Author Topic: ITS Spaceship Round-trip each Synod possible?  (Read 3269 times)

Offline 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 »

Online 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. 

Offline 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 »

Online 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?

Offline 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 »

Online 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 »

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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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Offline 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 »

Online 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?

Offline 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 »

Offline 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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Online 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.

Offline 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 »

Offline 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 »

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