to Earth.  I interpret any ambiguous comments from Musk in the most conservative manor possible and expect that system will be more complex then even Musk's original plans call for as he aims for the simplest solution first and has to compromise once he gets flight experience.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #712 on: 10/11/2015 08:56 AM »
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.
« Last Edit: 10/11/2015 12:03 PM by Burninate »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #713 on: 10/11/2015 11:26 PM »
Yes that is the escape manuver as I had calculated it, but I'm proposing Magneto-Plasma aerocapture which doesn't consume any propellant.  I'm not entirely sure that the MP system can provide enough drag to do such a capture primarily because it dose not look to be able to create lift, only drag so the atmospheric cord is quite small.

Withing this propellent-less braking system the trajectory clearly dose not work as it's beyond any feasible propulsive capture or any reasonable sized aeroshell.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #714 on: 10/12/2015 06:11 AM »

HEO (or even worse, a highly elliptical LEO-HEO transfer orbit) is a terrible assembly/rendezvous point, for a variety of reasons. Some that immediately come to mind:
 - Time to get there for SEP cargo runs - you need a LOT of SEP transfer vehicles to preposition cargo for a flotilla of your Mars transfer vehicles.
 - The number of runs you need to take through the Van Allen belts with a crew is significant, even if you dock and depart ASAP. It's not great for all the cargo spirals either.

So Im curious, what exactly do you mean by "HEO"?

Again what you propose does not have much left in common with what we know of the MCT architecture, so the discussion should probably move.

Our SEP tug is reusable and we expect to be launching continually from Earth to build up a High  Earth Orbit, aka something near escape velocity, could be Lagrange point, Lunar Distant Retrograde, highly elliptical orbits etc etc, basically something out at the very edge of Earth's Sphere of influence, the DeltaV is similar for them all.

Not all of them are the same - nor stable. The moon will influence many of them. But eccentricity matters a lot. Make it too high and you WILL make lots of passes through the Van Allen belts. And no, Delta-V is NOT similar for them all.

-  High Earth orbit is FAR better then LEO in every way, it is free of dangerous orbital junk, it is colder meaning propellant boil-off is minimized, the SEP won't be shaded as it would be in LEO meaning it can depart at full power, their is no appreciable drag which would require constantly re-boosting stuff.

In every way?  ::) Oh boy. Not for accessibility, that's for sure. Nor travel time for supplies. Nor for solar flare protection. There are good some valid trade-offs to be made by staging there, but pretending to that there are no down sides just makes your argument look silly and it taints the rest of your arguments.

- Time to conduct the raising of cargo is inconsequential because we expect a continual non-stop launch campaign on Earth putting cargo into a stream going to the assembly point, the SEP tugs and their propellants will be less massive then the equivalent propellants needed to move cargo to HEO and do TMI from their then doing it from LEO on chem even at the slowest transfer.

Inconsequential?  ;D As said by everyone when real world practicality gets in the way. But I suppose being so generic allows you to hand-wave away all concerns. And the number of SEP tugs that would be needed. You haven't even settled on a HEO staging point, so how can we challenge your math assumptions for a SEP tug and how many you will need, and how large such SEP tugs would be?

But since you say you have made some basic calculations, please offer some information about these tugs. 1. Name a HEO staging orbit. 2. Settle on a SEP spiraling transit time. (weeks? months?) 3. Settle on how much cargo per SEP tug. (if you don't have a suggestion, lets make it 100t) Now can your show how large/massive/complex a SEP tug would have to be to accomplish the transfer of said amount cargo in said amount time to the defined staging point?

- Why do I have to keep shooting down this straw-man, I have told you on several occasions that crew are sent up to the assembly point by a capsule on chemical propulsion, they do not make multiple passes through the Belt and I have never heard any evidence that the belt will be harmful to cargo.  Amorphous silicon solar collectors are basically immune to radiation degradation and can make plenty of passes through the belt as well.

I only mentioned it to make you define your HEO staging orbit. And since you have not - above you wrote that "highly elliptical orbits" were a possible staging place, and those could certainly intersect with Van Allen belts. (again, specificity about a staging orbit would help avoid us bringing up these "straw-men" arguments) As for the effect on computers, arrays, and other equipment, I'm not sure what the point would be to bring it up to you, since you have already hand-waved away such concerns.

Your INTERPRETATION of Musk's comments is different then my own, your interpretation is common, even dominant I grant that but it basically amounts to a single stage from LEO being then going all the way to mars surface and then back to Earth surface with refueling at LEO and mars surface. I've rejected this as physically impossible as it makes a SSTO vehicle on Earth look easy in comparison.  I interpret his landing comment to refer to just the lander, aka the lander dose not shed any parts during EDL, a transit vehicle in mars orbit will be necessary to return to Earth.  I interpret any ambiguous comments from Musk in the most conservative manor possible and expect that system will be more complex then even Musk's original plans call for as he aims for the simplest solution first and has to compromise once he gets flight experience.

Well, now that *you* have rejected it...  ;) So you admit that you are basically using this thread now as a soap box for your own Mars architecture, now that you have rejected the aspects of it that are unique? And are there any more exotic (theoretical with no actual work done on them) technologies like Magneto Plasma Aerocapture that you want to throw in? NEP?

Whatever shape SpaceX's Mars plans actually solidify into will likely follow their current paradigm. Better, more practical use of existing technologies instead of chasing the very cutting edge of performance. It will be something that is more affordable rather than exotic. Taking something like a Mars SSTO lander and realizing that with some added performance and propellant transfer it can be used for Mars transit. And it could boost itself into LEO. Who would have thunk? Your ideas are almost the opposite of the SpaceX approach. Multiple staging points. Multiple kinds of vehicles. Are you *trying* to make this as expensive as possible by mutating it into a NASA-ish Mars program?
« Last Edit: 10/12/2015 06:18 AM by Lars-J »

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #715 on: 10/12/2015 07:00 AM »
I agree @LarsJ, Impaler is not discussing a SpaceX MCT based on what we know. His ideas may work (I don't know) but they need a thread in the Mars forum instead of here.

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #716 on: 10/12/2015 07:16 AM »
Magnetic capture/deceleration may be included in the plan sooner than we think. It sounds like it is real and has very major advantages. We can assume though that it is not part of the plan now. There is much to do, lets wait it out.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #717 on: 10/12/2015 07:27 AM »
Magnetic capture/deceleration may be included in the plan sooner than we think. It sounds like it is real and has very major advantages. We can assume though that it is not part of the plan now. There is much to do, lets wait it out.

How is it "real" in any way? Some theoretical papers aside, nothing much has been done on it, am I wrong here? And you could say the same about ANY technology on the horizon. EM drive?  ;D At some point the discussion needs to be based on what we actually know instead of what we would like it to be.
« Last Edit: 10/12/2015 07:28 AM by Lars-J »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #718 on: 10/12/2015 07:45 AM »
In every way?  ::) Oh boy. Not for accessibility, that's for sure. Nor travel time for supplies. Nor for solar flare protection. There are good some valid trade-offs to be made by staging there, but pretending to that there are no down sides just makes your argument look silly and it taints the rest of your arguments.

No where did I say they were better in every way, please stop putting words in my mouth.

Inconsequential?  ;D As said by everyone when real world practicality gets in the way. But I suppose being so generic allows you to hand-wave away all concerns. And the number of SEP tugs that would be needed. You haven't even settled on a HEO staging point, so how can we challenge your math assumptions for a SEP tug and how many you will need, and how large such SEP tugs would be?

My preferred staging point would be EML1, are you happy now?  The number, power level and transit time for SEP tugs would all involve trade offs between the size of the launch vehicle, the propellant tankers used, and ISP.  The MCT will likely be the largest payload and how much that can be reduced or subdivided will be key to sizing them.  I expect around a year for transit.

I only mentioned it to make you define your HEO staging orbit. And since you have not - above you wrote that "highly elliptical orbits" were a possible staging place, and those could certainly intersect with Van Allen belts. (again, specificity about a staging orbit would help avoid us bringing up these "straw-men" arguments) As for the effect on computers, arrays, and other equipment, I'm not sure what the point would be to bring it up to you, since you have already hand-waved away such concerns.

You really seem to hate the idea that anyone might have any flexibility when developing an idea and want to use that as a mean of attack rather then as a basis for rational discussion.  I find your straw-manning to be quite intentional and disrespectful, you repeatedly forget explanations from a few pages ago and bring up repeatedly debunked criticisms.

Well, now that *you* have rejected it...  ;) So you admit that you are basically using this thread now as a soap box for your own Mars architecture, now that you have rejected the aspects of it that are unique? And are there any more exotic (theoretical with no actual work done on them) technologies like Magneto Plasma Aerocapture that you want to throw in? NEP?

Whatever shape SpaceX's Mars plans actually solidify into will likely follow their current paradigm. Better, more practical use of existing technologies instead of chasing the very cutting edge of performance. It will be something that is more affordable rather than exotic. Taking something like a Mars SSTO lander and realizing that with some added performance and propellant transfer it can be used for Mars transit. And it could boost itself into LEO. Who would have thunk? Your ideas are almost the opposite of the SpaceX approach. Multiple staging points. Multiple kinds of vehicles. Are you *trying* to make this as expensive as possible by mutating it into a NASA-ish Mars program?

Everyone's ideas here are their own architectures or else no one but Elon Musk could post in this thread.  My ideas are perfectly consistent with the few crumbs of information we actually have and I fully expect to be predictive of the eventual mission architecture.

We know for a FACT that SpaceX is considering SEP (which btw is a 40 year old technology) from quotes by Shotwell herself, and Musk has said that HALL thrusters are easy to manufacture (and solar panels are simpler then drywall).  The Magneto Plasma Aerocapture is the most ambitious tech but it is well worth the development efforts and something SpaceX could easily do if they tried.

Your comments about adding performance to a SSTO vehicle reveal the kind of 'DeltaV-only' thinking that I find misleads most speculators here.  I rejected direct Earth return not because of insufficient velocity (which is is just barely achievable) it is the re-entry at 12-14 km/s at Earth which is not survivable for anything other then a small capsule like vehicle with high structural mass fraction.  I pay attention to entry velocities and g-forces as significant factors in the vehicles performance envelop and try to minimize them as much as possible.

Offline Rocket Surgeon

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Re: MCT Speculation and Discussion Thread 4
« Reply #719 on: 10/12/2015 07:50 AM »
The biggest problem with using SEP in cislunar space is the Van Allen radiation belt. This makes it an issue as you'll be forced to dwell in this high radiation zone for a long time as you raise up. Bad idea for people, but not for inert material. So you either need to boost past the belt quickly, or have A LOT of heavy shielding.

So here's a fun idea. If we go with the idea that the MCT will need several refuelings before leaving the earth-moon system, could we make the tankers also SEP powered? Deploy them to LEO then SEP all the way to above the Van Allen radiation belt, lets say L1. Then launch the MCT either with enough fuel to reach fueling point or refuel in LEO, boost out of the Van Allen radiation belt, then refuel at L1 and Launch to Mars for a fast transfer. Once done, the SEP-tankers come back down and reenter for servicing.

Of course, this would only be worth while if it saved a significant amount of mass, if you're only saving a hand full of tons, it wouldn't be worth it as a fully reusable Super Heavy Lift Rocket would probably drop $/kg to orbit down a fair bit. If Elon seriously wants to hit the '$500,000 a Mars Ticket' goal, we're talking ~$50/kg.

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