Author Topic: MCT Speculation and Discussion Thread 4  (Read 621597 times)

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #480 on: 08/25/2015 11:28 AM »
I'm confident they will reach the same conclusion I have after figuring out (possibly the hard way) that Direct Earth return is impossible.  People are being way to fast to grasp at nebulous ideas and speculations even if they come from Musk as THE ONE AND ONLY way it will be done,  Musk like any good programmer tries to think of the simplest possible system he thinks could possibly work, we saw that with F9 reuse plans, they are now WAY more complex then originally planned, MCT will be the same.

I am with Gucky on this one. I dont think that the mission plan you described is realistic at the beginning.
1. Aerobraking has never been used before, for good reason.
2. SEPs are for small payloads, not ones of many mT. The amount of solar arrays needed to get any meaningful thrust would be ginormous.
3. Using first SEP and then chemical propulsion is really inefficient.
4. Your mission plan is as complicated as it possibly can get. Simple is most often more important than efficient. Any complication in the flight plan means a rats hole of additional design and engineering work and launch mass to avoid failures. The more things you have the more things can break. And if anything breaks, your mission is toast.
5. The answer to "direct return is impossible" would not be to make the plan more complicated but to reduce the payload mass. The 100 persons per MCT figure is probably something for the far future. Not something for the first try.

The plan you described is maybe something for the far future, when Humanity has figured Mars transport out and is on its way to optimize the process. When failure modes are known and engineers learn to cut the right corners. I am very skeptical for the beginning though.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #481 on: 08/25/2015 01:50 PM »
As opposed, I believe 50t could be the dry mass of a 820t fully loaded and fueled MCT, note that at most there would be 1.3km/s of ΔV or so a re-entry mass around 230t, and a landing mass approaching 150t.

Does it really need that much delta-v to go from terminal velocity to landing?

Note "At most there would be 1.3km/s of ΔV" my mission 'plan' was leaving Earth orbit with enough 'hyperbolic' velocity (600 m/s or so over escape TMI, varies between launch windows greatly) to leave 1.3 km/s of ΔV which includes the 1km/s I am expecting is needed for landing and 300m/s for margin/contingency.

EDIT: originally had BF and typed escape instead of TMI
« Last Edit: 08/25/2015 04:26 PM by nadreck »
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 Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #482 on: 08/25/2015 05:51 PM »

I am with Gucky on this one. I dont think that the mission plan you described is realistic at the beginning.
1. Aerobraking has never been used before, for good reason.

MCT will require at least 10 things that have never been done and Airobraking HAS been done with satellites, your thinking of Airocapture (which NASA is studying heavily and will probably do soon) but I am base lining propulsive capture with the airobraking just for scrubbing off some additional velocity.

2. SEPs are for small payloads, not ones of many mT. The amount of solar arrays needed to get any meaningful thrust would be ginormous.

I find it quite odd that people who don't bat an eyelash as SpaceX designing and building the largest launch vehicle in history, an interplanetary spacecraft larger then the shuttle and EDL technology able to land more then 3 orders of magnitude more mass then has ever been put on Mars suddenly go into conniptions at the idea that SEP vehicles might be made plus size as well.

3. Using first SEP and then chemical propulsion is really inefficient.

Yes their is an inefficiency their, but resent papers on Hybrid propulsion have shown considerable savings in time and required SEP power levels when chemical propulsion is narrowly focused on fly-by escape maneuvers and propulsive capture.  And obviously we must have propellant for landing.

4. Your mission plan is as complicated as it possibly can get. Simple is most often more important than efficient. Any complication in the flight plan means a rats hole of additional design and engineering work and launch mass to avoid failures. The more things you have the more things can break. And if anything breaks, your mission is toast.

I find this statement ridiculous, it is no more complex then Apollo and involves only 2 vehicles and 1 mission critical rendezvous at Mars.  Their are Earth orbital rendezvouses but these are so routine now I don't see how they can be considered a barrier, not to mention their are fewer of them when you realize that every tanker visiting the giant alternative 1000 mt MCT vehicle is a rendezvous too.

5. The answer to "direct return is impossible" would not be to make the plan more complicated but to reduce the payload mass. The 100 persons per MCT figure is probably something for the far future. Not something for the first try.

Payload is irreverent because it is a question of dry mass fraction being impossibly low, that comes from the DeltaV and the Earth Entry velocity requirements.

The plan you described is maybe something for the far future, when Humanity has figured Mars transport out and is on its way to optimize the process. When failure modes are known and engineers learn to cut the right corners. I am very skeptical for the beginning though.

The second part with the 'mother-ship' would be for a far future, but the same basic landing ferry is going to be needed in both architectures.  The future improvements will all be in SEP as chemical performance is basically maxed out so we can create now a lander which will not become obsolete for decades.

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #483 on: 08/25/2015 06:29 PM »
I agree with having large SEP tugs.  Why not?  Only the solar panels would be the limiting factor.  A Falcon Heavy could deliver 50 tons of propellant to a large SEP tug after an earlier Falcon Heavy delivered a 50 ton tug to LEO.  With one docking maneuver to either attach the propellant, or load the propellant.  Then this 100 ton tug could take a large cargo to Mars, especially a cargo that is not affected by the radiation belt. 

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #484 on: 08/25/2015 06:54 PM »
Posting my spreadsheet for fun & amusement "designing" various BFRs
Please inform me of any errata.  Thanks!
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #485 on: 08/25/2015 06:58 PM »
This has been posted previously but here's two nice papers describing SEP/Chemical hybrid approach to Mars vehicles. 

I believe Gwenn Shotwell when she said "We're looking at SEP" for the MCT.
« Last Edit: 08/25/2015 06:59 PM by philw1776 »
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Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #486 on: 08/25/2015 07:51 PM »
Ok, lets make a back-of-the-envelope calculation.

A recent all-electric satellite Eutelsat  115 WEST B (2,205 kg) uses about 18 kW (4x 4.5 kW thrusters) to reach GSO. It takes 8 Month, so that is a reasonable timeframe for the not yet crewed MCT. Each thruster weights below 16 kg, say 15. So the mass of the spacecraft without the thrusters is 2141 kg. The solar arrays provide 18 kW of power for the thrusters. Wikipedia says that one gets about 300W/kg and 300W/m^2. That makes 60 kg of solar panels and 60 m^2 area. So the spacecraft without the engines and without the panels is 2081 kg.

Now, scaling that to the mass of the MCT of about 475mT in your design. Then the SEP tug would need 475/2* 18 kW= 4.25MW of solar, or 472/2 * 60kg approx 14 mT of solar panels which have a surface area of about 14000 m^2. Say launch that thing with an BFR that has a payload bay of 20m hight. Then the solar arrays need to have a wing span of about 700m. Since the individual elements cant be larger than the diameter, say 10m, that are 70 segments. Say we have 2 equal sized 350m panels (one on either side) and you use ISS type solar arrays, that would fill two boxes of about 5m height each.

I admit, I thought that the solar panels alone would be far larger than that. I thought that this alone would invalidate your concept. It is at these considerations borderline possible. But I did not include everything that needs to be included. I did not include the Xenon mass, the mass of the SEP tug structure, any electronics or transformers. No heat control system or other things that are needed.

sources:
https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electric
https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft
https://en.wikipedia.org/wiki/Integrated_Truss_Structure#Truss_subsystems

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #487 on: 08/25/2015 11:14 PM »
Ok, lets make a back-of-the-envelope calculation.

A recent all-electric satellite Eutelsat  115 WEST B (2,205 kg) uses about 18 kW (4x 4.5 kW thrusters) to reach GSO. It takes 8 Month, so that is a reasonable timeframe for the not yet crewed MCT. Each thruster weights below 16 kg, say 15. So the mass of the spacecraft without the thrusters is 2141 kg. The solar arrays provide 18 kW of power for the thrusters. Wikipedia says that one gets about 300W/kg and 300W/m^2. That makes 60 kg of solar panels and 60 m^2 area. So the spacecraft without the engines and without the panels is 2081 kg.

Now, scaling that to the mass of the MCT of about 475mT in your design. Then the SEP tug would need 475/2* 18 kW= 4.25MW of solar, or 472/2 * 60kg approx 14 mT of solar panels which have a surface area of about 14000 m^2. Say launch that thing with an BFR that has a payload bay of 20m hight. Then the solar arrays need to have a wing span of about 700m. Since the individual elements cant be larger than the diameter, say 10m, that are 70 segments. Say we have 2 equal sized 350m panels (one on either side) and you use ISS type solar arrays, that would fill two boxes of about 5m height each.

I admit, I thought that the solar panels alone would be far larger than that. I thought that this alone would invalidate your concept. It is at these considerations borderline possible. But I did not include everything that needs to be included. I did not include the Xenon mass, the mass of the SEP tug structure, any electronics or transformers. No heat control system or other things that are needed.

sources:
https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electric
https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft
https://en.wikipedia.org/wiki/Integrated_Truss_Structure#Truss_subsystems

Your basing your calculations on a freight SEP vehicle on a com-sats performance?  That's like using a water bottle rocket to estimate the performance of a launch vehicle.  Try using one of a million SEP design concepts for lunar tugs, ARM or Boeing's recent paper rather then this nonsense.  I'll get back with a concept of my own shortly.

Second who said anything about payload doors on the BFR, I said BFR would be a conventional 2 stage rocket which means normal clam-shell payload fairings probably on the order of 15 m diameter and >40 m long, the thing would have the same launch configuration as ARM with solar arrays on long boom arms that simply fold out.  And as for propellants I said they would be on a separate launch and are almost certainly going to be Krypton, as their is not enough production of Xenon to supply this vehicle.
« Last Edit: 08/25/2015 11:33 PM by Impaler »

Offline Rocket Surgeon

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Re: MCT Speculation and Discussion Thread 4
« Reply #488 on: 08/26/2015 04:36 AM »
Something I haven't seen many discussions on: Abort Modes
Is this somewhere where SEP may be superior? If you use a 6 months fast Hohmann transfer, you will naturally return to Earth in 2 years if you miss Mars for what ever reason. Swing out to 2 AU then back in again to meet up with Earth (source: the Case for Mars/Mars Direct).

This is fine for initial exploration, as you will only have a small crew (~4 people) as you only need to carry 2 years worth of supplies for 4, and you would arguable have to do that anyway.

But trying to carry 2 years worth of supplies for 100 people, and things get out of hand. Roughly 1000kg per person of supplies and you're looking at over 100 tonnes of just food, water and oxygen. We could just assume that nothing will go wrong, but it's not going to be fun when something does.

Offline Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #489 on: 08/26/2015 04:44 AM »
Something I haven't seen many discussions on: Abort Modes
Is this somewhere where SEP may be superior? If you use a 6 months fast Hohmann transfer, you will naturally return to Earth in 2 years if you miss Mars for what ever reason. Swing out to 2 AU then back in again to meet up with Earth (source: the Case for Mars/Mars Direct).

This is fine for initial exploration, as you will only have a small crew (~4 people) as you only need to carry 2 years worth of supplies for 4, and you would arguable have to do that anyway.

But trying to carry 2 years worth of supplies for 100 people, and things get out of hand. Roughly 1000kg per person of supplies and you're looking at over 100 tonnes of just food, water and oxygen. We could just assume that nothing will go wrong, but it's not going to be fun when something does.

That's why IMO it makes sense to launch several MCT's in the same launch window - a small fleet - that way they could support and assist each other if a problem should occur.

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #490 on: 08/26/2015 05:24 AM »

That's why IMO it makes sense to launch several MCT's in the same launch window - a small fleet - that way they could support and assist each other if a problem should occur.

Yes in the first few synods with human presence I expect the crewed MCTs to be launched with enough extra room that a single one could be lost and everyone be accommodated on the remaining ones. For example, the first crewed expedition, I see two MCTs with about 10 people on each launched at almost exactly the same time, and each capable of providing ECLSS for 20.  At the first return window those short timers (scientists maybe 6 to 8 ) would return on one, but leave a craft behind that could still evac everyone there. The next arrivals would have at least as many people and and ships and as few returning.
« Last Edit: 08/26/2015 08:12 PM by nadreck »
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 TetraOmni

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Re: MCT Speculation and Discussion Thread 4
« Reply #491 on: 08/26/2015 07:21 AM »
Not sure this has been discussed here. Is there a chance BFR gets a grasshopper type development vehicle?

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #492 on: 08/26/2015 09:05 AM »
Your basing your calculations on a freight SEP vehicle on a com-sats performance?  That's like using a water bottle rocket to estimate the performance of a launch vehicle. Try using one of a million SEP design concepts for lunar tugs, ARM or Boeing's recent paper rather then this nonsense.

In the contrary. I am basing my calculations on what I know is not nonsense but existing technology. Also can you please refer to the paper?

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #493 on: 08/26/2015 01:51 PM »
Ok, lets make a back-of-the-envelope calculation.

A recent all-electric satellite Eutelsat  115 WEST B (2,205 kg) uses about 18 kW (4x 4.5 kW thrusters) to reach GSO. It takes 8 Month, so that is a reasonable timeframe for the not yet crewed MCT. Each thruster weights below 16 kg, say 15. So the mass of the spacecraft without the thrusters is 2141 kg. The solar arrays provide 18 kW of power for the thrusters. Wikipedia says that one gets about 300W/kg and 300W/m^2. That makes 60 kg of solar panels and 60 m^2 area. So the spacecraft without the engines and without the panels is 2081 kg.

Now, scaling that to the mass of the MCT of about 475mT in your design. Then the SEP tug would need 475/2* 18 kW= 4.25MW of solar, or 472/2 * 60kg approx 14 mT of solar panels which have a surface area of about 14000 m^2. Say launch that thing with an BFR that has a payload bay of 20m hight. Then the solar arrays need to have a wing span of about 700m. Since the individual elements cant be larger than the diameter, say 10m, that are 70 segments. Say we have 2 equal sized 350m panels (one on either side) and you use ISS type solar arrays, that would fill two boxes of about 5m height each.

I admit, I thought that the solar panels alone would be far larger than that. I thought that this alone would invalidate your concept. It is at these considerations borderline possible. But I did not include everything that needs to be included. I did not include the Xenon mass, the mass of the SEP tug structure, any electronics or transformers. No heat control system or other things that are needed.

sources:
https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electric
https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft
https://en.wikipedia.org/wiki/Integrated_Truss_Structure#Truss_subsystems
What you did is assume away several factors: required delta V & burn duration, which determine both required power and required propellant.  The SEP mass fraction of the MCT being identical to the 702SP is not a safe assumption.
« Last Edit: 08/26/2015 01:52 PM by Burninate »

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #494 on: 08/26/2015 02:58 PM »
Ok, lets make a back-of-the-envelope calculation.

A recent all-electric satellite Eutelsat  115 WEST B (2,205 kg) uses about 18 kW (4x 4.5 kW thrusters) to reach GSO. It takes 8 Month, so that is a reasonable timeframe for the not yet crewed MCT. Each thruster weights below 16 kg, say 15. So the mass of the spacecraft without the thrusters is 2141 kg. The solar arrays provide 18 kW of power for the thrusters. Wikipedia says that one gets about 300W/kg and 300W/m^2. That makes 60 kg of solar panels and 60 m^2 area. So the spacecraft without the engines and without the panels is 2081 kg.

Now, scaling that to the mass of the MCT of about 475mT in your design. Then the SEP tug would need 475/2* 18 kW= 4.25MW of solar, or 472/2 * 60kg approx 14 mT of solar panels which have a surface area of about 14000 m^2. Say launch that thing with an BFR that has a payload bay of 20m hight. Then the solar arrays need to have a wing span of about 700m. Since the individual elements cant be larger than the diameter, say 10m, that are 70 segments. Say we have 2 equal sized 350m panels (one on either side) and you use ISS type solar arrays, that would fill two boxes of about 5m height each.

I admit, I thought that the solar panels alone would be far larger than that. I thought that this alone would invalidate your concept. It is at these considerations borderline possible. But I did not include everything that needs to be included. I did not include the Xenon mass, the mass of the SEP tug structure, any electronics or transformers. No heat control system or other things that are needed.

sources:
https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electric
https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft
https://en.wikipedia.org/wiki/Integrated_Truss_Structure#Truss_subsystems
What you did is assume away several factors: required delta V & burn duration, which determine both required power and required propellant.  The SEP mass fraction of the MCT being identical to the 702SP is not a safe assumption.

I know, its a back of the envelope analysis. Its quick and dirty, thats what estimations are for. Its a calculation to see if the concept makes sense in the most simplistic way. I expected that an SEP tug for such a huge payload is physically impossible to launch and I tried to prove that. I did not achieve that goal.
But that is not a prove that the concept works. Its just a failure to prove that it does not work. I hope you understand the difference.

Also, my initial information on Impalers design did only state "from LEO to HEO" which is as fuzzy as it can get and I cant possibly deduce dV requirements from that. I just assumed that GSO is roughly equivalent to what he envisions as HEO.

I still dont like Impalers concept for the other reasons I stated, even if a SEP tug is physically possible.

Offline Manabu

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Re: MCT Speculation and Discussion Thread 4
« Reply #495 on: 08/26/2015 09:00 PM »
Not sure this has been discussed here. Is there a chance BFR gets a grasshopper type development vehicle?
It has been discussed here: Developing BFR reusability first

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #496 on: 08/27/2015 01:02 AM »
I know, its a back of the envelope analysis. Its quick and dirty, thats what estimations are for. Its a calculation to see if the concept makes sense in the most simplistic way. I expected that an SEP tug for such a huge payload is physically impossible to launch and I tried to prove that. I did not achieve that goal.
But that is not a prove that the concept works. Its just a failure to prove that it does not work. I hope you understand the difference.

Also, my initial information on Impalers design did only state "from LEO to HEO" which is as fuzzy as it can get and I cant possibly deduce dV requirements from that. I just assumed that GSO is roughly equivalent to what he envisions as HEO.

I still dont like Impalers concept for the other reasons I stated, even if a SEP tug is physically possible.

Ok, so you admit that you've already decided you don't like my proposed mission architecture before you did any actual research (and regardless of what the research says), and are now trying to 'prove' it is not feasible via some sloppy back of a napkin calculations that seem to be uninformed by ANY actual studies on large SEP vehicles of which their are dozens.  And you expect me to do your research for you by providing these links?  I think not.

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #497 on: 08/27/2015 08:06 AM »
Ok, so you admit that you've already decided you don't like my proposed mission architecture before you did any actual research (and regardless of what the research says), and are now trying to 'prove' it is not feasible via some sloppy back of a napkin calculations that seem to be uninformed by ANY actual studies on large SEP vehicles of which their are dozens.  And you expect me to do your research for you by providing these links?  I think not.

Well in that case, go on. I have nothing to say to you on that level.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #498 on: 08/27/2015 11:28 PM »
Here are some of my initial thoughts on a large SEP tug.  First off Alpha should be around 5 kg/kw by combining 300 W/kg solar and 1.5 kg/kw Nested Hall thrusters with power processing units.  Sizing would be 5.75 MW which is admittedly very big.  29 of the X3 Hall thrusters would be used and the total hardware mass would be some 28 mt, possibly small enough to fit on a F9H launch depending on the packing density of solar arrays.

SEP Propellants would be delivered by a BFR flight which delivers a single propellant unit (likely a series of spherical tanks inside a cage frame) massing 215 mt total or which 183 is propellant and 32 are tanks which is 30% of departure mass.  This would be enough to get do the whole round trip.

Transit times would be around 400 days to spiral up from LEO, the lander would then travel to mars in around 200 days and stay 500 days on the surface, meanwhile the SEP tug would take about 600 days to travel to mars and reach low orbit.  After rendezvousing in Mars orbits the combined vehicle would take just 200 days to return to low Earth orbit due to the greatly reduced mass which will be just in time to leave again.

I'll do a more detailed breakdown and examine the area of panels needed.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #499 on: 08/30/2015 06:47 AM »
I don't see why direct earth return is impossible.

Even if it really is 8 km/s, that's not at all unachievable with chemical -- due to the exponential nature of the rocket equation, that's much easier than the ~ 9.5 km/s for SSTO (which is clearly possible).

If the return delta-v is that high, the vehicle would only ever be fully fueled on Mars, as it doesn't need nearly as much delta-v to get to Mars.

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