Author Topic: ITS Spaceship Round-trip each Synod possible?  (Read 8465 times)

Online Peter.Colin

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ITS Spaceship Round-trip each Synod possible?
« on: 07/07/2017 10:41 AM »
We know that ITS ships will leave Earth every 26 months to Mars.
But what about the return trip?
A lot of people generally assume that the return trip therefore is also every other 26 months.

Is it also possible to refuel and return unmanned as soon as the ship arrives using a "Low energy transfer"?
(fly the location Earth will be in roughly a year later)

So that the same ship is ready to fly to Mars every synodic period, instead of every other Synodic period?

What amount of fuel is needed? When flying back unmanned, slower speed?

Are other flight strategies of returning the ships possible?



https://en.m.wikipedia.org/wiki/Low-energy_transfer


Edit: Changed Tittle to be more descriptive


« Last Edit: 07/08/2017 04:56 PM by Peter.Colin »

Offline high road

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #1 on: 07/07/2017 11:14 AM »
The name of the thread is not related to your first post. Yearly launches would require opposition as well as conjunction launches. Opposition launches have a longer transit time, but allow/require the spacecraft to launch again after a month or so. Their total mission time would be longer than a conjunction mission, so there would be gaps of over a year followed by a gap of less than a year between launches.

For conjunction missions, the ship launching to Mars and the one coming back launch at approximately the same time, pass each other along the way and land at approximately the same time. There is no way a ship can go both ways without waiting for the next launch window.

As for 'slower speed': humans require food, so going faster means saving payload mass but spending more fuel. Returning without or with less people on board will save that fuel. However, the balance will not be shifted dramatically.

Edit: having read your other post in L2, I now get your point: you're talking about launching during conjunction, but returning during opposition. Or maybe even whenever, and taking as long as needed. This doesn't solve the problem however. The travel time will be longer than staying on Mars and leaving at the next conjunction if you use the same fuel or less. So there will always be two spacecraft travelling at the same time.

I have a few other remarks to your L2 post. But considering they are not particularly 'L2 level', I'll post them in the public version of that thread:

http://forum.nasaspaceflight.com/index.php?topic=42053.0

edit: I ended up cutting everything out to make sure I wouldn't end up on Chris' black list. So I replied in L2 anyway. (He already liked the post, so I assume he 'hoped I would be able to let go of' reacting to L2 in a public thread).
« Last Edit: 07/07/2017 12:31 PM by high road »

Offline spacenut

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #2 on: 07/07/2017 11:43 AM »
Musk once said it would take 10 cargo trips for every one human trip.  The cargo can go in one synod, and begin producing return fuel.  The one human ship can then go the next synod, land, refuel, and return.  Depending on how fast the cargo is off loaded, the cargo ships can return after off loading some of their fuel to the passenger return ship.  The humans, make keep their return ship to return anytime during the same or next synod. 

I think for safety and in the beginning, each ship should carry say, 90 tons of cargo, and 10 people.  All identical ships.  If some people need to return to earth, there are spots for them to return.  As the colony is built up, then new ships for 100 colonists can come without cargo. 

Online Peter.Colin

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #3 on: 07/07/2017 01:09 PM »

This doesn't solve the problem however. The travel time will be longer than staying on Mars and leaving at the next conjunction if you use the same fuel or less. So there will always be two spacecraft travelling at the same time.



True, but the real problem is getting getting the ships back as fast as possible for relaunch, not the longer travel time of the empty return ships.

Having them back on earth within a year and a few months is better from a reusability perspective, than having them back within 3 years and a few months.

If the people on mars haven't setup a living quarters and depend on their ship than of course we have no choice but to leave the ship on the surface.

« Last Edit: 07/07/2017 01:14 PM by Peter.Colin »

Offline guckyfan

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #4 on: 07/07/2017 01:33 PM »
It is going to be a mix of different solutions. ITS can send 450t to Mars. That is what Elon Musks calculated the cost/t from. But that mass does not allow for a fast transfer and 1 synod reuse. It will still be more cost efficient than sending 200t on a fast trajectory. It will also be less strenuous on the resources of the settlement, sending one ship back instead of two with lower cargo.

To me the presentation looked like it will be mostly the passenger ships that go fast and return fast. They will be more expensive too.

Regarding that remark by Elon Musk about a ratio of 10 cargo for 1 passenger ship. That was at a time when the goals were given as 100 passengers or 100t. One BFS replaces 4 to 5 of these 100t cargo ships. It is a very rough estimate anyway, not something carefully calculated, so who knows.

Offline M.E.T.

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #5 on: 07/07/2017 01:54 PM »
What I seem to have missed is who actually pays for the cargo flights - and for the cargo itself - to make the colony work? The $200k/passenger cost seems to be for the people transport to Mars only, and maybe whatever personal luggage they take with them.

Who pays for producing the bulk cargo needed by the colony and flying it to Mars?

Online Peter.Colin

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #6 on: 07/07/2017 02:29 PM »
Who pays is a good question.

The first return ship could be loaded full with Mars Samples.
I'd pay for a real Mars stone from the first Mision.
This is probably worth arready as much as the ship cost.

Or selling fuel or water to NASA could be profitable

From a profit perspective its probably best to try not bringing the ships back empty.
And try to utilize a low energy transfer to save fuel/costs.

« Last Edit: 07/07/2017 02:32 PM by Peter.Colin »

Offline high road

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #7 on: 07/07/2017 02:31 PM »

This doesn't solve the problem however. The travel time will be longer than staying on Mars and leaving at the next conjunction if you use the same fuel or less. So there will always be two spacecraft travelling at the same time.



True, but the real problem is getting getting the ships back as fast as possible for relaunch, not the longer travel time of the empty return ships.

Having them back on earth within a year and a few months is better from a reusability perspective, than having them back within 3 years and a few months.

If the people on mars haven't setup a living quarters and depend on their ship than of course we have no choice but to leave the ship on the surface.


This doesn't solve the problem however. The travel time will be longer than staying on Mars and leaving at the next conjunction if you use the same fuel or less. So there will always be two spacecraft travelling at the same time.


True, but the real problem is getting getting the ships back as fast as possible for relaunch, not the longer travel time of the empty return ships.

Having them back on earth within a year and a few months is better from a reusability perspective, than having them back within 3 years and a few months.

Yeah, I meant for low energy transfer, the total travel time of launching instantly is longer than the sum of hanging around on Mars until the next conjunction and the travel time of the Hohmann transfer orbit itself. Add enough power, problem solved. However, having ITS return empty but with full fuel tanks is only in the order of shaving a few weeks off of a Hohmann transfer orbit travel time. No hopping around the solar system just yet. But I let others with more than 'I can do orbital mechanics me' levels of experience do that calculation.

To make it visual: a Hohmann transfer orbit needs you to launch from one planet to the other (no matter what way) about six months before the planets are in conjunction, so you arrive at the destination when they are in conjunction. If you fuel up and leave again, assuming the same fuel and total mass for simplicity, you will end up not at earth but trailing it, because you started 6 months late. The planet now has to catch up with the rocket (the rocket isn't going to stay on Earth's orbit around the sun, but hey, simplicity), which is going to take longer than catching up with Mars, because the rocket is moving faster.

You could slam on the brakes and travel the opposite way, meeting Earth head on rather than chasing it. However, the only point in time when this actually intersects with Earth's orbit at the moment when Earth is actually at that location, is if you start 6 months before conjunction. Anything faster requires more energy. Or less energy, so the earth can catch up to your rocket at a later date than the Hohmann transfer.

Edit: one more orbital mechanics remark: the speed of chemical rockets, or any thrusters we have today, is such that if you would try to chase Earth when starting from Mars, you'll end up going towards Jupiter.
« Last Edit: 07/07/2017 02:43 PM by high road »

Offline AC in NC

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #8 on: 07/07/2017 05:26 PM »
What I seem to have missed is who actually pays for the cargo flights - and for the cargo itself - to make the colony work? The $200k/passenger cost seems to be for the people transport to Mars only, and maybe whatever personal luggage they take with them.

Who pays for producing the bulk cargo needed by the colony and flying it to Mars?

Forgive if this observation is reading too much into, but this kind of question always seems to be premised on Mars Colonization paying for itself. 

IMHO that may ultimately occur but I see Musk funding a lot of it out of revenue streams he will develop from ultra-low internal launch costs.

Offline meekGee

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #9 on: 07/08/2017 08:03 AM »
A few years ago I wrote a simple simulator and played with the orbits.

If you have enough dV, you can launch three months too early, travel a higher energy transfer orbit, refuel very quickly, and fly back, launching about 3 months too late.

It's expensive, but you get your ship back in time for the next synod.

These trajectories result in very high entry speeds.

Whether it is a worthwhile exercise is questionable, but it is possible.
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Online Peter.Colin

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #10 on: 07/08/2017 08:24 AM »
A few years ago I wrote a simple simulator and played with the orbits.

If you have enough dV, you can launch three months too early, travel a higher energy transfer orbit, refuel very quickly, and fly back, launching about 3 months too late.

It's expensive, but you get your ship back in time for the next synod.

These trajectories result in very high entry speeds.

Whether it is a worthwhile exercise is questionable, but it is possible.


Good to know it's at least a simulated possibility to reuse fast ships the next Synod!
Some people wonder why ITS spaceships are so fast, because this sacrifices payload,
Robert Zubrin famously among them.

Would your trajectory take less than a year to go
to Mars and return to Earth or more than a year?

And do you see any possibility that a low energy transfer return trip could also be used to reuse the ship the next Synod?
« Last Edit: 07/08/2017 08:35 AM by Peter.Colin »

Offline high road

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #11 on: 07/08/2017 09:27 AM »
If I get meekGee right, the idea is to launch 9 months before conjunction, and return to earth 3 months after, so about a year for a round trip.

The kicker is here: how many ITS's would you have to fly expendable (drifting in space for all eternity or on their own low energy transfer orbit for several years) to propel a single ITS to the required dV?

Online Peter.Colin

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #12 on: 07/08/2017 11:00 AM »
A hint that it might have been a design requirement from the start to bring the ITS ships back in time for the next Synod. (Delta V return = 7.5-9.0 km/s)
It would be typically for Elon Musk to design a ship which does the seemingly impossible, or highly unlikely, and than to not brag about it:


« Last Edit: 07/08/2017 11:30 AM by Peter.Colin »

Online IRobot

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #13 on: 07/08/2017 11:27 AM »
This is an important topic. A Mars colony will depend a lot on "tourists" that stay for a year. Ideally even below 4 months,  but that is not currently possible.

Otherwise we end up with a 95% male-nerd colony. That's how I see it in my head: nerd males perhaps with some minor autism or social awkwardness. Some sort of Comic-con, perhaps even with females prostitutes.

Bottom line: either there is a 1 year "package" for tourists or this can become a joke. "Go to Mars" or "you come from Mars" could easily become catchphrases to call someone a lonely nerd.

Everybody is talking about the technical challenges but nobody is talking about the social challenges. And my experience in management taught* me that people management is the hardest thing you can face. And I only manage them during working hours! I don't manage their private life! But any plan for Mars colonization must face the social challenges: maintaining male-female ratio, maintaining a healthy social environment, establishing rules (actually, laws), justice, social security, etc.

*edit:typo
« Last Edit: 07/08/2017 04:18 PM by IRobot »

Offline meekGee

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #14 on: 07/08/2017 12:50 PM »
I can't find the sim files again, but I remember turn-around had to be very quick.

You're basically launching 3 month away from the nominal time in both directions  and that is right a the cusp of totally missing the window. Any delay and you get into "might as well wait" territory.

Back when  I did this, I stumbled on some papers by Paul Wooster on Mars trajectories.  Imagine my delight when I found out back then that he was hired into a senior position at SpaceX.

This was at the time people were still arguing that SpaceX's Mars ambitions were a fake-out, so it was a reality confirmation for me.

Back to trajectories - I see the high entry rates as a bigger problem than the dV, because they make Carbon Composite designs more difficult.

dV can be gotten around using aggressive Oberth maneuvers. SpaceX's talk of refueling on the Mars side reinforces that.

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Offline high road

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #15 on: 07/08/2017 02:27 PM »
To meekGee: excellent. Maybe another explanation for the size of ITS?

This is an important topic. A Mars colony will depend a lot on "tourists" that stay for a year. Ideally even below 4 months,  but that is not currently possible.

That's why I said the title does not represent the subject well. We're still talking about one trip per synod. Even meek's idea would have people stay longer, not shorter. Unless they're willing to travel for a year only to stay on Mars for a few days at the most.

Quote
Otherwise we end up with a 95% male-nerd colony. That's how I see it in my head: nerd males perhaps with some minor autism or social awkwardness. Some sort of Comi-con, perhaps even with females prostitutes.

Bottom line: either there is a 1 year "package" for tourists or this can become a joke. "Go to Mars" or "you come from Mars" could easily become catchphrases to call someone a lonely nerd.

So no people doing their doctorate in geology, biology, or whatever new sciences see the light on Mars, funded by government grands and big business looking for new ways to make money? And their, perhaps nerdy, supporting staff who'se presence on Mars is also paid for by the aforementioned scientists? I don't kno aboit you, but around here, girls are.quite well represented.


Quote
Everybody is talking about the technical challenges but nobody is talking about the social challenges. And my experience in management tough me that people management is the hardest thing you can face. And I only manage them during working hours! I don't manage their private life! But any plan for Mars colonization must face the social challenges: maintaining male-female ratio, maintaining a healthy social environment, establishing rules (actually, laws), justice, social security, etc.

Quite right. On a hostile world like Mars, normal earth behaviour gets you killed. That's why the important question is 'where's the money coming from' because that determines the kind of people that are attracted. Luckily for Mars, the long travel and staying times and huge expenses will prevent people without useful skills or enough money to compensate, to get there.

Online Peter.Colin

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #16 on: 07/08/2017 02:50 PM »
@high road, your right about the tittle, changed it.
My first line of reasoning was the Earth is at the same place every year, unlike mars.
So that could mean you have more frequent options for return trajectories.
Also ballistic capture and low energy transfer, could have other possibilities back from Mars than towards.

« Last Edit: 07/08/2017 05:08 PM by Peter.Colin »

Offline Oli

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #17 on: 07/08/2017 05:10 PM »
I can't find the sim files again, but I remember turn-around had to be very quick.

How did your trajectories differ from those that can be found with NASA's trajectory browser?

E.g.

Shortened URL!
« Last Edit: 07/09/2017 03:12 AM by Oli »

Offline Nomadd

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #18 on: 07/08/2017 05:25 PM »
@high road, your right about the tittle, changed it.
My first line of reasoning was the Earth is at the same place every year, unlike mars.
So that could mean you have more frequent options for return trajectories.
Also ballistic capture and low energy transfer, could have other possibilities back from Mars than towards.


You might want to think about that again. It doesn't matter where Earth or Mars are during the year. Only in relation to each other.
« Last Edit: 07/08/2017 05:28 PM by Nomadd »

Online Peter.Colin

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #19 on: 07/08/2017 09:21 PM »
@Oli: Thanks for the link very useful!

So it seems possible for the ships to be back to Earth before the next departure.
It's typical that for all the calculated trajectories that the stay time on Mars is either 30 days or 112 days.

What's unclear to me is whether a re-entry speed would prohibit the trajectory or not?
The BFS can decelerate from entry velocities in excess of 12,5km/s.
I suposse before re-entry it could flip and slow down with its engines first, how much depends on multiple things.


« Last Edit: 07/08/2017 09:22 PM by Peter.Colin »

Offline meekGee

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #20 on: 07/08/2017 09:43 PM »
I can't find the sim files again, but I remember turn-around had to be very quick.

How did your trajectories differ from those that can be found with NASA's trajectory browser?

E.g.

Shortened URL!

I expect they'd be similar.  (I didn't include 30 days on the surface)  Physics is physics.

But, mine didn't have long links that broke the site formatting.
« Last Edit: 07/08/2017 10:53 PM by meekGee »
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Offline Oli

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #21 on: 07/09/2017 03:10 AM »
@Oli: Thanks for the link very useful!

So it seems possible for the ships to be back to Earth before the next departure.
It's typical that for all the calculated trajectories that the stay time on Mars is either 30 days or 112 days.

What's unclear to me is whether a re-entry speed would prohibit the trajectory or not?
The BFS can decelerate from entry velocities in excess of 12,5km/s.
I suposse before re-entry it could flip and slow down with its engines first, how much depends on multiple things.

Minimal Mars departure dV from C3=0 for each opportunity is somewhere between 3.4km/s and 4.2km/s (unless you can do a Venus flyby, e.g. in 2036). Add to that 5.5km/s from Mars surface to C3=0, and you get somewhere between 8.9km/s and 9.7km/s. 9.7km/s is more or less what ITS can do without payload. There won't be enough fuel to slow down to 12.5km/s.

As for reentry velocity, the heating rate is proportional to the cube of the velocity. So for example entering with 12.5km/s instead of 7.5km/s (LEO) results in 4.6x the heating rate. Entering with 18km/s instead of 12.5km/s  means 3x the heating rate, etc. I'm not an expert on heat shields but...they don't seem to be the most reusable hardware in the first place.

2 other points:
- You need a lot more fuel on Mars for fast returns. 2x to almost 3x more.
- ITS refurbishment is more "concentrated" with fast returns.

Anyway, you get my point. I think fast returns, while theoretically possible, won't happen for practical reasons (assuming ITS will happen, if not it's always nice to speculate). Except when Venus is at the right place of course.
« Last Edit: 07/09/2017 03:11 AM by Oli »

Offline Nathan2go

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #22 on: 07/09/2017 05:13 AM »
A few years ago I wrote a simple simulator and played with the orbits.

If you have enough dV, ...

It's expensive, but you get your ship back in time for the next synod.

I'm suspicious that these one synod orbits might be impractical unless they are slow (i.e. 9-10 months each way).

I read an article on cyclers, JBIS-2007, which described a one-synod semi-cycler (with unpowered Mars flyby) in section 3.4, and I don't see why things would change much with a stay of only a couple of weeks.  The semi cycler departs Earth with Vinf of typically 6 km/s but it can be as high as 7.3 (= 13.0 km/s, or V_LEO+2.8 km/s),  partway there, it does a deep-space maneuver of 2.2 km/s, then it arrives at Mars with typical Vinf =4.8 km/s (Vinj= 3.4 km/s) but as high as 4.9 (Vinj= 3.5 km/s), with a 300 day time-of-flight.  The JBIS article lists Vinfinity, but I have converted to Vinjection, the equivalent speed in LEO or low Mars orbit for Oberth burns.

The zero-stay version would depart Mars with Vinf= 4.8 km/s (Vinj= 3.4 km/s), do a deep-space maneuver of 2.2 km/s, then arrives at Earth with Vinf= 6 km/s (Vinj= 12.4km/s), with a 300 day time-of-flight.

I suspect the deep-space burns are only needed to make the unpowered fly-by work, so they could probably be optimized out without much change in the injection velocities.   The article says that 1.2 km/s more delta-V would shorten flight time to 265 days.
---
Anyway, I haven't seen hard number for this one-synod mission, perhaps because it does not fit a NASA mission profile.  So it could be that the 80-150 day transits that Musk mentioned are for 2-synod round trips.

If the one-synod trip really does have a 300 day flight-time, there is a little pressure on the ground crew and spaceports, since all the ship arrivals, refurb, and launches happen in a 3 month window (except the tanker flights could go months early, if an on-orbit fuel depot is added).
[edited, to clarify that a 26 month synod with two 300 day flights leaves a combined 6 months for stay-overs].
« Last Edit: 07/10/2017 01:02 PM by Nathan2go »

Online Peter.Colin

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #23 on: 07/09/2017 09:43 AM »
Interesting, why do you you suspect both have to be slow (10 months each way)?

Is there no possibility you go to Mars fast, and go back slow, so you are still back in the same Synod?

I'm no expert, just asking question. ;-)
This orbital mechanics stuff in hard, taking for example also deep space burns into account.
« Last Edit: 07/09/2017 10:40 AM by Peter.Colin »

Offline high road

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #24 on: 07/09/2017 06:46 PM »
Going slow is counterintuitive here. It takes longer because Mars and Earth are further away from each other, not because the spacecraft is actually "going slower". In fact, the rocket would have to move much faster to catch up with Earth during the same synod, while avoiding to be flung out to Jupiter or dropping towards the sun.

Online ThereIWas3

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #25 on: 07/09/2017 10:01 PM »
With the number of trips Musk is talking about, I would think they will have enough spacecraft for two fleets.  Fleet "A" departs Earth the first Synod, waits on Mars for next opportunity, and returns.  Meanwhile Fleet "B" does the opposite, departing Mars the first Synod.  This way you have traffic in both directions each Synod without having to rush the turnaround, and allowing for repairs, etc.  A little juggling is required the first time, to position the "B" fleet.
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Offline Robotbeat

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #26 on: 07/09/2017 10:11 PM »
Except Musk explicitly mentions a full reuse cycle every synod. You can read it in the recently released white paper version of the IAC presentation.
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Offline Nomadd

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #27 on: 07/09/2017 11:02 PM »
Going slow is counterintuitive here. It takes longer because Mars and Earth are further away from each other, not because the spacecraft is actually "going slower". In fact, the rocket would have to move much faster to catch up with Earth during the same synod, while avoiding to be flung out to Jupiter or dropping towards the sun.
Just to give you a hint of the counter-intuitiveness, slowing down while orbiting the sun would make you go faster.
« Last Edit: 07/22/2017 05:34 AM by Nomadd »

Offline Nathan2go

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #28 on: 07/10/2017 02:56 AM »
Interesting, why do you you suspect both have to be slow (10 months each way)?

Is there no possibility you go to Mars fast, and go back slow, so you are still back in the same Synod?

I'm no expert, just asking question. ;-)
This orbital mechanics stuff in hard, taking for example also deep space burns into account.

Your intuition is correct.  The "Aldrin Cyclers" (also described in the JBIS article I linked above) come in two flavors, each of which visit Earth and Mars every 26 months.  The Up Cycler takes 147 days to reach Mars, and 21 months to get back; the Down Cycler is roughly the opposite.  Again, I would expect that switching from a cycler trajectory to one with briefs stay-overs on each end would cause only a small change in departure/arrival velocity, and probably get rid of the deep-space burns as well as speeding the slow leg a bit. 

Aldrin's Up cycler has an Earth departure Vinfinity is 6.1 km/s (Vinj= 12.4 km/s = Vcirc+4.8 km/s), and the Mars Vinfinity is a whopping 9.3 km/sec (Vinj = 7.1 km/s), which the ITS ship can barely do with only a 130 ton payload.
---
It will be interesting to hear if Musk has an opinion on the issue, but I would think in the early years, most Mars passengers would be planning on a stay of 2-4 years, with only 10% or so being "lifers".  The lifers would presumably prefer a fast outbound, and slow return of the empty ship.  The round trip people might prefer the more balanced 10month/10month out/return, or perhaps 8/12 (assuming that's also possible).
---
With the number of trips Musk is talking about, I would think they will have enough spacecraft for two fleets.  ...  each Synod without having to rush the turnaround, and allowing for repairs, etc.
Well, Musk did an optimistic analysis to show how low the cost might go, and assuming the ship is calendar-life limited to say 21 years, flying each ship every synod  gets you twice as many flights per ship life; this saves about 20% off the average trip cost. 
« Last Edit: 07/10/2017 03:23 AM by Nathan2go »

Offline guckyfan

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #29 on: 07/10/2017 05:20 AM »
Well, Musk did an optimistic analysis to show how low the cost might go, and assuming the ship is calendar-life limited to say 21 years, flying each ship every synod  gets you twice as many flights per ship life; this saves about 20% off the average trip cost.

Which indicates if you can carry 20% more cargo going slow is economical. It also reduces propellant production needs on Mars because of fewer flights.

It seems going fast for passengers and slow for cargo may be a good choice. Also scientists may pay more for fast flights in both directions so occasionally a return would be scheduled for not returning in one synod but providing fast transfer for passengers.

Online Semmel

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #30 on: 07/10/2017 05:53 AM »
Going slow is counterintuitive here. It takes longer because Mars and Earth are further away from each other, not because the spacecraft is actually "going slower". In fact, the rocket would have to move much faster to catch up with Earth during the same synod, while avoiding to be flung out to Jupiter or dropping towards the sun.

I agree.  Any flight time that is not ~8 month takes more energy from the rocket too. Whether longer or shorter doesn't matter. More energy means faster. So if a trajectory takes 10 month instead of 8, it takes more fuel to fly it.
« Last Edit: 07/10/2017 01:36 PM by Semmel »

Offline Robotbeat

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #31 on: 07/10/2017 01:16 PM »
Going slow is counterintuitive here. It takes longer because Mars and Earth are further away from each other, not because the spacecraft is actually "going slower". In fact, the rocket would have to move much faster to catch up with Earth during the same synod, while avoiding to be flung out to Jupiter or dropping towards the sun.

I agree.  Any flight time that is not ~6 month takes more energy from the rocket too. Whether longer or shorter doesn't matter. More energy means faster. So if a trajectory takes 10 month instead of 6, it takes much much more fuel to fly it.
That isn't true. There are complicated routes that are very efficient but which take longer than 6 months.
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Offline Nathan2go

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #32 on: 07/10/2017 01:19 PM »
...Any flight time that is not ~6 month takes more energy from the rocket too. Whether longer or shorter doesn't matter. More energy means faster. So if a trajectory takes 10 month instead of 6, it takes much much more fuel to fly it.
Actually, the minimum energy trajectory is the Hohmann transfer, which takes 258 days (8.4 months); from Zubrin's The Case for Mars, p. 79.  The issues is that the Hohmann transfer rounds trip does not complete in one synod (I don't know the exact total, but the normal "piloted conjunction" mission with 6 month transfers has a 910 day round trip = 1.2 synods). 

The one-synod transfers (including the 10 month transfers) use more propellant because the departure windows are sub-optimal.
« Last Edit: 07/10/2017 01:21 PM by Nathan2go »

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #33 on: 07/10/2017 01:37 PM »
...Any flight time that is not ~6 month takes more energy from the rocket too. Whether longer or shorter doesn't matter. More energy means faster. So if a trajectory takes 10 month instead of 6, it takes much much more fuel to fly it.
Actually, the minimum energy trajectory is the Hohmann transfer, which takes 258 days (8.4 months); from Zubrin's The Case for Mars, p. 79.  The issues is that the Hohmann transfer rounds trip does not complete in one synod (I don't know the exact total, but the normal "piloted conjunction" mission with 6 month transfers has a 910 day round trip = 1.2 synods). 

The one-synod transfers (including the 10 month transfers) use more propellant because the departure windows are sub-optimal.

I actually meant the Hohmann but didnt remember the correct number (apparently). I edited my post accordingly, thanks!

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #34 on: 07/10/2017 01:40 PM »
I agree.  Any flight time that is not ~6 month takes more energy from the rocket too. Whether longer or shorter doesn't matter. More energy means faster. So if a trajectory takes 10 month instead of 6, it takes much much more fuel to fly it.
That isn't true. There are complicated routes that are very efficient but which take longer than 6 months.

I heard about these gravity-well transfers which seem to work if you have very much time at your disposal. Or maybe some Venus flyby. But these windows do not occur every synode or are not practical every time.

Online Peter.Colin

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #35 on: 07/10/2017 05:25 PM »
Except Musk explicitly mentions a full reuse cycle every synod. You can read it in the recently released white paper version of the IAC presentation.

Exactly!

In the paper he also mentions a trip to Mars eventually taking only 30 days.
What trajectory would that be?
I gues a very short trajectory, beginning to approximate a straight line?

If you are able to get to Mars in 30 days, no doubt you can be back before the next synod?
In either slow or fast, measured in time per return trip ;-)

The question is what maximum travel time (as a rule of thumb...) for a trip to mars would allow for getting back to Earth in time for the next Synod?
(Taking all aspects of reasonable practicality into account)


« Last Edit: 07/10/2017 07:49 PM by Peter.Colin »

Offline guckyfan

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #36 on: 07/11/2017 07:45 AM »
Or maybe some Venus flyby. But these windows do not occur every synode or are not practical every time.

These trajectories are not available every synod for Inspiration Mars style free return trajectories. But I do not see why they would not work every synod with propulsion at Mars. They do not require a Venus flyby, though that would be a bonus, available sometimes. They do require dipping down to ~Venus orbit but don't depend on Venus being there at that time.

If I understand correctly, those trajectories can be flown fast and direct to Mars and slow with a long flight back to earth. Or they can be flown slow and dipping down to Venus on the way to Mars and then fast back to earth.

I wonder if they can use the slow way to Mars with cargo and passengers go on the fast path back to earth. That way passengers could use fast transfers both directions but not in one round trip.

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #37 on: 07/21/2017 02:39 PM »


From 40.45

Elon Musk:  "If you can get the ship to and from Mars inside that six month window, than you can get to re-use it twice as often. So there's actually a lot of merit in to be able to get to Mars in under three months"

So round-trip every synod probably is the reason the ship is designed to be that fast.

This quote however doesn't really answer the possibility of fast to Mars and slow cargo-return within the same Synod ... This is quite important to calculate at which point Mars cargo like methalox, water and polymers are less expensive in space (Near Earth) than launching them from Earth to space with its deep gravity well.
« Last Edit: 07/21/2017 10:24 PM by Peter.Colin »

Online KelvinZero

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #38 on: 07/23/2017 11:59 AM »
Someone who knows something about orbits might be able to immediately dismiss this, but I don't suppose there is any possibility of more than one round trip each Synod? I imagine you would be alternating totally different sorts of transfers such as opposition and conjunction, going overboard on one and using the other only for cargo.

If you are getting there and back within 6 months, I suppose another way of getting more use out of the ITS would be lunar missions or LEO tourism. Otherwise it is just sitting around for more than a year.

Offline Robotbeat

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #39 on: 07/23/2017 12:38 PM »
It's take a lot more delta-V, like twice as much as even a really fast transfer.
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Offline Hotblack Desiato

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #40 on: 07/23/2017 05:05 PM »
Interesting, why do you you suspect both have to be slow (10 months each way)?

Is there no possibility you go to Mars fast, and go back slow, so you are still back in the same Synod?

I'm no expert, just asking question. ;-)
This orbital mechanics stuff in hard, taking for example also deep space burns into account.

Your intuition is correct.  The "Aldrin Cyclers" (also described in the JBIS article I linked above) come in two flavors, each of which visit Earth and Mars every 26 months.  The Up Cycler takes 147 days to reach Mars, and 21 months to get back; the Down Cycler is roughly the opposite.  Again, I would expect that switching from a cycler trajectory to one with briefs stay-overs on each end would cause only a small change in departure/arrival velocity, and probably get rid of the deep-space burns as well as speeding the slow leg a bit. 

Aldrin's Up cycler has an Earth departure Vinfinity is 6.1 km/s (Vinj= 12.4 km/s = Vcirc+4.8 km/s), and the Mars Vinfinity is a whopping 9.3 km/sec (Vinj = 7.1 km/s), which the ITS ship can barely do with only a 130 ton payload.
---
It will be interesting to hear if Musk has an opinion on the issue, but I would think in the early years, most Mars passengers would be planning on a stay of 2-4 years, with only 10% or so being "lifers".  The lifers would presumably prefer a fast outbound, and slow return of the empty ship.  The round trip people might prefer the more balanced 10month/10month out/return, or perhaps 8/12 (assuming that's also possible).
---
With the number of trips Musk is talking about, I would think they will have enough spacecraft for two fleets.  ...  each Synod without having to rush the turnaround, and allowing for repairs, etc.
Well, Musk did an optimistic analysis to show how low the cost might go, and assuming the ship is calendar-life limited to say 21 years, flying each ship every synod  gets you twice as many flights per ship life; this saves about 20% off the average trip cost.

That actually solves another problem I have with flights once every synode. And besides, it solves the problem that people who want to leave Mars have to stay on the ITS back home for a long time.

The problem I see is: with the fleet of ITS cycling every 26 monts, they will all stay together for a brief period of time on Mars and a similar period on Earth. During that time, they need to receive maintenance etc. Sure, you can use the production personell for the mainentance, leading to 25 months working on new ITS and spare parts, and 1 month of actual maintenance for the fleet in use. By using the up-cycles and the down-cycles, the time available for maintenance doubles. Cargo-ITS go to Mars via down-cycles (18m trajectory), and allow people to travel back to Earth on a quick 6m trajectory (or even faster). And people who want to go to Mars can use the up-cycles with fast trajectories to Mars and slow ones back to Earth.

This will also help the fuel production and storage on Mars, since it lowers the amount of fuel that needs to be kept in stock on Mars.

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #41 on: 07/23/2017 10:08 PM »
Someone who knows something about orbits might be able to immediately dismiss this, but I don't suppose there is any possibility of more than one round trip each Synod? I imagine you would be alternating totally different sorts of transfers such as opposition and conjunction, going overboard on one and using the other only for cargo.

If you are getting there and back within 6 months, I suppose another way of getting more use out of the ITS would be lunar missions or LEO tourism. Otherwise it is just sitting around for more than a year.

If you can get to mars in under 30 days, like Elon said future ships wil be able to do, you could theoretically do 2 or 3 round trips per Synod, 2 of them in a 6 months window,  with the same ship.
The 3rd trip back could be a long indirect trip.
« Last Edit: 07/23/2017 10:10 PM by Peter.Colin »

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #42 on: 07/23/2017 10:28 PM »
If you can get to mars in under 30 days, like Elon said future ships wil be able to do, you could theoretically do 2 or 3 round trips per Synod, 2 of them in a 6 months window,  with the same ship.
The 3rd trip back could be a long indirect trip.
I was hoping for something less brute force, eg where you at least leave at the right time for whatever transfer type in both cases. I was wondering if by freak coincidence the 6month round trip time Elon mentioned (opposition-class?) would leave time for a slower conjunction-class mission.

Anyway, I guess using it for a couple of cis-lunar flights during the off-season would be a lot less complicated (leo tourism or lunar base). I guess if the trip takes 6 months including return you have about 1.5 years when it is just sitting around.

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #43 on: 09/04/2017 12:49 PM »
Hi just had another random idea. Too small for it's own thread.

A big problem with cost effective mars flights is that due to roughly two years between launch windows, you cannot get many uses out of your ITS before it has to be retired.

Once you get a significant number of flights, how about a new class of ITS which is really just an ITS-shaped DSH with a heat shield. It has just enough engines and tankage to get to orbit, empty.

passengers could dock with it in high orbit. They are packed like sardines in a standard ITS. The DSH version has more space.

A full tanker is used as a booster to push it towards mars, on a trajectory that allows the tanker to aerobrake at earth.

The DSH is aerocaptured at mars but the passengers have to be shuttled down to mars with another ITS. (Alternatively, this version does have engines and tankage, but only equivalent to what an ITS needs to land on mars

The point is that the ITS at each end can be significantly reused. The DSH version could be much less expensive per passenger since it does not need to maintain engines or store fuel. (or alternatively, if it lands at mars, still not as many or as much)

Also, because it's task is much simpler, it may be possible to reuse it a few more synods.

Offline oldAtlas_Eguy

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #44 on: 09/04/2017 07:06 PM »
Hi just had another random idea. Too small for it's own thread.

A big problem with cost effective mars flights is that due to roughly two years between launch windows, you cannot get many uses out of your ITS before it has to be retired.

Once you get a significant number of flights, how about a new class of ITS which is really just an ITS-shaped DSH with a heat shield. It has just enough engines and tankage to get to orbit, empty.

passengers could dock with it in high orbit. They are packed like sardines in a standard ITS. The DSH version has more space.

A full tanker is used as a booster to push it towards mars, on a trajectory that allows the tanker to aerobrake at earth.

The DSH is aerocaptured at mars but the passengers have to be shuttled down to mars with another ITS. (Alternatively, this version does have engines and tankage, but only equivalent to what an ITS needs to land on mars

The point is that the ITS at each end can be significantly reused. The DSH version could be much less expensive per passenger since it does not need to maintain engines or store fuel. (or alternatively, if it lands at mars, still not as many or as much)

Also, because it's task is much simpler, it may be possible to reuse it a few more synods.
The alternate to all of this is that the ITS is likely to have more usage of about 60-90% within cis-lunar space. Using the ITS multiple times in local space (like 20+ times) prior to using the used vehicle on a Mars trip reduces the build rates and the costs for Mars since the cost of going to Mars is the cost of just one more launch of this used vehicle.

Online KelvinZero

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #45 on: 09/05/2017 12:31 AM »
The alternate to all of this is that the ITS is likely to have more usage of about 60-90% within cis-lunar space. Using the ITS multiple times in local space (like 20+ times) prior to using the used vehicle on a Mars trip reduces the build rates and the costs for Mars since the cost of going to Mars is the cost of just one more launch of this used vehicle.
I think you are very likely right.

I still sort of like this ITS+Tanker+ trajectory that immediately recovers Tanker by skimming earth. We will have to think up some use for that :)

Offline QuantumG

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #46 on: 09/05/2017 12:34 AM »
Not yet mentioned on this thread, as far as I can see. Fast transfers are good for mitigating both radiation and zero-g exposure concerns. Also this plot.




Jeff Bezos has billions to spend on rockets and can go at whatever pace he likes! Wow! What pace is he going at? Well... have you heard of Zeno's paradox?

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #47 on: 09/06/2017 08:32 PM »
Elon has previously confirmed out-of-synod return trips being possible with the 12m ITS.

How feasible is a longer trajectory of maybe 3-4 months from Mars, past the sun (maybe even a Venus flyby for sightseeing) and back to the earth? Or the other way around. E.g. 1 fast trip, then two slow. Perhaps cargo only in the 4 month trips.

That could increase lifecycles from approximately 15 to 45 uses, which would significantly reduce the number of ITS ships needed to build the colony and leave more money for building the colony.

Offline Rei

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Re: ITS Spaceship Yearly Mars return possible?
« Reply #48 on: 09/29/2017 02:04 PM »
Who pays is a good question.

The first return ship could be loaded full with Mars Samples.
I'd pay for a real Mars stone from the first Mision.
This is probably worth arready as much as the ship cost.

There's lots of different ways to help pay for offworld settlement activity.  This (ch. 8, p.213-228) is for Venus, but a lot of the same aspects can also apply to Mars as well. As for the specific one mentioned above: certainly public sales of mineral sample sales are among the ways for a colony to earn income. An even larger market is if you can return minerals that are workable in the super-premium end of the decorative stone market, for a low enough price (reasonable hardness / workability / aesthetics). "Oh, your countertop is made from brazilian agate?  Mine is made from polished basalt from freaking Mars.;)  It's not a limitless market, but when you're talking about the sort of payload return costs that SpaceX is, there's potential.

That said, there's no question that someone is going to have to put forward the capital costs. In Musk's previous IAC presentation, they left it as a big blank as to who was supposed to pay for it. In the current one, it sounds like SpaceX themselves thinks that they can cover the initial capital costs.

Offline Nathan2go

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #49 on: 10/01/2017 06:36 PM »
Hi just had another random idea. Too small for it's own thread.

A big problem with cost effective mars flights is that due to roughly two years between launch windows, you cannot get many uses out of your ITS before it has to be retired.

Once you get a significant number of flights, how about a new class of ITS which is really just an ITS-shaped DSH with a heat shield. It has just enough engines and tankage to get to orbit, empty.

passengers could dock with it in high orbit. They are packed like sardines in a standard ITS. The DSH version has more space.
...
The point is that the ITS at each end can be significantly reused. The DSH version could be much less expensive per passenger since it does not need to maintain engines or store fuel. (or alternatively, if it lands at mars, still not as many or as much)
...
There are two issues: passenger volume, and ship reuse.

For ship reuse, maybe you make the ship as two modules: a passenger compartment, and a separable propulsion module.  The propulsion module, with an added nose cone (with nose tank for balance at landing), could be used as a tanker.  You'd use it as a tanker for say 50 flights, then convert it to passenger duty, for 10 more flights to complete its service life.   

Musk said last year that the tanker was the same as the ship, but filled all the way with propellant: I don't think this has to be done this way.  If the tanker propellant volume is the same as for the passenger ship, the payload capacity only drops a bit (7% ?), since the 2nd stage gross mass is 85% propellant anyway, and reducing the mass of the 2nd stage will boost the performance of the 1st stage.

Or maybe you use the ship for monthly Lunar tourism flights for a few years to improve utilization.

As far as having the passengers ride a tightly packed ship to orbit, then meet a larger ship there (i.e. mother ship):  yes, it "feels" right.  But in practice, there are many problems.  If the larger ship has more mass, then perhaps it will need in-space construction (e.g. maybe it launches with the big passenger volume empty, and the interior features get installed through a large cargo hatch), which implies the in-space labor must be cheap.  Also, maybe you need a dedicated flight to load consumable, in addition to the passenger flight.  Also, if it is too heavy to land, it must receive all maintenance while in orbit.

Note that is not a very effective way to reduce the amount of fuel that gets lifted off planet surfaces.  Starting in low Mars orbit, you still need a methalox burn with mass_ratio = 2 to return to Earth.  If you get the fuel from Mars surface, it's about a 2x savings if the ship starts in orbit instead of on the surface; easy to eat up that savings with a heavy ship.  So returning spacious (heavy) ships will still take a lot of fuel. 

The mother ship idea gets a lot more appealing when the mother ship is nuclear powered.

Online KelvinZero

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #50 on: 10/01/2017 10:59 PM »
...
The mother ship idea gets a lot more appealing when the mother ship is nuclear powered.

Hi Nathan,

Just to clarify, the central goal I was aiming for was to solve the problem of a super expensive vehicle only being reusable every 2 years.. so I was setting out to make that bit as cheap and low maintenance as possible.

My solution was an interplanetary vehicle that was little more than an empty husk of a BFS.. a BFS with no engines or fuel tanks. Just one big pressurised volume inside the exact same aerodynamic hull.. It is only bigger in the sense of having more living space because you took the tanks out. All it has is a heat shield to brake into orbit at the other end and some minor rockets to correct its trajectory.

The expensive part would be a conventional tanker as Elon Musk has presented. This aerobrakes back to earth immediately after pushing the husk ship on its way, so it's part of the mission is mere days and it can immediately  be used again.

Offline Rei

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #51 on: 10/01/2017 11:41 PM »
Quote
Just to clarify, the central goal I was aiming for was to solve the problem of a super expensive vehicle only being reusable every 2 years

Isn't the solution simply "have more destinations", as has been discussed?  Earth-Earth, Earth-Moon, and Earth-Venus, etc.  So long as a given craft is multipurpose and any internal reconfigurations for different missions don't require an multiyear retrofit....  A retrofit that takes a couple days, weeks, even months would be fine, so long as the craft itself can handle different missions.
« Last Edit: 10/01/2017 11:50 PM by Rei »

Online KelvinZero

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #52 on: 10/01/2017 11:54 PM »
Quote
Just to clarify, the central goal I was aiming for was to solve the problem of a super expensive vehicle only being reusable every 2 years

Isn't the solution simply "have more destinations", as has been discussed?  Earth-Earth, Earth-Moon, and Earth-Venus, etc.  So long as a given craft is multipurpose and any internal reconfigurations for different missions don't require an extensive retrofit....
That is true.. it is not 2 years that the vehicle is out of use, it could be as low as 6 months if we have 3 months both ways. Still, 6 months is a lot longer than typical mission times of a day to LEO tourism or a week to the moon and back. (passengers to the moon could stay as long as they like, but it is cheaper to reuse the BFR immediately bringing other passengers home)

This is also dedicated to just one purpose, shovelling passengers to mars as cheaply as possible, and it is not something you would consider until a long time in the future: when you had ITS being used purely as shuttles around mars, and so also repaired and serviced at mars.

Offline wes_wilson

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Re: ITS Spaceship Round-trip each Synod possible?
« Reply #53 on: 10/02/2017 03:33 AM »
Wonder how the return trip time would improve if:
- a couple of the tanker BFS's were taken to Mars in addition to a passenger ship (these could remain for reuse at mars)
- passenger ship flies into mars orbit and is refuelled by the tankers (just like at earth)
- BFS leaves from Mars orbit for Earth with full tanks of fuel

Basically all the same process they're proposing at Earth; just without the need for BFR due to lower gravity.



 
@SpaceX "When can I buy my ticket to Mars?"

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