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#76 2018-06-06 04:12:32

kbd512
Administrator
Registered: 2015-01-02
Posts: 4,610

Re: NASA's planning for Mars human missions - interesting stuff.

Louis,

Do you mean bouncing off the atmosphere and flying off into space, never to be seen again, or reentering at too steep an angle and becoming an Aurora BFRalis?

That sounds like a kamikaze mission to me.  A reentry like that is an act of desperation because you don't have the dV required for orbital insertion.  If you use electric propulsion, then you don't have to do any of that nonsense.

The design and maintenance of the TPS to protect the Space Shuttle was akin to pulling teeth, but BFR will take the thermal management problems to a new level.  Doable?  Sure, but get your popcorn ready because it could get toasty.

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#77 2018-06-06 05:39:40

louis
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From: UK
Registered: 2008-03-24
Posts: 6,856

Re: NASA's planning for Mars human missions - interesting stuff.

Well Space X seem to this they can do with 2-3 G forces on the crew.  We haven't seen their calculations but I presume they have a few people on board who can calculate these things. I suspect there's going to be an additional element of retropulsion that perhaps people aren't factoring in. I'd also like to add that for many years several people here argued  that the sort of landing now proposed for Mars was not possible - that you needed chutes as well as retropulsion. You never hear that argument anymore. I suspect the objections to Space X's return to Earth will similarly fade away.

kbd512 wrote:

Louis,

Do you mean bouncing off the atmosphere and flying off into space, never to be seen again, or reentering at too steep an angle and becoming an Aurora BFRalis?

That sounds like a kamikaze mission to me.  A reentry like that is an act of desperation because you don't have the dV required for orbital insertion.  If you use electric propulsion, then you don't have to do any of that nonsense.

The design and maintenance of the TPS to protect the Space Shuttle was akin to pulling teeth, but BFR will take the thermal management problems to a new level.  Doable?  Sure, but get your popcorn ready because it could get toasty.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#78 2018-06-06 05:59:59

kbd512
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Registered: 2015-01-02
Posts: 4,610

Re: NASA's planning for Mars human missions - interesting stuff.

Louis,

For a 2g to 3g reentry, they must be using retro-propulsion.  There's no other way to do it.  You either slow your reentry velocity or the acceleration forces go up when you slam into the atmosphere.  I'm not sure who said landing on Mars using retro-propulsion was impossible since that's exactly how the rovers were soft landed, but they were mistaken.  The idea behind the use of parachutes was to obtain a terminal descent solution with less mass.  It turns out that that's either very difficult or impossible.

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#79 2018-06-06 09:14:35

Oldfart1939
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Registered: 2016-11-26
Posts: 2,221

Re: NASA's planning for Mars human missions - interesting stuff.

Louis-
We are dealing with the tyranny of the Tsilkovsky Rocket Equation and Newton's Laws. It's all about delta(V). The returning BFS must use retropropulsion for deceleration in order to get the numbers of 2-3 g's. GW Johnson has elsewhere provided the numbers, and I tend to believe the calculations over and above the hyperbole of some SpaceX promotalk. Or of wishful thinking. Numbers simply DO NOT LIE!

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#80 2018-06-06 13:45:55

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,571
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Re: NASA's planning for Mars human missions - interesting stuff.

At average orbital conditions:  Earth’s V about sun = 29.771 km/s,  Mars’s V about sun = 24.121 km/s.  For an average-condition Hohmann min-energy transfer orbit,  perihelion V = 33.4 km/s,  and aphelion V = 22.4 km/s.  Earth escape = 11.179 km/s (surface),  and Mars escape = 5.0282 km/s.  The one-way transfer time at average conditions is 259 days. 

Returning to Earth,  the perihelion speed of the spacecraft is higher than Earth’s orbital velocity by 33.4 – 29.771 = 3.629 km/s.  This increases due to Earth’s gravity as the spacecraft nears the interface altitude to V = (3.629^2 + 11.179^2)^0.5 = 11.75 km/s at entry interface.  (Worst case is over 12 km/s.) 

If you use a higher-speed trajectory,  the speed at entry interface is much higher.  The 6-or-7-month trajectory is nearer 17 km/s at entry.  This is an off-angle vector addition,  not simple addition. 

I do not have an entry trajectory code,  but I do have a spreadsheet form of the 1950’s warhead entry model based on ballistic coefficient and scale height.  For a more-or-less cylindrical spacecraft 9 m diameter and 48 m long entering at 45 degree angle-of-attack,  the effective blockage area is something like 300 sq.m,  with a drag coefficient that is partly crossflow,  and nearer 1.0 than 0.5 or 2.0. 

The weight at entry interface includes the final touchdown propellant and 50 tons payload weight:  something pretty closer to 192 metric tons.  For that weight on that area at that 1.0 coefficient,  the ballistic coefficient is about 640 kg/sq.m.  The effective “nose” radius for the heat shield shape would be about the radius of the cylindrical shape:  4.5 m.

When I run those through the entry spreadsheet at 11.75 km/s and various shallow angles below local horizontal,  I get nice high end-of-hypersonics altitudes,  but also fairly high peak gees. 

    Angle, deg    peak gee    h, km at M3  (all 11.75 km/s at entry interface)
        1.0      6.04        ~37
        2.0     12.08        ~32
        3.0     17.79        ~28
        4.0     24.72        ~26

One might say just shallow-out to limit gees,  but that does nothing for the risk of bouncing off the atmosphere into space,  while still traveling faster than escape speed.  If I run 0.5 degrees,  I really do get 3.02 peak gees and a Mach 3 altitude ~ 42 km.  But,  Apollo used about 2.3 degrees returning from the moon,  at about 11 km/s,  and saw 11 gees peak. They came in that steep to avoid bouncing off.   

Running a nominal 15 km/s for a faster trajectory,  at an Apollo-like 2 degrees,  I get 19.68 peak gees,  and an altitude at Mach 3 near ~ 32 km. 

I have to conclude a BFS crew will be a lot more likely to see 12-15 gees,  possibly as much as 20 gees for a very small error in angle toward a steeper entry,  depending upon just how fast the entry interface speed is,  and just how much bounce-off risk they wish to take.  Something nearer 2-4 gees seems very unlikely indeed.  Not impossible,  but very unlikely due to the greater bounce-off risk.

Remember,  there is no propellant budget available to reduce the entry speed at interface.  The entire load is consumed by the escape from Mars and the terminal landing burn at Earth,  with only a tiny kitty for midcourse correction.  I have all that reverse engineering posted over at "exrocketman". 

My entry spreadsheet also estimates peak heating rates and the time integral heat accumulated,  but that is another topic for another time.

GW

Last edited by GW Johnson (2018-06-06 13:50:28)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#81 2018-06-06 15:15:26

kbd512
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Registered: 2015-01-02
Posts: 4,610

Re: NASA's planning for Mars human missions - interesting stuff.

Everyone is missing the big picture.  SpaceX must fly 4 tankers for each BFS they send to Mars.  Well, guess what?

The type of craft I proposed can easily be delivered to ISS with 1 cargo flight and then loaded with 1 passenger / cargo flight (for the initial missions) if some space in BFS is reserved for food and water.  The first crew to setup the LOX/LH2 plant is 6 to 12 astronauts.  The BFS-MP to take humans to the surface is strapped to the nose of the ITV-P's rigid module after the crew are aboard.  ISS has a manipulator to do that work.  The BFS-MP and BFS-MC stay on Mars and only fly to and from LMO to pick up passengers or cargo and drop them off on Mars or return them to the ITV-P.  Apart from humans, anything sent to the surface of Mars shall never return to Earth.  The ITV-P will come back, with or without anyone aboard.  The first BFS-MC never leaves the surface, so its engine is a spare for the BFS-MP.  ITV-C's, if anyone is wondering, is just a power and propulsion module with a BFS-MC strapped to its nose.  Again, once the BFS-MC is on Mars, it's never coming back to Earth.  If the vehicle must be cannibalized for spare parts on the surface of Mars or in LMO, then it will be.

Ever after, the ITV-P's passengers and consumables are replenished, a camera inspects it for orbital debris damage, and repair or replacement of damaged components is performed as required.  This is the last step of any mission.  After you return home, you clean (take out the trash, perform PMS on avionics and life support equipment, etc) and repair your ship in preparation for the next crew's mission.  The next crew inspects the ship to determine if anything was missed.  ISS maintains a stock of replacement parts and consumables.  The astronauts aboard ISS maintain and monitor the ship in the absence of a mission crew.  At least three separate crews inspect the ship before each subsequent re-flight.  This is exactly how operations work aboard American submarines and we haven't lost one in living memory.

If you want to carry more passengers and cargo on subsequent missions, then the crew loads passengers and cargo separately.  You don't need to continually send 8 BFS-OT to send 1 BFS-MP and 1 BFS-MC to Mars.  That's 10 launches to accomplish what a maximum of 2 launches can accomplish for 1/5th the price.  Flying more often only makes monetary sense when paying customers are lined up.  Make the price of the trip equivalent to the price of a car and you'll have an endless supply of paying customers.

The modules, solar panels, and other equipment are long duration space flight proven.  The MPD engine is not, but ground testing is complete.  All it takes is a series of tests to confirm long duration function.  At this point, it's no more or less proven to work than BFR.

The MPD thruster's cathode slowly dies from erosion over a period of continuous use, but it's ~20kg for a complete 1MWe MPD thruster vs 227kg for 1 immortal Aerojet-Rocketdyne X3.  If you just replace the cathode, then maybe 2kg.  Carry spare cathodes and it's never going to be a problem.  You need 5 X3's to produce the same thrust and those engines use expensive and highly pressurized Argon / Xenon / Krypton instead of Hydrogen electrolyzed from water.  The more electrical power you make, the more thrust and Isp you get from MPD and the weight of the thruster and output does not scale linearly.

We can do this.  It's the most efficient, therefore least costly, and least hazardous way to get the job done using near term or current technology.

* Substantially lower reentry velocities from LMO mean the BFS-MP and BFS-MC heat shields last longer
* Engines accumulate fewer total seconds of firing time and the firing time that is accumulated is doing useful work
* BFS can be reused as soon as fuel is available
* Spare parts are available from other vehicles on Mars, if required
* Acceleration forces experienced are sedate in comparison to an interplanetary reentry
* Crew arrives in good condition for reentry because they've been in 1g the entire time
* No severe mass constraints placed upon the ITV-P since it's not a rocket upper stage, so systems redundancy and radiation shielding are beefed up for greater protection
* ITV's have enough dV and consumables to abort to Earth without ever landing on Mars, even if they insert into orbit first
* The launch and reentry events are the only really dangerous parts of the mission

Stop trying to do insane things with chemical propulsion technology.  It certainly won't work well and may not work at all.

Just build a BFR that can go to and from low orbit and make reasonable rough field landings at a reasonable cost and with a reasonable level of reliability.  That's hard enough.  Don't complicate matters further with additional capabilities.  Forget about all the nuttier stuff that may somehow be technically possible using chemical propulsion.  It's still insanely dangerous.  Just because you can doesn't mean you should.

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#82 2018-06-06 15:23:04

Oldfart1939
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Registered: 2016-11-26
Posts: 2,221

Re: NASA's planning for Mars human missions - interesting stuff.

GW-

Thanks for once again providing some sanity and hard numbers to what turned into something of a pissing contest over reentry g levels to anticipate. I accurately recalled the 11 g number, but failed on the higher energy return numbers. I

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#83 2018-06-06 17:53:37

louis
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From: UK
Registered: 2008-03-24
Posts: 6,856

Re: NASA's planning for Mars human missions - interesting stuff.

kbd512 wrote:

Everyone is missing the big picture.  SpaceX must fly 4 tankers for each BFS they send to Mars.

Where does this come from. I've seen you write this before but it makes no sense to me. What do you mean?

Can you supply some figures/references?

And even if they do, why is such a big deal for you?  These are reusable rockets we are talking about.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#84 2018-06-06 17:54:56

louis
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From: UK
Registered: 2008-03-24
Posts: 6,856

Re: NASA's planning for Mars human missions - interesting stuff.

Well that confirms what I think. Space X says they are coming in at 2-3 Gs. So they must be using retropulsion. Problem solved.

Oldfart1939 wrote:

Louis-
We are dealing with the tyranny of the Tsilkovsky Rocket Equation and Newton's Laws. It's all about delta(V). The returning BFS must use retropropulsion for deceleration in order to get the numbers of 2-3 g's. GW Johnson has elsewhere provided the numbers, and I tend to believe the calculations over and above the hyperbole of some SpaceX promotalk. Or of wishful thinking. Numbers simply DO NOT LIE!


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#85 2018-06-06 17:57:23

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,856

Re: NASA's planning for Mars human missions - interesting stuff.

I just keep getting deja vu, all those years of being told you can't have a no-parachute landing on Mars. Lots of figures and assertions were made then, but now everyone accepts you can have a retropulsive landing.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#86 2018-06-06 18:56:01

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,741
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Re: NASA's planning for Mars human missions - interesting stuff.

louis wrote:

I just keep getting deja vu, all those years of being told you can't have a no-parachute landing on Mars. Lots of figures and assertions were made then, but now everyone accepts you can have a retropulsive landing.

We have long known no-parachute landing on Mars with retropropulsion is possible. It's just not desirable. I posted this chart several times. It shows total mass to Mars required for 40 metric tonnes payload landed on Mars. Note 23 metre diameter heat shield with supersonic retropropulsion requires 81 mT total mass. Adding a parachute reduces total mass to 78 mT. And this doesn't include a heat shield as narrow as the Red Dragon.
ADEPTsequence.jpg

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#87 2018-06-06 19:36:47

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,856

Re: NASA's planning for Mars human missions - interesting stuff.

1. "Desirable" is meaningless word as far as EDL goes. Doable is the word we need to use I think.

2.  No point in discussing 40 mt payload if the payload is 150 mt.

3.  Complexity can be as deadly to a mission as anything else.

RobertDyck wrote:
louis wrote:

I just keep getting deja vu, all those years of being told you can't have a no-parachute landing on Mars. Lots of figures and assertions were made then, but now everyone accepts you can have a retropulsive landing.

We have long known no-parachute landing on Mars with retropropulsion is possible. It's just not desirable. I posted this chart several times. It shows total mass to Mars required for 40 metric tonnes payload landed on Mars. Note 23 metre diameter heat shield with supersonic retropropulsion requires 81 mT total mass. Adding a parachute reduces total mass to 78 mT. And this doesn't include a heat shield as narrow as the Red Dragon.
http://chapters.marssociety.org/winnipe … quence.jpg


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#88 2018-06-06 20:42:39

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,073

Re: NASA's planning for Mars human missions - interesting stuff.

louis wrote:

Well that confirms what I think. Space X says they are coming in at 2-3 Gs. So they must be using retropulsion. Problem solved.

Oldfart1939 wrote:

Louis-
We are dealing with the tyranny of the Tsilkovsky Rocket Equation and Newton's Laws. It's all about delta(V). The returning BFS must use retropropulsion for deceleration in order to get the numbers of 2-3 g's. GW Johnson has elsewhere provided the numbers, and I tend to believe the calculations over and above the hyperbole of some SpaceX promotalk. Or of wishful thinking. Numbers simply DO NOT LIE!

Retro propulsion is only once in the atmosphere the 11 g's are prior to beginning that entry. The rocket would need lots of fuel to be able to break far enough down the speed before doing the entry from orbit.

Something that would not use as much fuel would be to do a reverse spiral with busts from the engines to begin the gradual slow down if we have the food and oxygen to do so. Combine it with the ion engines firing continously while doing this spiral should do the trick.

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#89 2018-06-06 20:53:44

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,073

Re: NASA's planning for Mars human missions - interesting stuff.

louis wrote:

1. "Desirable" is meaningless word as far as EDL goes. Doable is the word we need to use I think.

2.  No point in discussing 40 mt payload if the payload is 150 mt.

3.  Complexity can be as deadly to a mission as anything else.

RobertDyck wrote:
louis wrote:

I just keep getting deja vu, all those years of being told you can't have a no-parachute landing on Mars. Lots of figures and assertions were made then, but now everyone accepts you can have a retropulsive landing.

We have long known no-parachute landing on Mars with retropropulsion is possible. It's just not desirable. I posted this chart several times. It shows total mass to Mars required for 40 metric tonnes payload landed on Mars. Note 23 metre diameter heat shield with supersonic retropropulsion requires 81 mT total mass. Adding a parachute reduces total mass to 78 mT. And this doesn't include a heat shield as narrow as the Red Dragon.
http://chapters.marssociety.org/winnipe … quence.jpg

The issue with parachutes is that you need to wait until deep in the thicker part of the atmosphere before they can work which allows the ship to continue to pick up speed if not using retro propulsion to slow the ship on the way down before they can open. The hypercone which inflates to give aid for the atmospheric drag slows the ship and its works via expanding once we are in the thicker part to aid in reducing the amount of fuel used as the drag from the hypercone does slow the ship. The bigger it can get the more drag it will have which mean that tonnage can go up and still achieve the landing requirement speed..

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#90 2018-06-06 21:55:44

RobertDyck
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From: Winnipeg, Canada
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Re: NASA's planning for Mars human missions - interesting stuff.

louis wrote:

2.  No point in discussing 40 mt payload if the payload is 150 mt.

I never said a parachute would work for 150 mt. What I did say, several times, is we need something substantially smaller for the first mission. Something about the size of Mars Direct. Actually, we first need a robotic Mars sample-return mission to demonstrate ISPP. That would actually demonstrate all technologies, end-to-end, to land on Mars and return to Earth. Robert Zubrin long argued against robotic sample-return, until I had a one-on-one discussion with him at a Mars Society convention. I pointed out all technologies were carefully tested by Apollo before committing human lives to them. Actually that started with Pioneer 0-4, Ranger 3-9, Surveyor 1-7, Lunar Orbiter 1-5, Mercury, Gemini, then Apollo. Robert Zubrin responded that he would accept sample-return as a technology demonstrator only. I also pointed out that Mars 2020 is stupid; he criticized it first but I said we could send something the size of Mars Phoenix with a rover the size of Sojourner. It could include the return rocket, which would use ISPP and return a capsule directly to Earth like Star Dust or Genesis. Note that after this conversation with Dr. Zubrin, he argued for a Mars mission the size of Curiosity with a rover the size of Spirit or Opportunity. That's larger than I said, but still self-contained, and would demonstrate ISPP. I could list other precursor missions, but most have already been completed. After that we need a small human mission to Mars. Then we need a small human mission to prepare for the first BFS. We need a fuel factory and depot, as well as living quarters with life support for 100 people. All before the first BFS lands on Mars.

Last edited by RobertDyck (2018-06-06 22:00:36)

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#91 2018-06-06 22:43:31

kbd512
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Registered: 2015-01-02
Posts: 4,610

Re: NASA's planning for Mars human missions - interesting stuff.

louis wrote:

Where does this come from. I've seen you write this before but it makes no sense to me. What do you mean?

BFR-OC (Orbital Cargo) Concept: specifically designed to deliver cargo (spacecraft) to orbit
* small habitable section, unpressurized cargo bay
* intended primarily to deliver satellites or other spacecraft components
* first variant to be flown and tested

BFR-OT (Orbital Tanker) Concept: specifically designed to deliver propellant to an orbital propellant depot
* small habitable section, larger propellant tanks
* intended primarily for lunar exploration
* second variant to be flown and tested

BFR-MC (Mars Cargo) Concept: delivers heavy cargo (LOX/LH2 plants, PV arrays, robots, materials, etc) to Mars
* BFR-MC is just an internally reconfigured BFR-MP (has life support, but no seats or very few seats, just cargo space)
* Stays on Mars and continues to fetch heavy cargo from LMO
* third variant to be flown and tested

BFR-MP (Mars Passenger): delivers humans and light cargo (consumables) from the ITV to Mars
* Stays on Mars and continues to fetch people and consumables from LMO (has seats and personal living quarters)
* fourth variant to be flown and tested

ITV-P (Passenger): long duration deep space habitation
* beefed up for durability, redundancy, and protection
* first variant to be flown and tested

ITV-C (Cargo): long duration deep space cargo delivery
* ITV-C is just the engineering section of ITV-P and does not provide artificial gravity
* second variant to be flown and tested

If BFR-OC can launch 150t to LEO, then that's great.  Let's use that capability to build an ITV that enables operations anywhere in the inner solar system and has the mass margins and performance to go where we want to go and come back to Earth.  The ITV-P is an inflatable wheel connected to an ISS node module, except the node module is either a SLS or BFS propellant tank.  The inflatable provides artificial gravity and the node module contains all the stuff required for habitation, power, and propulsion.

louis wrote:

Can you supply some figures/references?

I get my information from Elon Musk.  Skip to 11:43 in the video link below.

YouTube - SpaceX 'BFR' Spaceship: Elon Musk Takes You Under the Hood

The BFR booster with a fully loaded BFS delivers 150t to LEO.  There's no propellant left once BFS gets its 150t payload to LEO.  It's not possible to go anywhere else.  To actually deliver 150t to the surface of Mars, then BFS must perform 4 butt-to-butt refueling operations in LEO, in rapid succession, without damaging either vehicle.  The tanker consumes some of its fuel to get to orbit and the rest is transferred to the Mars bound BFS.  That's 5 launches to LEO for 1 BFS to Mars.  That's $200M spent delivering gas to LEO, each and every time BFS goes to Mars.  Elon Musk wants to send 2 BFS to Mars per opportunity, so $400M.  Now we're up to the price of a Space Shuttle launch.  Then he wants to return the ships to Earth two years later to repeat the process.  That's just spending money on gas to spend money on gas.  If you have the equipment to make gas on Mars, then the gas is free.  A MPD thruster equipped ITV can move an equivalent payload with about 25 times less fuel.  The passengers, their consumables, and the fuel can be delivered in 1 flight.

If you damage either vehicle's exposed body flap or the attached heat shield in the refueling process, then your passengers may not even make it back to Earth alive without a rescue BFR and the two BFR's you damaged are write-offs unless you can repair the heat shield and body flaps in orbit.  Again, just because you can does not mean you should.  Critical thinking is required here.  Your paying customers are sucking down food and water while they're waiting for their fuel.

Governments can afford $200M to deliver gas to space, but mere mortals can't.  If 100 people go, that's $2.5M per person.  Fabulously rich people or governments can afford to go to Mars, but John Q. Public never will at that price point.  With my concept, recurring launch costs are $500K to send 100 people to Mars or $50K to send 1,000 people to Mars.  The government pays the initial costs for the BFS and ITV family of vehicles and writes off the cost as public transportation infrastructure improvement.  All vehicles are reusable, so the government and the service providers makes a profit after the vehicles are paid off.  Ordinary people can afford $50K and so can a business or university.  Now we have a business model, thus the beginnings of a space economy.

louis wrote:

And even if they do, why is such a big deal for you?  These are reusable rockets we are talking about.

The US government, via the US Air Force, is underwriting the cost of the critical technologies for BFR and BFS.  There are economical ways to use the vehicles that squeezes every last dollar of value out of them.  It's not about reuse of the rocket if it's just as expensive to go anywhere as it ever was.  There's still no point in anyone going anywhere except for flags and footprints, but at least we get some science.  Everyone in the world watched Apollo 11.  It was clear that nobody but military test pilots would ever go to the moon, so by Apollo 17, the public was no longer interested.  See the problem there?  No airline corporation in existence flies their jets every two years, either.

The new reusable vehicles just created a new problem (the gas bill) to replace the old problem (the cost of building new rockets) that makes a space economy impossible.  Governments proved it was possible to send people to the moon more than decades ago, yet there are still no businesses that send people into space to make money.  We need to come up with a reality-based business model.  I think it involves some combination of colonization and mining.  To do that, the costs associated with transportation have to plummet.  Reusability is the first step and superb fuel economy is the second step.

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#92 2018-06-07 09:41:20

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,571
Website

Re: NASA's planning for Mars human missions - interesting stuff.

I don't know where the "4 tankers for a BFS in LEO" is coming from.  I saw no clear estimates for that in the version of the film on Spacex's own website.  Some of the pictorials seemed to show perhaps 4,  but no numerical data was given at all. 

When I did the reverse-engineering posted over at "exrocketman",  I got 6 tankers needed to fill the BFS up to a full 1100-ton propellant load in LEO.  Actually 6 and a fraction,  but my analysis is likely slightly conservative. 

And,  I used a full 1100-ton propellant load for a BFS coming home from Mars.  I allowed for a small course correction kitty,  the effects of boil-off,  and a final landing burn,  and used almost all the rest just getting away from Mars.  I found very little capacity for a higher-energy,  shorter-duration ride home.  Louis,  you can forget "3-4 months home";  it'll be closer to the Hohmann 8 months. 

Perhaps 6 might be possible if less than the 50 ton return payload is carried.  But if so,  what's the point of the trip?  Returning crew plus the life support supplies to sustain them,  that's many tons. 

In the updates to my reverse engineering over at "exrocketman",  I addressed multiple issues,  and identified some things Spacex has probably not fully thought through yet.  A biggie was rough-field landing stability with a relatively tall craft.  Another was thrown-debris damage to adjacent items,  from stuff thrown by the jet blast landing and taking off. 

One was how to provide artificial spin gravity by tail-to-tail docking of two BFS's traveling together.  I'm pretty sure they haven't looked at that,  but they should.  So I hope they see the "exrocketman" site as well as these forums, which we know they do look at. 

As for the entry analysis posting I did just above,  bear in mind that most of the delta-vee from entry speed down to zero is killed by aerodynamic drag,  the bulk of it at hypersonic conditions.  For the ride home to Earth's surface,  they will decelerate below Mach 3 in the thin air between 30 and 40 km altitude,  and as they fall further down,  they will continue to decelerate aerodynamically,  to something just above Mach 1 if end-on,  possibly well subsonic if they can survive being dead-broadside.  No engine thrust has been used at all to this point in the entry trajectory.

They fall transonically like that to low altitudes (the last few km),  reversing somewhere on the way down to tail-first,  and start the final landing burn.  If they were falling at Mach 1,  the theoretical delta vee left to kill is only about 0.33 km/s.  You probably ought to at least double that for trajectory adjustments and hover capability kitty for your landing.  I was using something closer to 0.7 km/s,  if memory serves,  but that is explicitly posted over at "exrocketman" as the final landing analysis.

The point of aerobraked landings is not to use fuel until you have to,  use aerodynamic forces instead for the great bulk of your deceleration.  For a BFS landing on Earth returning from Mars,  there is crudely 12 km/s to kill.  All but a fraction of a km/s is killed without running engines at all,  that's ~95%!!!.  So I do NOT see any significant efficiency gain for an object with chutes instead of a jet propulsive landing.  I DO see a utility gain for the retropropulsive landing: no splashdown damage or ocean recovery costs (and they are huge).

As to technological readiness:  PICA heat shields fly all the time now.  High-pressure liquid rocket engines fly all the time now.  A couple of companies are doing retropropulsive landings all the time now.  Only the specific airframe design needs to be verified in test.  One might question the switch to composites,  but many companies are starting to do this now.  Some of the other technologies,  most notably the in-situ propellant production gear,  still needs maturity and verification testing in large scale devices.  But in a handful of years,  this thing could be flying.

Kbd512 is right,  we could do even better with very high-power electric propulsion available.  But it is not.  And I don't think it will be ready before BFS starts flying. 

So,  to me,  the common-sensical thing to do is use what you have right now and get on with the job.  Add the better strokes later as they become available.  I don't see the sanity in waiting several more years for a better engine than the ones we already have.  Just get on with the war. 

That's what NASA can no longer do,  being hamstrung by pork-barrel mandates from an ignorant Congress,  and being infected with the bureaucratic bloat that inevitably strikes all very large organizations.  (Which is why Spacex and Blue Origin need to get on with their wars,  before they grow too large.) 

GW

Last edited by GW Johnson (2018-06-07 10:08:18)


GW Johnson
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#93 2018-06-07 09:56:32

Void
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Re: NASA's planning for Mars human missions - interesting stuff.

I have a lack of understanding of G.W. Johnsons numbers.  I trust them, but cannot comprehend them.
I further trust Oldfart 1939 when he says he trusts G.W. Johnson.

However, I think I see a loophole that exists:

Quote: (From "kbd512")

BFR-OT (Orbital Tanker) Concept: specifically designed to deliver propellant to an orbital propellant depot
* small habitable section, larger propellant tanks
* intended primarily for lunar exploration
* second variant to be flown and tested

Gwynne Shotwell mentioned "Propellant Depot" usage as well in a video.

So, there are two loopholes at least.

1) You could use 1 or more BFS-OT devices to serve as "Tugs" to push a Mars or deep space BFS out, conserving that BFS's fuel, and extending it's capabilities.

2)You can have a fuel depot(s).

*It only makes sense to load a fuel depot 4 to 6 or more times from a BFR-OT, and then join the Fuel Depot to the Mars bound device to:
a) Fill the Mars bound device, or;
b) Take the fuel depot with you, that is if you want to really have propellant options.  (It would be an external propellant tank in this case).

*For "b" BFS will have to be modified, if you are not going to "Butt reload the fuel".  But you could butt load the fuel periodically.  But of course you have to clamp the fuel depot to the BFS.

And for further flexibility, per a post from Oldfart 1939, previously and elsewhere, you may contemplate a fuel depot sent to Mars orbit previously.

And as Kbd512 has informed us, the "BFS-OT (Orbital Tanker) can earn it's keep when not filling depots and performing other tasks, by flying Lunar missions.

I see a whole lot of flexibility and also capability in such a system.

Done.

Last edited by Void (2018-06-07 10:09:47)


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#94 2018-06-07 12:48:51

GW Johnson
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Re: NASA's planning for Mars human missions - interesting stuff.

I really don't think you will see the BFR/BFS system used to go to Mars orbit instead of the surface, whether or not there is some sort of propellant depot there.  The delta-vee to go from interplanetary trajectory to Mars orbit is quite a bit larger than that for their mostly-aerobraked direct landing.  You would have to drastically-reduce payload in order to mave the mass ratio necessary to achieve Mars orbit. 

The same argument applies to the notion of capturing into Earth orbit from an interplanetary trajectory returning from Mars.  The delta vee into orbit is just very much larger than the delta vee to land retropropulsively.  I'm not even sure the mass ratio is there at zero return payload.  (Zero payload is an unmanned ship,  by the way.)  This would be true regardless of whether there was some sort of propellant depot in LEO. 

That's not to say that a propellant depot in LEO wouldn't be a good thing,  because it would.  That's because BFS arrives in LEO with dry tanks when carrying full useful payload.  Docking once with the depot to take on the full load is easier and safer than docking with 6 tankers in rapid succession to take on the same load. 

The same is not true at Mars.  This is because no BFR first stage is required to reach Mars orbit (or escape).  Just need a fully-fueled BFS. 

The tyranny of the rocket equation strikes again.  It ain't symmetrical,  because Mars is lower gravity,  almost no air,  and farther out in the sun's gravitational well.  Earth has much more gravity,  a much thicker atmosphere,  and is deeper in the sun's gravitational well.  Unsymmetrical situation. 

GW

Last edited by GW Johnson (2018-06-07 12:52:24)


GW Johnson
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#95 2018-06-07 13:12:49

GW Johnson
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Re: NASA's planning for Mars human missions - interesting stuff.

A followup about capturing into orbit at Mars rather than direct descent to the surface.

Viewed with respect to the sun,  your velocity in your transfer orbit out at Mars is less than Mars's orbital velocity about the sun.  You need to arrive just ahead of Mars in its orbit,  and let the planet run over you from behind. 

The difference in velocities about the sun contributes directly to your relative velocity (relative to Mars) for orbit or landing,  plus another contribution from potential energy relative to Mars as the distance closes.  Direct from an interplanetary trajectory,  this falls in the 6-7 km/s range,  and needs to be located sort of tangential to the planet to avoid too-steep an entry.

To reach the surface directly,  you can kill all but about the last 0.7 km/s delta vee with hypersonic aerobraking alone,  no jet thrust at all,  which is 90+% of all the delta vee. If you are tiny,  or have some sort of extendible heat shield to reduce your ballistic coefficient,  you will come out of hypersonics at Mach 3-ish (about 0.7 km/s) something like 20 km above the surface.  If you are big and heavy,  you come out of hypersonics about 5 km (or lower) above the surface,  seconds from impact.

The ringsail chutes we have will work supersonic,  but just barely to Mach 2.5.  It takes some some to aerobrake to Mach 2.5 from 3,  some small time to deploy the ringsail,  and some very-significantly-more time for that chute to slow you to about Mach 1 in that thin air.  It won't slow you down any more than that,  in that thin air.  The rest (about 0.25-ish km/s)  has to come from terminal retropropulsion to avoid a crash.

You don't have time for any of that if you are big and heavy.  That takes about 2-3 minutes,  you have seconds,  because you come out of hypersonics so low.  So you just bite the bullet and do 0.7 km/s worth of retropropulsion to land. 

0.7-1.0 km/s vs 0.25 to 0.5 km/s terminal retropropulsion.  Out of 6-7 km/s total.  Big deal,  he says sarcastically. 

Just get on with it.  That's what Spacex has done.  This applied to Red Dragon the same way it applies to BFS.  Or to anything else any of you want to land on Mars that weighs over a ton.  It ain't really the rocket equation,  but it is tyranny of physics.

If instead you go into low Mars orbit,  you must burn an engine for 3-4 km/s worth of delta-vee.  It's vacuum,  there is no aerobraking to supply the force.  3-4 km/s really is quite a lot more than 1 km/s delta vee for the direct landing.  Now do you understand why Spacex designed its BFS the way it did?

Some have promoted repeated-pass aerobraking from the interplanetary trajectory into some sort of Mars orbit.  Yes,  this is a theoretical possibility.  It is not something we have really done before. It is NOT YET ready to apply. 

An observation from an old engineer who spent decades developing things never done before into salable products.  Use only technologies that are off-the-shelf ready-to-apply on your build-and-fly programs.  If you don't,  you will never fly.  There is definitely a place and a need for technology development programs.  But that place has nothing to do with build-and-fly programs.

GW

Last edited by GW Johnson (2018-06-07 13:21:00)


GW Johnson
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#96 2018-06-07 14:59:26

kbd512
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Re: NASA's planning for Mars human missions - interesting stuff.

GW,

To the extent that I understand the math, I've tried to think this all the way through.  Now let's consider how this could possibly work in everyone's favor.  Feel free to smack me over the head with you knowledge of how this works if I get something wrong.

Tankers Cost Real Money

The 4 tankers for 1 Mars bound BFS comes from Musk's presentation video that I provided a link to in response to the question posed by Louis.  I also presume the BFS-OT will have slightly larger tanks to transfer a bit more propellant.  If you realistically have to fly 6 tankers for 1 Mars bound BFS-MP or BFS-MC, then that makes the value proposition even worse and now we're talking about a $500M gas bill penalty associated with Earth bound launch services, which is more than the cost of a Space Shuttle launch.  Once again, there is no possibility of anyone but governments being able to afford such space-fare prices simply to pay for the gas to go to Mars.  This is not practical for paying customers.  The price of the launch doesn't magically come down.  The standing army on Earth still has to be paid with profits from other launch services, but there's not $1B worth of new business from satellites to pay for the program.

Electric Propulsion, The Mission Enabler

My $50K per seat price comes from loading a BFR-MP with 1,000 seats and no cargo because the consumables have to come from Mars if you intend to support that many more people on Mars and is direct evidence of your colony's ability to support another 1,000 mouths to feed.  The BFS stay on both ends (Earth/Mars, Earth/moon, or Earth/asteroid) and never return to Earth once the ITV delivers them to their destination because BFS is optimized as short flight duration surface-to-orbit transport only.  The use of LOX/LH2 (the devil we know), reduced length, 2/3rds payload, much wider landing gear (the split-strut concept I proposed in another thread), and replacement maintenance parts available at the destination are what enable this.  The gas from Mars, the moon, or the asteroid is F-R-E-E after you pay to send the machinery there and the machinery pays for itself with free gas.

Electric Propulsion Actual Progress

Aerojet-Rocketdyne's 200kWe X3 Hall thruster completes testing this year, has already been under test for a year, the results have been published, and they look spectacular.  X3 met or exceeded all expectations.  Orbital ATK's MegaFlex design for 300kWe arrays already completed ground testing.  Advanced MPD thrusters have been in continuous testing by universities and NASA for decades.  Recent advances have solved the fundamental problems with electrode erosion, but the electrode is a piece of Tungsten that weighs a couple of kilograms.

Going To and From LMO Only

1. The ITV-P transports the crew and BFS-MP to LMO.  BFS-MP stays on Mars ever after.  For subsequent missions, BFS-MP is reused on Mars.
2. The ITV-C transports the cargo and BFS-MC to LMO.  BFS-MC stays on Mars ever after.  For subsequent missions, BFS-MC is reused on Mars.
3. When the BFS-MP or BFS-MC arrives at Mars, the header tanks are full, but the main tanks are empty.
4. The Mars fleet of BFS is 2 BFS-MP and 2 to 4 BFS-MC.

How Does This Work?

BFR = Booster
BFS = Ship; MP = Mars Passenger, MC = Mars Cargo; EP = Earth Passenger
ITV = Interplanetary Transport Vehicle; P = Passenger, C = Cargo
{} = {Starting Point}; Earth, LEO, LMO, Mars
() = vehicle(s) at the starting point
*** = transfer or docking/undocking event
---> = transit event

Outbound Transit to Mars:
{Earth}(BFR + BFS-MP)--->{LEO}***{ITV-P}
{Earth}(BFR + BFS-MC)--->{LEO}***{ITV-C}
{LEO}(ITV-P + BFS-MP)--->{LMO}(ITV-P)***(BFS-MP)--->{Mars}
{LEO}(ITV-C + BFS-MC)--->{LMO}(ITV-C)***(BFS-MC)--->{Mars}

Inbound Transit to Earth:
{Mars}(refueled BFS-MP)--->{LMO}(BFS-MP)***(ITV-P)
{LMO}(empty BFS-MP)--->{Mars}
{LMO}(ITV-P)--->{LEO}
{LMO}(ITV-C)--->{LEO}
{LEO}(ITV-P)***(BFS-EP)--->{Earth}

All BFS that go to Mars stay on Mars to continue to fetch passengers and cargo from LMO to take to the surface of Mars.  Subsequent ITV's don't have to carry BFS with them except to add vehicles to the Mars BFS fleet.  Spare parts are shipped, as required, aboard ITV-C.  BFS are only shipped back to Earth for overhaul or replacement, as required.  This works because while the main tanks are depleted by going to orbit, the header tanks required to land again are not.  The efficiency of high powered electric propulsion is such that heavy payloads can be shipped for many times less fuel.  The ITV fuel is Hydrogen, but in practice this means water.  Perhaps residual LH2 from the BFS or an orbital depot can be used as cryocooler technology improves.

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#97 2018-06-07 15:05:33

RobertDyck
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Re: NASA's planning for Mars human missions - interesting stuff.

GW Johnson wrote:

Some have promoted repeated-pass aerobraking from the interplanetary trajectory into some sort of Mars orbit.  Yes,  this is a theoretical possibility.  It is not something we have really done before. It is NOT YET ready to apply.

Of course you realize I argue for developing aerocapture. That has to be demonstrated by robotic spacecraft. We know why Mars Climate Orbiter failed: metric conversion error. NASA fixed that, they now use all metric. So all orbiters sent to Mars should use aerocapture. Build the experience we need for something bigger. Just do it.

Last edited by RobertDyck (2018-06-07 15:06:21)

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#98 2018-06-07 17:14:52

louis
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Re: NASA's planning for Mars human missions - interesting stuff.

kbd,

Thanks for explaining that. Looking at the presentation...

https://www.youtube.com/watch?v=tdUX3ypDVwI

At 27:38 I'm seeing 5 tanker loads represented.

I was slightly confused by your statement that:

"SpaceX must fly 4 tankers for each BFS they send to Mars."

It sounded like you were claiming there had to be 4 separate tankers built in addition to the Mars bound craft. But maybe that was my misinterpretation.

As Musk makes clear in his presentation this doesn't necessarily mean you have to have multiple tankers if you have fully reusable tankers - you could in theory use just one flying 4 or 5 times as necessary. As Musk points out the fuel and propellant are very cheap.

I'm guessing that if you have 2 human and 2 cargo BFRs headed for Mars in the same cycle, it's unlikely they would use just one tanker to make possibly as many as 20 flights in a short period.  But on the other hand you are not going to build 20 tankers - maybe a fleet of between 3 and 5 tankers would suffice. Although, I guess you would have to have at least one sub on the bench as well.



kbd512 wrote:

The 4 tankers for 1 Mars bound BFS comes from Musk's presentation video that I provided a link to in response to the question posed by Louis.

Last edited by louis (2018-06-07 17:16:02)


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#99 2018-06-07 17:58:33

Void
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Re: NASA's planning for Mars human missions - interesting stuff.

GW Johnson

Per your post #94:
Quote:

I really don't think you will see the BFR/BFS system used to go to Mars orbit instead of the surface, whether or not there is some sort of propellant depot there.  The delta-vee to go from interplanetary trajectory to Mars orbit is quite a bit larger than that for their mostly-aerobraked direct landing.  You would have to drastically-reduce payload in order to mave the mass ratio necessary to achieve Mars orbit. 
The same argument applies to the notion of capturing into Earth orbit from an interplanetary trajectory returning from Mars.  The delta vee into orbit is just very much larger than the delta vee to land retropropulsively.  I'm not even sure the mass ratio is there at zero return payload.  (Zero payload is an unmanned ship,  by the way.)  This would be true regardless of whether there was some sort of propellant depot in LEO. 
That's not to say that a propellant depot in LEO wouldn't be a good thing,  because it would.  That's because BFS arrives in LEO with dry tanks when carrying full useful payload.  Docking once with the depot to take on the full load is easier and safer than docking with 6 tankers in rapid succession to take on the same load. 
The same is not true at Mars.  This is because no BFR first stage is required to reach Mars orbit (or escape).  Just need a fully-fueled BFS. 
The tyranny of the rocket equation strikes again.  It ain't symmetrical,  because Mars is lower gravity,  almost no air,  and farther out in the sun's gravitational well.  Earth has much more gravity,  a much thicker atmosphere,  and is deeper in the sun's gravitational well.  Unsymmetrical situation. 
GW

I appreciate the reply, but perhaps I need to communicate a bit more.

1=Least important.
2=Next.
3=Most important.

1) I have presumed that the stated intention of SpaceX to do a Hohmann Transfer with a nasty aeroburn will be done.
So, if somehow a depot were used in Martian orbit, it would be for the return trip.  I guess it's best benefit would be to reduce the amount of propellants that need to be manufactured for the first mission.  However through it is a notion that might have merit, I don't expect that they will employ it.  I just mentioned it to be complete.  That is often how I operate.

2) I think that we agree that a depot in LEO makes plenty of sense.

Why launch a BFS to orbit with 100 or so passengers and wait for 4 to 6 dangerous dockings and fueling actions?  Really dumb and dangerous.

3) I don't see that you studied/noticed the notion of a "Tug" to help the passenger BFS go to Mars.  I have tried to test this idea here before, but it is as if I never mentioned it.  Here it is again.  Take a "Tanker" BFS, and fill it completely.  Take a "Passenger" BFS, and fill it completely.  Join them, perhaps at the near nose airlocks, end to end.  (Other options could be considered).

Use the "Tanker" BFS in LEO, connected to the "Passenger" BFS (In LEO of course), as a booster to get the assembly to some higher Earth orbit.  Disconnect the two, send the "Tanker" BFS back down to Earth (Or LEO for refueling to land), and then light up the "Passenger" BFS.

If this were possible then the capability of the mission of the "Passenger" BFS will have been enhanced, as it will have consumed less propellants to get to high Earth orbit, and so will have more to complete its mission, perhaps faster, and/or safer.

......

Yes, more expensive, but if they are having various BFS ships to move people around from place to place on Earth and also to go to the Moon and back, then every 26 months for the adorable window of Hohmann transfers, some could be put temporarily into this service.


And I am done.

Last edited by Void (2018-06-07 18:13:57)


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#100 2018-06-07 18:28:56

kbd512
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Re: NASA's planning for Mars human missions - interesting stuff.

Louis,

I was tired when I watched it, so 5 tanker flights it is.  GW says 6, I thought I counted 4.  Either way, we're still proving my point.  12 BFR launches to send 2 vehicles to Mars is unsustainable.  Absent real interplanetary transport vehicles that use electric propulsion and artificial gravity, this sort of mission architecture is already flags and footprints.  STS was unsustainable as a function of the program cost.  SLS has simply been entirely unaffordable from the word "go".  The continual use of chemical propulsion for in-space transportation is less affordable on a per mission basis than STS was.  The excitement won't capture public attention for more than the first few missions.  Apollo proved that beyond any reasonable doubt.

The cost of launching doesn't go away because you have a reusable rocket.  It's more affordable than expendable rockets in certain cases, but still solidly within the realm of the extravagantly wealthy and governments.  If BFR was flown in expendable mode, then you'd get enough tonnage in LEO to fly a single mission.  Doing that also defeats the purpose of reusable space launch vehicles.

It's not about the cost of the propellant even though that quickly adds up.  The maintenance forever and always costs real money.  The $50M for each flight means each flight of BFR costs $50M, not $50M to send 1 BFS to Mars.  That's $600M for a single mission to Mars.  I'm trying to lower the operational costs to roughly double that of a two hour 747 flight, on a per person basis.  Normal people can afford to pay those prices and normal people are your recurring revenue stream.  IIRC, 747's cost $24K to $27K per flight hour.  You can think of this BFR and ITV enabled architecture in terms of cost per mission instead of cost per flight hour.  My full mission architecture has a recurring cost of $50K to send a person to Mars when the number of people sent is 1K per mission.  For every hour that a 747 flies, you have tens of hours of associated maintenance that must be performed, roughly on the order of the costs stated, or the jet stops working.  The same is true for launch vehicles.

Void,

An empty Orbital Propellant Depot (OPD) could be delivered to Mars aboard a Mars-based ITV-C.  If Phobos has water on it, then Phobos could serve as a refilling station for the Mars-based ITV-C and OPD.  The ITV-C would take the full OPD back to LMO and serve as a Mars Orbital Station for Mars-bound but Earth-based ITV-P's and ITV-C's.  If inbound BFS or ITV's need to top off their tanks, then they can top off in LMO at the Mars Orbital Station.  Eventually, you only need fuel for one-way transits to or from Mars.  That saves even more tanker flights to LEO if Earth-based ITV-P's and ITV-C's don't have to refuel at Earth.  This is another reason why the propellant selection has to be LOX/LH2.  Water is available lots of places.  CO2 is only available in certain places.  There's also a limitless supply of water further out.  Outside of the main belt, you may need nuclear power.  Anywhere between the Sun and Mars, there's plenty of solar power available from flexible thin film arrays.

If ITV-C's can return water and materials from asteroids to Earth / moon / Mars / Venus, then there are no more tanker flights required for operations and the transportation operation becomes self-sustaining.  Apart from being the elixir of life, water is also the best chemical and electrical propellant available in terms of cost and ease of transport.

The second generation ITV's will use high temperature superconductors and the solar wind for both power and propulsion.  Solar inks woven into the fabric shell of the rotating portion of the ITV will provide emergency power for life support.  These ITV's will make the transits in terms of days, rather than months.  For in-flight emergencies, robotic interceptor vehicles will fly in orbits between the Earth and Mars, chasing down wayward ITV's and dragging them back to Earth or Mars for repair at the orbital stations.  The flight crew will simply enter a code, press a button, and the interceptor will fix their position, plot a course to intercept, fire up its drive coils, and chase down the stricken vessel using multiple gee's of acceleration.  Orbital interceptors might follow ITV's in to assure the craft attain stable orbits.

The second generation BFS will land like an airplane on electromagnetic capture runways that use electromagnets to decelerate the vehicle from Mars local Mach 1 to a standstill using coils in the skid style landing gear.  The same device will accelerate departing BFS after the rocket engine has been run up to full power, much like the EMALS system aboard our aircraft carriers.

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