Author Topic: ITS Tanker Rendezvous vs Propellant Depot  (Read 5376 times)

Offline Lars-J

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #40 on: 06/28/2017 05:47 AM »
But you are still putting all the eggs in one basket, even if the total surface area is smaller.

It also causes a bit of a traffic jam, since it forces all spaceships for en entire launch window flotilla to be launched into the same orbital plane.

My point is that there is pros and cons of one central depot. Don't ignore the drawbacks.

Offline intrepidpursuit

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #41 on: 06/29/2017 05:16 PM »
If you build tankers designed to transfer fuel out and BFS designed to take fuel in, then you have two vehicles that share very much commonality that can accomplish the entire mission.

Making the tanker able to take fuel in as well makes it double as a depot to speed the process, but it is trading additional cost and on orbit operations for speed. Does that speed help? They'd have to decide.

Building any dedicated hardware for a depot makes no sense. Putting it closer to escape velocity makes engineering sense but no business sense when you could just launch a few more reusable tankers and actually SAVE time. Plus then the BFS has to have more delta-v to reach said orbit. What does this gain us? Anything at all?

SpaceX wants to build one or one and a half things that are fully reusable and accomplishes the entire mission and seems to believe that size is no object. That thinking is incompatible with a dedicated fuel depot.

Offline Oli

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #42 on: 06/30/2017 12:12 AM »
One advantage of SEP tug to assist with hauling propellant is it reduces the energy needed on Earth.

To fuel up 6 ITS vehicle/tanker launches every synod takes about 1 Megawatt each, or about 6 Megawatts total on Earth to produce methane. With 6 Megawatts on SEP, you can reduce that in half, effectively harvesting the energy in space instead of on Earth.

That doesn't make a difference at first, but imagine, say, 1000 ITSes departing at once to Mars. That's saving you like 3 Gigawatts of surface power on Earth, reducing the environmental footprint of the whole endeavor and exporting that effort to space. And potentially saving money as well.

I would use SEP for Earth-Mars transfer, not only to supply a depot in cis-lunar space. That saves a lot of fuel and the power requirement on Mars would be a lot lower. Moreover, "ITS" would not have to be designed as a deep space vehicle, instead it would serve as a highly utilized shuttle at both ends.
« Last Edit: 06/30/2017 12:14 AM by Oli »

Online Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #43 on: 06/30/2017 02:59 AM »
One advantage of SEP tug to assist with hauling propellant is it reduces the energy needed on Earth.

To fuel up 6 ITS vehicle/tanker launches every synod takes about 1 Megawatt each, or about 6 Megawatts total on Earth to produce methane. With 6 Megawatts on SEP, you can reduce that in half, effectively harvesting the energy in space instead of on Earth.

That doesn't make a difference at first, but imagine, say, 1000 ITSes departing at once to Mars. That's saving you like 3 Gigawatts of surface power on Earth, reducing the environmental footprint of the whole endeavor and exporting that effort to space. And potentially saving money as well.
Using SEP for Earth-Mars transfer means 90 day transfers are not really feasible.

I would use SEP for Earth-Mars transfer, not only to supply a depot in cis-lunar space. That saves a lot of fuel and the power requirement on Mars would be a lot lower. Moreover, "ITS" would not have to be designed as a deep space vehicle, instead it would serve as a highly utilized shuttle at both ends.
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Offline oldAtlas_Eguy

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #44 on: 06/30/2017 03:12 PM »
The key decision item that determines whether a Depot or Tanker only would be better economically is whether the target destination would offload less than a full tanker load of propellant.

In the ITS case multiple tankers are needed such that the target ITS(BFS) is the depot.

In a smaller tug stage than the tanker it would be better to have a depot that is occasionally refueled by a tanker. For lunar operations this may be the case. A depot makes sense when prop is sold to other parties, such as in a rich vehicle environment of tugs and Lunar landers.

But when the prop tanker is a designed integral part of a larger system ITS(BFS) depots just get in the way.

That is also why the Vulcan/ACES distributive launch does not go for a depot but just a tanker/End user target case. A depot adds unnecessary costs for such scenarios.

Offline TrevorMonty

Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #45 on: 06/30/2017 07:41 PM »
Predeployed tankers are way to go for infrequent missions. This used on earth for expedition like trans Antractic crossing in 50s. For frequent travel on same route fuel depots make more sense eg USA route 66.


Offline alexterrell

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #46 on: 06/30/2017 07:53 PM »
If it were a NASA question, then a propellant depot would have the advantage of being fuel-able by multiple providers. NASA could let contracts for the cheapest delivery of fuel, and not have to worry about achieving a 99% launch success rate.

However, for SpaceX, multiple providers isn't an issue. Tanker Rendezvous would be si,pler.


Offline alexterrell

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #47 on: 06/30/2017 07:59 PM »
More interesting: why not a depot on Mars? Or on both?

Let's rethink the whole approach. Let's say that for the first flights the current plan is the best approach.

But then, when scaling up, why not go for this:

- very large space-only transit vehicles, with no volume limitation. 1000 to 10.000 people per transit
- propellant depot both on Mars and Earth, using caught asteroids
- Crew/cargo shuttles both on Mars and Earth

Cons:
- requires large infrastructure upfront, including capturing asteroids to orbit, in-space building, depots, cargo/crew transfer stations, etc
- maintenance of transit ships

Pros:
- very large transit ships could potentially be much cheaper per transit.
- very large transit ships could potentially be safer, example having a full operation room
- if fuel mining from asteroid is cheap enough, faster transit trajectories and larger transit windows
- reduced stress on transit ship components (no hard acceleration from deorbits)

IF Phobos or Deimos contain fuel bearing materials, then they could become the petrol / gas stations of the solar system.

Fill up at Phobos with enough fuel to get to an elliptical Earth orbit AND back to Phobos. With aerocapture at Mars, round trip delta V is about 3,200 m/s. 1,000m/s of that could be done by Mars Orbital tugs.

Offline Peter.Colin

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #48 on: 07/07/2017 08:19 PM »
A propellant depot would we preferred if it is cheaper per volume, than fully (re-)loaded ITS tankers in orbit.

Suppose that as prerequisite you want a BFS to be fueled only once and fully from a fully re-loaded tanker in orbit.
You would need the number of BSF's +1 tanker.
First tanker goes into orbit and stays there.
The second tanker, completely fills up the first tanker (takes multiple launches)  than also stays in orbit.
The third tanker completely fills up the second and stays almost empty in orbit.
To be filled up by the fourth tanker and so on.


If you have a fleet of 10 BFS you would need 11 tankers.
If these 11 tankers are more expensive than a large depot it makes more sense to do a Depot.

Another option is:
Instead of 11 tankers for 10 BFS:
Use 3 tankers and 8 stripped down "Non-Landing tankers".
with no landing legs, no heat shield and less engines.
Maybe the NL-tankers can be made taller for more volume if their reduced weight permits it.


These NL-tankers could also be used in mars orbit, for storing
the produced Methalox fuel. It's more logical than storing all the produced fuel on the surface of Mars, since storage volume on the ground is likely more limited..

Storing fuel in Mars orbit has two main advantages:
- You don't need to store the fuel needed to get this fuel into mars orbit.
- You don't need to land your fuel tanks, so they are cheaper per volume.

Disadvantage: You would need at least one regular tanker ship on Mars to get all the fuel to the NL-tankers in orbit.



« Last Edit: 07/08/2017 08:02 AM by Peter.Colin »

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