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This presentation claims that Space X documentation indicates the average Earth to Mars journey time will be about 115 days - under four months. The BFS will eject out of Earth orbit at a much higher speed than is normal in Mars Missions.
https://www.youtube.com/watch?annotatio … HqRuB87nLA
People here (and elsewhere) seem to assume the transfer time is going to be more like 8 months...so this is quite a game changer if true, isn't it? Far less muscle atrophy for one thing.
Edit: Oops - just realised this refers to the old ITS...so is that faster transfer time no longer on offer?
Last edited by louis (2018-04-24 18:21:54)
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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More fuel, more engines without the mass penalty of max payloads to transport equals a faster time to mars on a direct course but at a big expense of fuel to slow down once near mars to be able to land.
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I ran the numbers for the 2017-presentation reduced-size BFR system over at "exrocketman". At max payload 150 tons, I show a little excess delta-vee capability on the outbound trip that could be used to decrease travel times a little.
I show almost none available on the return trip, at its (reduced) max return payload of 50 tons. But if these payloads are less than max, more propellant is available to speed the trip up. A little bit.
I don't have a way to estimate the higher energy orbits. All I know how to do are min-energy Hohmann transfer orbits. Those can be done pencil-and-paper, which is all I have.
GW
Last edited by GW Johnson (2018-04-25 12:21:57)
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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BFR or ITS obviously has more fuel than it needs to attain orbit from the surface of Mars. What if we kept the BFR's on Mars after we send them there and use them to fly to and from the surface to low orbit?
We could use a cycler that provides artificial gravity from spinning inflatable "wheels" designed to contain 100 passengers per unit. The center sections would contain life support, consumables storage, and docking rings for BFR to offload passengers from Earth. Instead of 100 colonists per trip, we could carry 1,000. To add or subtract passengers, add or subtract habitation rings and add or subtract propulsion modules. It's scalable, whereas BFR and ITS are not.
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Can anyone tell me how the cycler will pass Earth then Mars on every orbit? Earth's year is 365 days, Mars's is 688 days, not an even ratio. What cycler orbit period must we have to ensure those close passes, and just how big is that orbit? I honestly don't know.
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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Wikipedia: Mars cycler
Cycler trajectories between Earth and Mars occur in whole-number multiples of the synodic period between the two planets, which is about 2.135 Earth years. Among the first Earth–Mars cycler trajectories calculated were VISIT 1 and VISIT 2, with cycles repeating every 7 synodic periods or about 15 Earth years.
In 1985, Dr. Buzz Aldrin presented an extension of his earlier Lunar Cycler work which identified a Mars Cycler corresponding to a single synodic period. The existence of the now-eponymous Aldrin Cycler was calculated and confirmed by scientists at JPL later that year: a single eccentric loop around the sun from Earth to the Martian orbit in 146 days, spending the next 16 months beyond the orbit of Mars, and another 146 days from the Martian orbit back to Earth.
For each Earth–Mars cycler that isn't a multiple of 7 synodic periods, an outbound cycler intersects Mars on the way out from Earth while an inbound cycler intersects Mars on the way in to Earth. The only difference in these trajectories is the date in the synodic period in which the vehicle is launched from Earth. Earth–Mars cyclers with a multiple of 7 synodic periods return to Earth at nearly the same point in its orbit and may encounter Earth and/or Mars multiple times during each cycle. VISIT 1 encounters Earth 3 times and Mars 4 times in 15 years. VISIT 2 encounters Earth 5 times and Mars 2 times in 15 years.
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My idea isn't a true cycler, but you can't really call it "My idea" either. Many people have thought of it before me. That is, use aerocapture to enter planetary orbit. A reusable interplanetary craft that goes from Earth to Mars and back, over and over again.
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A rotating cycler could resolve the gee problems. Capsules with stores and passengers would need to catch up to the cycler in solar orbit and then decelerate to match its velocity and spin up to match rotation. Exposure to zero gee would then be limited.
You would need good radiation shelters as missing a rendezvous with one of the planets would mean you would be on the cycler for a very long time.
Last edited by elderflower (2018-04-28 03:17:30)
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The delta-vees to get onto and off-of the cycler at the appropriate times are the same as for any high-energy trajectory, and far larger than for for Hohmann transfer. The advantage of the cycler, once built and on its orbit, is that the craft that transfers to and from the cycler need not be all that large, if the long-term life support is built into the cycler. Quite a bit of effort and resources goes into building such a thing, though.
Now, this cycler must inherently pass close to both Earth and to Mars, in each of its orbits. It will be gravitationally perturbed by those close passages, and will require propulsion and the associated propellant to restore its intended trajectory fairly often. That's some modest delta-vee budget for a rather large item, which is a lot of thrust, and a lot of propellant. If you don't do that, the orbit becomes unusable for purposes of a cycler.
As with all ideas, there's both pluses and minuses. I'm not yet sure we know what they all are yet, since this has never yet been done.
The aerocapture idea in one form is what Musk plans for his BFS. He just goes for direct entry. At Mars, you let the planet run over you from behind, controlling the geometry of that encounter to produce exactly the entry angle geometry you want. This is how all the direct-entry probes have been landed. We have experience with this, and it works.
At Earth, you run into the planet from behind, again controlling the geometry of the encounter to produce the entry angle you want. While we haven't yet done exactly this, it is so close to the Mars scenario that this has been the free return mode in mission plans since before Apollo. Confidence that this will work seems to have been high all those decades since Apollo. Although I'd like to see it done with something unmanned before we risk a crew.
To enter orbit at either end instead of doing direct entry, is another step of increasing precision in the encounter geometry. You have to aerobrake away most but not all the excess energy to capture into an elliptical orbit, and then manipulate that orbit into what you want, which follows up with repeated aerobrakings at perigee, until you finally enter.
Theoretically, this save fuel, but it adds both flight time and some very demanding trajectory control requirements. To the best of my knowledge, this hasn't yet actually been done. I'd like to see some experience obtained with this on unmanned things, before we commit to risking crews with it.
GW
Last edited by GW Johnson (2018-04-28 09:17:06)
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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GW Could a close flyby of the moon deliver a reasonably large part of the delta V required to get to Mars? Also providing braking on return?
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The right answer is "I don't know". My gut feel says no, because the moon is 1/81 as massive as the Earth itself.
Vehicle velocities relative to the sun on interplanetary trajectories are going to be substantially higher than Earth's velocity about the sun. The higher-energy this trajectory (for shorter flight times), the higher the magnitude of this velocity, and in most cases, the more non-co-linear it is with the Earth's orbital velocity vector. That really raises delta-vee very fast.
From a min-energy Hohmann transfer, the difference in vehicle and Earth orbital velocities is the least, and the two are rather co-linear, so the numbers almost subtract algebraically. That delta-vee is a bit over 2 km/s to reach Earth escape speed, a bit over 5 km/s to reach low Earth orbit speed, and a bit over 13 km/s to land directly. Could 14 or 15, if the orbital details are unfavorable.
From a higher energy transfer trajectory, it is really easy to see that landing delta-vee hit 17 (or even more) km/s. Fortunately, there is an atmosphere to get most of that (all but the last ~1 km/s) from aerobraking. Unfortunately, that is inherently a high-gee ride, with a huge risk of bouncing off the atmosphere like a skipped stone on the lake.
Typical delta-vees from escape to orbit speed at the moon are ~1 km/s. Its escape speed is only about 2 km/s. So, I'd guess that swinging by the moon isn't going to adjust your speed by much, although you could get a small angle change that way.
Pencil and paper as I am, I don't have a good way to investigate such questions. That's best done with some sort of orbital computer program these days.
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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Oh, OK. Thanks for the prompt response, GW
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You're quite welcome!
I do this for the fun of it, and because curiosity leads me to investigate.
The abilities are there from 20 years in aerospace defense work doing new product development, mostly in propulsion. It was inherently multi-disciplinary.
But I am also old and obsolete. When I first went to work, we still used slide rules to design airplanes and missiles. Supersonic stuff, it was. Calculators replaced slide rules as a 1:1 drop-in replacement. Desktop computers came much, much later.
GW
Last edited by GW Johnson (2018-04-29 09:30:16)
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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I also used slide rules- still have one for nostalgia purposes. Then mainframes and calculators, then desk top computers and now a phone that does everything. My Markes is seventh edition, held together with sticky tape. Spent most of my career organising suppliers of equipment and then organising the organisers of same. Not a lot of maths involved, so I'm very rusty on that.
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Hi Elderflower:
I wish there was a dial-type cell phone. The push-button phones left me behind once they added the * and # keys.
I have never really understood any of that stuff. Most of today's voicemail systems frustrate me to the point of intense anger. Computers are even more arcane. You can imagine how I cuss this very laptop I am typing on right now.
That image should bring a smile, at least, if not a belly laugh.
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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Well I am now part of the 60's crowd and have been brought up with technology as well as tried to keep up though sometimes its more about making money by planned obsolence rather than something that is an improvement....
The real issue for a transit to mars is the distance to what we can get from an energy source to push man to Mars as fast as possible.
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