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#1 2018-10-30 03:35:14

louis
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From: UK
Registered: 2008-03-24
Posts: 7,208

Journey time to Mars...

I've never been quite able to tie down the BFR journey time to Mars...

This Wikipedia page states:

"The notional journeys outlined in the November 2016 talk would require 80 to 150 days of transit time, with an average trip time to Mars of approximately 115 days (for the nine synodic periods occurring between 2020 and 2037)."

https://en.wikipedia.org/wiki/SpaceX_Ma … astructure

An average trip time of 115 days = just under 4 months and the upper limit is 5 months. That seems much shorter than the sorts of times assumed here in discussions of space medicine, gravity effects etc.

Are those figures valid - in particular for the latest BFR?

If so, that's very encouraging I feel.


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

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#2 2018-10-30 15:59:22

GW Johnson
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From: McGregor, Texas USA
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Posts: 4,820
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Re: Journey time to Mars...

Louis,  if you zero-out the payload for a lower ignition weight,  the max loadable propellant weight is a higher fraction of the reduced ignition weight,  leading to higher mass ratio and higher delivered delta-vee. 

I ran a bunch of numbers for BFR/BFS while carrying useful payloads,  and I don't believe anything but something rather near min-energy Hohmann transfer times.  These are average 8 months,  at most 9 months,  and at least 7 months to Mars. 

You simply ain't going to Mars in 5 months or less in a BFS,  except maybe (maybe !!!!) one carrying no payload and not surviving the landing.

It ain't nothing but rocket equation dV = Vex LN (MR) work using published Isp's,  the approximation Vex = gc * Isp,  realism-factored ideal dV's,  and the inviolable constraint that Wpayload + Winerts + Wpropellant = Wignition.  It complicates slightly when you have to make 2 or more burns,  but not a whole lot. 

There's absolutely nothing hard about this.  But it makes rather egregious liars out of a lot of folks blogging on the internet,  or releasing advertising hype.  Or popular-science-type articles. 

Lesson:  run the numbers.  Believe no one.

GW


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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#3 2018-10-30 17:32:02

SpaceNut
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Re: Journey time to Mars...

Higher delta at mars also means higher heating load on the heatshield materials as well....

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#4 2018-10-30 20:23:09

Oldfart1939
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Registered: 2016-11-26
Posts: 2,296

Re: Journey time to Mars...

If we examine Robert Zubrin's table 4.2 for the free return trajectories in "The Case for Mars," we can answer this question posed by Louis. The departure velocity is not the same as deltaV, but close enough for the present discussion.

With a departure velocity of 3.34 km/sec, we have a Earth LEO Hohmann transfer to Mars of 250 days; a departure velocity of 5.08 km/sec, yields a transit time to Mars of 180 days, with an acceptable aeroentry in Mars' atmosphere. Kick up the departure velocity to 6.93  km/sec , and the transit time drops to 140 days, but with a dangerously high atmospheric entry velocity at the destination. Shorter transit times require still higher departure velocities, with nearly impossible conditions for aerocapture.

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#5 2018-10-31 02:35:25

louis
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From: UK
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Posts: 7,208

Re: Journey time to Mars...

I can't argue the rocket equation figures, but I can point you in the direction of this document (see the table on page 12):

https://ntrs.nasa.gov/archive/nasa/casi … 209887.pdf

That gives an outward journey time (in 2020 admittedly) of between 90 and 130 days - so between 3 months and just over 4 months.

How does that relate to what you both are saying?  Are you saying the BFR is travelling at slower speeds than those given in the table?


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

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#6 2018-10-31 07:54:54

GW Johnson
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From: McGregor, Texas USA
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Re: Journey time to Mars...

The answer is "yes".  Far slower. 

And I am not saying it.  The required delta vees,  the rocket's Isp, and its available weight statement are saying it.  It's just physics,  Louis.  Wishful thinking has nothing to do with it.

GW

Last edited by GW Johnson (2018-10-31 07:55:25)


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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#7 2018-10-31 12:18:37

Oldfart1939
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Re: Journey time to Mars...

Louis-

A 180 day transfer is "do-able," since it requires an approximate delta V of 5 km/sec. The rocket equation then determines the mass fraction of the vehicle when the Isp for propulsion is known and multiplied by the gravitational constant. I come up with the vehicle mass of 26% of the fully fueled spacecraft. Bumping this up to a 7 km/sec delta V, the mass of the entire vehicle launched into the Hohmann transfer trajectory is just  hair under 15%. Calculating vehicle mass as representing 10% , that doesn't leave much payload, maybe 5.5%? Using the desired payload of 100 Tonnes, the entire fully fueled vehicle in LEO must be 2,000 Tonnes, of which 1690 Tonnes is a combination of Methane and LOX.

Point of information in these calculations was Musk's stated Isp = 383 sec for the Methylox propellant combination in the vacuum Raptor engine. The above numbers for a 140 day transfer would require 11+ loads of fuel to LEO for said vehicle, based on 150 Tonnes to LEO using BFR/BFS. On the other hand, a transit time of 180 days requires 1/3 the fuel and LOX, or only 4.5 BFS tanker flights to accomplish.

I would personally guess that reduced complexity and fewer fuelling flights would swing the decision to a 180 day mission.

Everyone is free to check my calculations using the Tsilkovsky Rocket equation.

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#8 2018-10-31 19:26:02

kbd512
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Re: Journey time to Mars...

I see that Louis has discovered why we need SEP and LOX/LH2 for in-space propulsion.  The crossover point where SEP produces identical transit times to LOX/LH2 is an electric propulsion system capable of processing 600kWe to 700kWe of input power.  With 30t of Argon and a burnout mass of 100t, you can achieve the same 7km/s dV using Aerojet-Rocketdyne's X3, with a demonstrated Isp of 4000s.  X3 is a 200kWe thruster.  So BFS needs 6 of them and perhaps 2t to 3t worth of thin film solar arrays and deployment structures.  It's just an extra system incorporated into the vehicle.  The major difference is that if the BFS was LOX/LH2 powered, it could go from the burnout of the BFR booster to TMI using LOX/LH2 in conjunction with SEP, without orbital refueling.  There's nothing inherently wrong with orbital refueling, but why would you needlessly complicate the mission when modern technology makes refueling completely unnecessary?

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#9 2018-10-31 19:43:56

SpaceNut
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Re: Journey time to Mars...

Trading the mass of the fuels for the mass of the electrical system....adding complexity just as having multiple bfr refueling did

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#10 2018-10-31 19:58:34

kbd512
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Re: Journey time to Mars...

SpaceNut,

It's a self-contained propulsion system that doesn't require cryogenic fuel transfer in orbit.  The cost of not having to fly 5+ tankers is far in excess of incorporating another system into the vehicle.  Once the vehicle gets to Mars, it has it's own megawatt class solar array to provide electrical power for the cryocoolers to keep the LOX/LH2 cold and operate the LOX/LH2 plant.  At night, BFS can burn a little LOX/LH2 in a gas turbine to maintain power and keep the propellants cold.  Conceptually, there's even a way to recycle some of what was burnt during the previous night since the exhaust product is water.  Mars has lots of water in the ground and Argon in the atmosphere, so that's everything required to return to Earth without the need to operate a petrochemical refinery on another planet.  The propellant depots are useful for other purposes in CIS-Lunar space.  If you want to send something to Mars or Venus in one shot, this is the only practical way to do it without using something mind-blowingly gigantic, like ITS, or nuclear technology we don't have.

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#11 2018-11-01 14:44:46

GW Johnson
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From: McGregor, Texas USA
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Posts: 4,820
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Re: Journey time to Mars...

My two cents' worth is that the thrust level must be high enough relative to vehicle weight to achieve a realistic acceleration.  My figure of merit is 0.01 gee or greater.  The bigger the vehicle,  the bigger the thrust.  The bigger the thrust,  the higher the electric power level. 

I honestly don't know much about current electric thruster technology.  But I get nervous when we talk KW instead of MW.   For that practical acceleration level reason.  The longer it takes to add the delta vee,  the bigger the gravity loss you incur,  and the lower the midpoint speed you reach.

Somebody with better knowledge needs to put numbers to this,  with due allowance for the vehicle acceleration.

GW


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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#13 2018-11-02 13:42:10

Oldfart1939
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Re: Journey time to Mars...

At this point in time, there is only ONE GAME IN TOWN; SpaceX is betting the farm on BFS/BFR. I have considered having SEP as a adjunct to CH4 & LOX, but the technology isn't anywhere close to maturity. So far, there are demonstration projects. If we wait around for SEP, it'll add another 25 years to something that COULD have been done 30 years ago by the then-current technology.

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#14 2018-11-02 13:45:12

kbd512
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Re: Journey time to Mars...

GW,

To begin with, I'm talking about achieving TMI using chemical propulsion or coming close enough that the remainder of the dV increment required is minimal.  If that point wasn't clear before, then it should be now.  All the stuff that NASA wanted a decade ago for a viable high-power SEP-enabled mission that starts in LEO is the sort of stuff we either have today or is in a very advanced state of testing.

The following paper contains the relevant equations for parametric modeling of the problem, but software like MALTO is still required to integrate the thrust produced:

Fast Transits to Mars Using Electric Propulsion

Thereafter, the actual equations for the solution are required:

Study of Earth-to-Mars Transfers with Low-Thrust Propulsion

Fundamentals of Electric Propulsion: Ion and Hall Thrusters

The scripts used in "Study of Earth-to-Mars Transfers with Low-Thrust Propulsion" requires MATLAB.  After reviewing the code, I didn't see anything that couldn't be done using Java or a variety of other languages.

General Remarks

Transit times faster than 100 days that rely upon direct entry would produce vibration sufficient to kill the crew through traumatic brain injury before g-forces could, so any talk of 3 month transits to Mars in BFS using a direct entry flight profile is utter nonsense.  I'm only looking at 6 to 9 month transits with maximum delivered tonnage as a result.

Lockheed-Martin had to completely redesign the pressure vessel for Orion because they had structural failures upon reentry during EFT-1.  NASA even admitted that in a video I watched a few days ago.  Although composite structures can be designed to dampen vibration, carbon fiber has brittle failure modes since the materials tolerate so little elongation before failure.  The stiffness of CF doesn't come without a cost.  BFS is an all-CF composite primary structures vehicle, so SpaceX had better do some serious design analysis and testing.

Remarks on NTR

My prior arguments about reasons for further development of NTR were related to high-thrust and high-Isp nuclear technologies producing vehicle alphas comparable to current SEP technologies.  That means Americium fueled gas cores as a function of the propulsion system mass required to produce favorable neutronics (no reflector required to sustain fission) and the requisite volume of the core to minimize shielding mass.  We've haven't pursued gas core NTR technology since initial experimentation performed by both the US and former USSR in the 1960's.  We have reliable chemical and high-power SEP because that's what NASA pursued.  At this point, I'm only interested in the sort of things we have now.  If focus is permitted to stray, we won't live to see the first mission happen.

Remarks on High-Power SEP

The 600kW to 700kW figure I cited comes from trade studies appearing in a NASA document from a few years back.  The power output level and vehicle alphas (power-to-weight ratios for the fueled vehicle plus payload) for vehicles with total wet masses in the 100t to 150t range.  I'm presuming that the vehicle in question uses LOX/LH2 to achieve or nearly achieve TMI from LEO, whereupon SEP takes over to provide the rest of the dV increment to reach Mars in a timely manner.  The system I proposed for BFS would nominally provide 2MWe at 1AU.

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#15 2018-11-02 14:10:07

louis
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From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

Re the earlier discussion on this thread, I think with full refuelling the outward bound journey time would be around 130 days in 2020...

https://www.nextbigfuture.com/2018/06/s … ation.html

Can those who argue that it is going to be a minimum of 150 days but likely more, explain where the tables are going wrong? 

I am assuming the BFS achieves a Delta V of 6.4 Kps.


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

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#16 2018-11-02 18:07:51

Oldfart1939
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Re: Journey time to Mars...

Louis-
The outbound velocity could be very high, but would make an aerocapture into the Mars atmosphere impossible without a braking burn. The table in The Case for Mars I referenced earlier addresses that very issue. It's PHYSICS and ORBITAL MECHANICS! The rocket equation also points out the fuel and oxidizer issues.

Last edited by Oldfart1939 (2018-11-02 18:09:09)

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#17 2018-11-02 19:40:44

SpaceNut
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Re: Journey time to Mars...

https://www.space.com/24701-how-long-do … -mars.html

https://space.stackexchange.com/questio … te-to-mars

Earth to mars is not a straight line journey for man....

The fastest launch ever (New Horizons) reached about 16 km/s, Atlas V rocket directly into an Earth-and-solar escape trajectory.
The craft flew at 58,000 kph (36,000 mph), and would be able to reach Mars in 39 days when Mars is the closest to Earth in its orbit or 289 days when Mars is at its the farthest.
Rocket: Atlas V (551) AV-010
https://en.wikipedia.org/wiki/Atlas_V


https://en.wikipedia.org/wiki/New_Horizons

payload Launch mass of the probe is 478 kg (1,054 lb)

http://www.pluto.jhuapl.edu/Mission/The … meline.php

as you can see the flight path is curved for quite a long time after launch

new_horizons_flight_path_animation.jpg

Flight to Mars: How Long? Along what Path?


Scale the payload to the size of the bfr and you can get a a true idea of just how small it really is....

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#18 2018-11-03 10:29:48

Oldfart1939
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Registered: 2016-11-26
Posts: 2,296

Re: Journey time to Mars...

Louis-
The Tsilkovsky rocket equation is really tyrannical since it involves an exponential function. I just number-crunched the payload fractions of 3 different delta V scenarios: delta V = 7000meters/sec; payload of 5% (assuming 10% of total vehicle weight is structural); delta V of 8000 meters/sec; payload = 1.8% (same assumption regarding vehicle structure); delta V of 8500 meters/sec; payload = 0.2 %. These are the limits of the CH4/LOX system with an Isp of 383 sec. No matter what you would LIKE, the limits of the physics do not ALLOW the sort of transit times Musk tosses around without some serious modification to the mission architecture.

As GW has stated above, the entry velocities into Mars atmosphere MUST be considered, otherwise the possibility of skipping off into interplanetary space looms for those riding the spacecraft. The numbers in the Zubrin table that I have referenced are for FREE RETURN to Earth, should the vehicle miss the planet due to mistakes in trajectory refinement.

I don't care what Musk SAYS at this juncture, but only what the Tsilkovsky equation tells us what MUST happen and the constraints involved.

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#19 2018-11-03 11:25:18

SpaceNut
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Re: Journey time to Mars...

https://en.wikipedia.org/wiki/Tsiolkovs … t_equation

https://en.wikipedia.org/wiki/Orbital_mechanics

The second equation to a fast transit to mars is the
https://en.wikipedia.org/wiki/Aerocapture
Application of a Fully Numerical Guidance to Mars Aerocapture

https://en.wikipedia.org/wiki/Orbit_insertion

The air break is where the size diameter of the heatshield, how deep into the atmosphere you dive and where the mass come into play.

File:Aerocapture.svg

bigidea.nianet.org/wp-content/uploads/2018/07/Mars-Architecture-Document.pdf

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#20 2018-11-03 11:41:50

Oldfart1939
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Registered: 2016-11-26
Posts: 2,296

Re: Journey time to Mars...

Other considerations are the "human" limitations w/r to gees the body can withstand in an atmospheric entry after being in a weightless condition for X months!
I personally see the transit time being around 160-170 days in order to keep the payload at a reasonable percentage of the vehicle weight minus fuel.

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#21 2018-11-03 13:46:17

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 25,309

Re: Journey time to Mars...

http://nasa.wikia.com/wiki/Spacecraft_propulsion

That g force is just one that we might experience on trying to slow down and may experience if we make multiple plunges in the atmosphere to slow down even more as we will see another on mars final entry to land.

G-FORCE TO MARS

https://en.wikipedia.org/wiki/Atmospheric_entry

https://ntrs.nasa.gov/archive/nasa/casi … 207380.pdf
A Comparative Study of Aerocapture Missions with a Mars Destination

Its not really all that funny but we have been here before :
http://newmars.com/forums/viewtopic.php?id=7806&p=3

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#22 2018-11-03 17:45:52

louis
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From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

Have you looked at this chart?

Are you saying there is something wrong with it?

https://i.imgur.com/vTjmEa1.png


SpaceNut wrote:

https://www.space.com/24701-how-long-do … -mars.html

https://space.stackexchange.com/questio … te-to-mars

Earth to mars is not a straight line journey for man....

The fastest launch ever (New Horizons) reached about 16 km/s, Atlas V rocket directly into an Earth-and-solar escape trajectory.
The craft flew at 58,000 kph (36,000 mph), and would be able to reach Mars in 39 days when Mars is the closest to Earth in its orbit or 289 days when Mars is at its the farthest.
Rocket: Atlas V (551) AV-010
https://en.wikipedia.org/wiki/Atlas_V


https://en.wikipedia.org/wiki/New_Horizons

payload Launch mass of the probe is 478 kg (1,054 lb)

http://www.pluto.jhuapl.edu/Mission/The … meline.php

as you can see the flight path is curved for quite a long time after launch

https://www.theplanetstoday.com/images/ … mation.jpg

Flight to Mars: How Long? Along what Path?


Scale the payload to the size of the bfr and you can get a a true idea of just how small it really is....


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

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#23 2018-11-03 17:58:12

louis
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From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

I've made no claim to be qualified to debate this!  You're not engaging with the stuff I posted.

Can you state explicitly what is wrong in the post that heads up this sub-reddit on Space X:

https://www.reddit.com/r/spacex/comment … _analysis/

Here is the chart the reddit lead post refers to:

https://i.imgur.com/vTjmEa1.png

The chart does not support your conclusions, so I am wondering where you think the reddit guy has gone wrong.



Oldfart1939 wrote:

Louis-
The Tsilkovsky rocket equation is really tyrannical since it involves an exponential function. I just number-crunched the payload fractions of 3 different delta V scenarios: delta V = 7000meters/sec; payload of 5% (assuming 10% of total vehicle weight is structural); delta V of 8000 meters/sec; payload = 1.8% (same assumption regarding vehicle structure); delta V of 8500 meters/sec; payload = 0.2 %. These are the limits of the CH4/LOX system with an Isp of 383 sec. No matter what you would LIKE, the limits of the physics do not ALLOW the sort of transit times Musk tosses around without some serious modification to the mission architecture.

As GW has stated above, the entry velocities into Mars atmosphere MUST be considered, otherwise the possibility of skipping off into interplanetary space looms for those riding the spacecraft. The numbers in the Zubrin table that I have referenced are for FREE RETURN to Earth, should the vehicle miss the planet due to mistakes in trajectory refinement.

I don't care what Musk SAYS at this juncture, but only what the Tsilkovsky equation tells us what MUST happen and the constraints involved.


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

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#24 2018-11-03 18:00:02

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 25,309

Re: Journey time to Mars...

Each color band represents the min to max distance that mars has when it launches. Min being on the upperside of the band and max at the lower of that same color.

you have a zero intercept as the BFR can not do anything until its got some fuel regardless of the amount.

pick the payload and where it intercepts is the time to mars at the minimum distance.

notice that when your distance increases (color Band) the time will be longer for that payload.

Now pick you time and going up on the line notice that the max distance means less payload to mars

carefully reading the link says that 150 t for less than 3 months which means to get there fast means lower payload to mars which is why they are now looking at 100 t to be able to achieve a faster time. to which I think the yellow colored band is that version of the bfr with 6 refills being needed for a mars trip.

notice the other image for titan with regards to how the payload drops with distance....

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#25 2018-11-03 18:28:12

Oldfart1939
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Registered: 2016-11-26
Posts: 2,296

Re: Journey time to Mars...

Louis-

I don't generally debate YouTube video presentations. I debate the absolute physics that apply. It's science, mathematics, and engineering. And you simply asked a question as the original poster. I answered to best of my ability, as did GW. Sorry it doesn't agree with wishful thinking and cheerleading.

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