### Author Topic: ITS Spaceship Round-trip each Synod possible?  (Read 9675 times)

#### 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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#### 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 »

#### 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 »

#### 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 »

#### 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.

#### 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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#### 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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#### 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 »

#### 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 »

#### 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.

#### 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 »

#### 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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#### 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 »

#### Semmel

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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!

#### Semmel

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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.

#### 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 »

#### 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.

#### Peter.Colin

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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 »

#### 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.

#### 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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