Author Topic: ITS Tanker Rendezvous vs Propellant Depot  (Read 6961 times)

Online sanman

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ITS Tanker Rendezvous vs Propellant Depot
« on: 06/18/2017 03:52 PM »
What are the pro's and cons of having a propellant depot in LEO, versus just purely doing orbital rendezvous between tanker and Mars ship?


One the one hand, Musk has pointed out that with 2 years between synod/departure-cycle, you've got plenty of time to do tanker rendezvous flights with Mars ship.

Perhaps in the early phase a depot wouldn't be so useful. But if you're scaling up to many Mars ships per synod in the long run, then wouldn't it make more sense to have a propellant depot in orbit, to avoid complications  and buffer against shortfalls due flight delays, etc? A depot might allow you to get by with a smaller proportion of tanker ships. Plus it would minimize docking events for the Mars ships, to minimize the risk/hazard to them.

If the propellant depot is methane-LOX, then what are the technical challenges?
Can solar arrays provide enough power to cool the methane & LOX to avoid boiloff?

Would it be possible for a propellant depot at a LaGrange point to fit into the ITS architecture?

Offline spacenut

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #1 on: 06/18/2017 04:46 PM »
To me, propellant depots would be better overall.  A Mars trip would during a timed restricted synod would require the tanker trips to be on time with no problems.  What if there is problem, weather where ever it launches from, an explosion, an engine failure, etc.  The ITS spacecraft would have to wait for fuel, or be stuck in orbit until it could get enough to land. 

I think during the 18 months Mars is further away waiting for the shortest launch synod, a fuel depot, or several fuel depots could be filled by anyone wanting to either go to Mars or governments wanting a piece of the action helping fuel these depots.  They could be filled by any number of rockets that wanted a stake in Mars colonization.  ESA, Russia, China, India, Japan, NASA, other private companies.  Any number of existing rockets could fill the tanks during the off synod. 

Then ITS can take off, dock, fill up, and go.  Then the next one can do the same. 

Maybe SpaceX wants to develop a tanker version so they will not be dependent on others.  Maybe they will have tanker flights from Boca Chica since there is an LNG facility proposed at Brownsville, and ITS flights from the cape or other launch site.  Seems they should have at least two launch sites, one for humans and cargo, one for tankers.  Seems more efficient.  Cost for duplicate facilities would be a problem without some government help, or other private companies help. 
« Last Edit: 06/18/2017 04:47 PM by spacenut »

Offline rakaydos

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #2 on: 06/18/2017 05:55 PM »
A dedicated propellant depot is a dedicated, specialized design, when tanker rendevous is "good enough" to start with.

Perfect is the enemy of good, and SpaceX is balancing "get it done" with "in our lifetimes", resulting in a few inefficencies that they can overpower now and iron away later.

Online Semmel

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #3 on: 06/18/2017 06:49 PM »
The depot would be at one specific orbital planes. One, not necessarily optimal for Mars insertion. When using the tankers, the orbital plane can be chosen case by case. Best option of both worlds would be in my opinion to launch the tankers first, filling up one tanker and cycling one other. When full, ITS launches and is refilled by the tanker in orbit. This minimises crew transfer time to Mars, utilises an optimal Mars insertion vector and does not require dedicated designs.

Offline AC in NC

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #4 on: 06/19/2017 04:08 AM »
Do we have a Deport primer here anywhere?  Although not for the short-term, there are so many compelling opportunities.  I just don't have any sense of how realistic some of them are with respect to long-term depot storage and remote depots like on Mars Orbit.

Online AncientU

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #5 on: 06/19/2017 11:59 AM »
The depot would be at one specific orbital planes. One, not necessarily optimal for Mars insertion. When using the tankers, the orbital plane can be chosen case by case. Best option of both worlds would be in my opinion to launch the tankers first, filling up one tanker and cycling one other. When full, ITS launches and is refilled by the tanker in orbit. This minimises crew transfer time to Mars, utilises an optimal Mars insertion vector and does not require dedicated designs.

That is if you only have one depot.  Not sure why singular is so commonly used with depot...
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Offline spacenut

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #6 on: 06/19/2017 12:52 PM »
My thinking is, once SpaceX gets the BFR/ITS launched and in testing stage, more governments and entities may want to contribute to a Mars colonization/outpost/research center campaign.  The best way, especially for NASA is design a propellant depot using existing rockets from all American competitors, and build a depot with docking, fueling etc connections all standardized so anyone can fill the station, and get a seat on the ITS to Mars.  The station could be placed in the best LEO position for ITS to dock and fuel. 

NASA could use huge 8.4m tanks launched and assembled in space for a very large storage facility.  Shading, solar power, and refrigeration equipment can be maintained by NASA or international partners like the ISS.  Russia may have to launch from South America to access the station since it would not be in their high latitude launch. 

This station with many 8.4m tanks could fuel multiple ITS for a synod trip to Mars and can be fueled on a routine (monthly) basis by all parties during the 18 month off synod. 

It might not start out that way.  It will probably start as SpaceX envisions, alone.  However, if others want part of the action, a fuel depot will be a necessity.  A fuel depot that can be fueled up on a continuous basis can support a moon program as well as Mars.  Win-win for everyone.  Less cost for SpaceX alone.  Standardize on unleaded metholox and go. 

Online AncientU

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #7 on: 06/19/2017 05:33 PM »
8.4m is not huge when refueling a 12m vehicle.
Huge is when ITS launches a depot that can refuel several (3-5 or more?) spaceships -- something of order 10,000t capacity.
« Last Edit: 06/19/2017 05:34 PM by AncientU »
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Offline Eer

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #8 on: 06/19/2017 05:52 PM »
The original post doesn't distinguish between various sources of propellant for the depot, but the discussion so far has dealt with propellant launched from earth.

SpaceX' colonization plans are such that I wonder if they cross the breakeven for comet mining for propellant to fill the various depots envisioned (Earth, Mars, Lunar, Jupiter??) to support high traffic (75 years from now) destinations.

I'm NOT talking about doing so in the next decade - I don't expect SpaceX to be launching hundreds or thousands of BFS per synod for a few years.  But when they do ...

If this is too off topic, moderators are welcome to move it to its own thread.

I just don't think heavy lift of propellant should be the focus of discussion for how to fill depots for high volume colonial migration or industrial traffic.

Offline DusanC

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #9 on: 06/19/2017 06:20 PM »
IMHO there would be no special orbital depot but there would be ITS tanker serving as depot because of unification savings.

IMHO there would exist 1 type of ITS with same tanks, engines, structure but with following versions of upper ,,cargo'' part:
1. Human transport
2. ISRU
3. Tanker
4. Cargo
.
.
.

So in this configuration you'd have ITS tanker orbiting Earth, getting filled up with multiple ITS tankers, and when it gets filled up ITS Human transport would dock to it and get refueled for Mars transit. On Mars there would be multiple ITS cargo and ITS ISRU already filled up with fuel waiting for ITS human transport.

Unification! Same type of ship serving multiple purposes.

Offline CuddlyRocket

Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #10 on: 06/20/2017 04:29 AM »
If you're launching propellant from Earth into LEO in order to refuel a Mars-bound spacecraft ('the ITS') why introduce the additional inefficiencies and points of failure of transferring the propellant to an orbital depot first and then transferring it to the ITS when you can simply transfer it directly?

An orbital depot could continue refueling the ITS if there's an interruption to the launching of propellant from Earth. But you would have to dedicate tanker launches to stocking the depot in the first place (so the ITS is not as refueled as it would otherwise have been) and once launching from Earth resumes you'll have to dedicate some tanker launches to re-stocking the depot rather than refueling the ITS.

I don't think the trades work in favour of the depot. One exception may be is if the ITS suffers a large loss of propellant very close to the time for Mars departure. However, would you really want to send to Mars a ship that's just suffered such a serious failure?

Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #11 on: 06/20/2017 05:31 AM »
I was just doing some math. One of the best reasons to use a depot is you can transport by various means.

For instance, you could launch propellant in LEO and haul it to near-escape with SEP.

Two reasons to do this: reduce launch mass (number of tanker launches) and increase delta-V.

I did some math, and for crewed launches at best you'd save about half your propellant, so 2 or 3 refueling launches. Maybe you could do better with cargo since the whole ship could be pushed with SEP. But anyway, this barely saves any money, but perhaps if you had a very large tanker, you'd get enough scale to make it cheaper.

And you can add another 3km/s to your delta-V budget.
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Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #12 on: 06/20/2017 05:33 AM »
8.4m is not huge when refueling a 12m vehicle.
Huge is when ITS launches a depot that can refuel several (3-5 or more?) spaceships -- something of order 10,000t capacity.
You could use the BFR booster itself, barrel-stretched to as tall as the full ITS. That'd be about 10,000 tons capacity. Maybe put end of life Raptors on it to save on cost. Without reusable bits on it, should cost about  same as an ITS booster, or ~$250 million.
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Offline rakaydos

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #13 on: 06/20/2017 05:54 AM »
8.4m is not huge when refueling a 12m vehicle.
Huge is when ITS launches a depot that can refuel several (3-5 or more?) spaceships -- something of order 10,000t capacity.
You could use the BFR booster itself, barrel-stretched to as tall as the full ITS. That'd be about 10,000 tons capacity. Maybe put end of life Raptors on it to save on cost. Without reusable bits on it, should cost about  same as an ITS booster, or ~$250 million.

Perhaps even pull some of the Raptors off entirely, and use the displaced raptor's fuel hookups for the fuel transfer attachment points.

Offline TrevorMonty

Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #14 on: 06/20/2017 05:59 AM »
I was just doing some math. One of the best reasons to use a depot is you can transport by various means.

For instance, you could launch propellant in LEO and haul it to near-escape with SEP.

Two reasons to do this: reduce launch mass (number of tanker launches) and increase delta-V.

I did some math, and for crewed launches at best you'd save about half your propellant, so 2 or 3 refueling launches. Maybe you could do better with cargo since the whole ship could be pushed with SEP. But anyway, this barely saves any money, but perhaps if you had a very large tanker, you'd get enough scale to make it cheaper.

And you can add another 3km/s to your delta-V budget.

RLVs favour high launch rates and with fuel launches there is no payload to process, fairing to recover/replace Simply reattach US stage to booster, refuel and launch. While ITS video is a little over simplified it's not far of what a RLV should be capable of.

The choice is trade a expensive SEP for another RLV tanker launch or two. In case of SpaceX RLV costs are all internal.

Offline DusanC

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #15 on: 06/20/2017 06:31 AM »
If you're launching propellant from Earth into LEO in order to refuel a Mars-bound spacecraft ('the ITS') why introduce the additional inefficiencies and points of failure of transferring the propellant to an orbital depot first and then transferring it to the ITS when you can simply transfer it directly?
...
Because with ''depot'' ITS with passengers has only one docking and transfer in orbit. Also it will take lot less time to load propellant from depot than from multiple ITS tanker launches. Less time = less mass for supplies for people onboard.


Online Semmel

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #16 on: 06/20/2017 08:32 AM »
I was just doing some math. One of the best reasons to use a depot is you can transport by various means.

For instance, you could launch propellant in LEO and haul it to near-escape with SEP.

Two reasons to do this: reduce launch mass (number of tanker launches) and increase delta-V.

I did some math, and for crewed launches at best you'd save about half your propellant, so 2 or 3 refueling launches. Maybe you could do better with cargo since the whole ship could be pushed with SEP. But anyway, this barely saves any money, but perhaps if you had a very large tanker, you'd get enough scale to make it cheaper.

And you can add another 3km/s to your delta-V budget.

I dont think SEP is a good option here. If you want a highly elliptical orbit as you do for interplanetary transfer, you need to burn at perigee. Since the tanker would spent only very little time at perigee as compared to the upper part of the elliptical orbit, it would take ages to get the almost escape velocity orbit. You might say that you have all the time in the world but you dont. The moon is going to screw your orbit up pretty fast. Chemical is probably better in this case, simply because its faster.

Offline IRobot

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #17 on: 06/20/2017 08:36 AM »
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)
« Last Edit: 06/20/2017 08:36 AM by IRobot »

Offline spacenut

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #18 on: 06/20/2017 04:17 PM »
Yes, either an ITS booster with just enough Raptor Vacuum engines to make orbit.  Fuel it up during the off synod.  It would become a fuel depot.  This is where others may help fuel it up.  Maybe several of these could be placed in orbit.  Then during the Mars travel synod, several ITS vehicles can fill up and go to Mars without having multiple launches going on at the same time.  Reduces the need for ground infrastructure like multiple launch pads. 

Offline Lars-J

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ITS Tanker Rendezvous vs Propellant Depot
« Reply #19 on: 06/20/2017 05:09 PM »
What people seem to miss is that a tanker can actually be used as a depot - you can do both with the architecture. Using a tanker as a depot allows you to launch the transit ship last. You could also use a hybrid approach.

It all depends on how much check-out time in LEO you want for your Mars-bound ship.
« Last Edit: 06/20/2017 05:10 PM by Lars-J »

Offline TrevorMonty

Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #20 on: 06/20/2017 06:31 PM »
What people seem to miss is that a tanker can actually be used as a depot - you can do both with the architecture. Using a tanker as a depot allows you to launch the transit ship last. You could also use a hybrid approach.

It all depends on how much check-out time in LEO you want for your Mars-bound ship.
This is ULA approach, they add extra insulation and sun shade to tank. The cost savings over a dedicate depot is huge, more than enough to cover boil off losses. Being US it can reposition its self depending on mission eg fill up in LEO (180km) then more to EML1, EML2 or HEO.

Offline RonM

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #21 on: 06/20/2017 06:41 PM »
What people seem to miss is that a tanker can actually be used as a depot - you can do both with the architecture. Using a tanker as a depot allows you to launch the transit ship last. You could also use a hybrid approach.

It all depends on how much check-out time in LEO you want for your Mars-bound ship.
This is ULA approach, they add extra insulation and sun shade to tank. The cost savings over a dedicate depot is huge, more than enough to cover boil off losses. Being US it can reposition its self depending on mission eg fill up in LEO (180km) then more to EML1, EML2 or HEO.

The question becomes which is more important, time or money.

If it doesn't matter that the spaceship has to wait in LEO for four or five tanker flights, then it's cheaper to build one tanker per spaceship.

If they don't want the spaceship loitering a long time in LEO for whatever reason, then build two tankers per spaceship. The first acts as the depot, is filled up with several flights from the second tanker, and then is used to refuel the spaceship.

Offline Jcc

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #22 on: 06/21/2017 01:58 AM »
What people seem to miss is that a tanker can actually be used as a depot - you can do both with the architecture. Using a tanker as a depot allows you to launch the transit ship last. You could also use a hybrid approach.

It all depends on how much check-out time in LEO you want for your Mars-bound ship.
This is ULA approach, they add extra insulation and sun shade to tank. The cost savings over a dedicate depot is huge, more than enough to cover boil off losses. Being US it can reposition its self depending on mission eg fill up in LEO (180km) then more to EML1, EML2 or HEO.

The question becomes which is more important, time or money.

If it doesn't matter that the spaceship has to wait in LEO for four or five tanker flights, then it's cheaper to build one tanker per spaceship.

If they don't want the spaceship loitering a long time in LEO for whatever reason, then build two tankers per spaceship. The first acts as the depot, is filled up with several flights from the second tanker, and then is used to refuel the spaceship.

For the early years you can have 2 tankers per ship, launch one, fill it up completely with a couple of launches of the second, and be ready with a full complement of propellant on orbit so the ship can launch with passengers, fill up quickly and go.

For the later years when 100 or 1000 ships launch per synod, it would be more efficient to use those tankers to fill up the ships over a period of many months before they all go to Mars. This is no problem for cargo ships, but with passengers, you may want to bring them up by another means and transfer them to ITS just before they go. Or, maybe not. The logistics of transferring that many people to orbit other than on an ITS ship would be tough.

Probably, 2 tankers per passenger ship, and have the cargo ships fuel first and loiter on orbit waiting for the passenger ships.

Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #23 on: 06/21/2017 03:03 AM »
I was just doing some math. One of the best reasons to use a depot is you can transport by various means.

For instance, you could launch propellant in LEO and haul it to near-escape with SEP.

Two reasons to do this: reduce launch mass (number of tanker launches) and increase delta-V.

I did some math, and for crewed launches at best you'd save about half your propellant, so 2 or 3 refueling launches. Maybe you could do better with cargo since the whole ship could be pushed with SEP. But anyway, this barely saves any money, but perhaps if you had a very large tanker, you'd get enough scale to make it cheaper.

And you can add another 3km/s to your delta-V budget.

RLVs favour high launch rates and with fuel launches there is no payload to process, fairing to recover/replace Simply reattach US stage to booster, refuel and launch. While ITS video is a little over simplified it's not far of what a RLV should be capable of.
Yes, this would be a future upgrade beyond what is shown. You wouldn't bother until you had already maxed out your launch rate to already take full advantage of a full, large RLV. And it's not a slam-dunk: You'd have to get costs very low for the SEP itself for the trade to close, but it DOES close with what I think is reasonable. But it's only a slight augment to the architecture (except potentially for cargo, where it could be even better), you can only save at most about half your propellant this way. (Although it does enable a meaningful bump in delta-v capability without much increased cost.) Still, it is worth it if we get to the rates SpaceX hopes to get to.

Quote
The choice is trade a expensive SEP for another RLV tanker launch or two. In case of SpaceX RLV costs are all internal.
SpaceX is going to build 12000 satellites every 4 years, each with Hall thrusters and kilowatts (maybe tens of kilowatts) of electric power. It need not be "expensive" for SpaceX to do SEP, in fact they'll have the most aggregate experience (i.e. satellite-hours) with SEP of anyone in the industry in a few years from now. And SEP costs would ALSO be internal to SpaceX.

I ran the numbers, and if SpaceX can build a 30-80 MW SEP vehicle for less than $1billion (and perhaps they can do it for significantly less since they'll have a LOT of experience with SEP from building tens of thousands of huge SEP satellites... huge hinted at by Mueller), it actually is cheaper than just doing more tanker flights even with the low costs SpaceX assumes). Otherwise, not worth it. So not worth it in the early days.


I was just doing some math. One of the best reasons to use a depot is you can transport by various means.

For instance, you could launch propellant in LEO and haul it to near-escape with SEP.

Two reasons to do this: reduce launch mass (number of tanker launches) and increase delta-V.

I did some math, and for crewed launches at best you'd save about half your propellant, so 2 or 3 refueling launches. Maybe you could do better with cargo since the whole ship could be pushed with SEP. But anyway, this barely saves any money, but perhaps if you had a very large tanker, you'd get enough scale to make it cheaper.

And you can add another 3km/s to your delta-V budget.

I dont think SEP is a good option here. If you want a highly elliptical orbit as you do for interplanetary transfer, you need to burn at perigee. Since the tanker would spent only very little time at perigee as compared to the upper part of the elliptical orbit, it would take ages to get the almost escape velocity orbit. You might say that you have all the time in the world but you dont. The moon is going to screw your orbit up pretty fast. Chemical is probably better in this case, simply because its faster.
This is false, though perhaps non-intuitive. A very highly elliptical and a circular orbit are actually very close to each other delta-v wise if both are very close to escape. You can spiral out with SEP then do a very small burn to bring your perigee very low. (Although a spiral may not be the optimum.)

Additionally, I was being very general when I said near escape. You could also do this at a distant lunar orbit or EML1 or EML2 or highly elliptical Earth orbit or what have you. They're all fairly close to each other delta-v wise. So you can actually take advantage of the Moon if you want.
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Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #24 on: 06/21/2017 03:23 AM »
Note:

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.
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Offline TrevorMonty

Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #25 on: 06/21/2017 05:01 AM »
SEP tanker will need active cooling with all extra mass, complexity and costs that go with that. For Mars trips of months it makes sense but for LEO to EML1 a normal tanker can do it in 4 days with for little boil off.

Offline Jcc

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #26 on: 06/21/2017 12:05 PM »
Note:

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.

The tanker still needs to get to orbit before SEP can work, and needs to be able to return to earth, so the form of the "SEP tanker" may need to be very similar to a regular tanker with the addition of SEP engines. Actually SEP is  good idea to augment thrust during Mars transit as well, so it may be an integral part of ITS. If there were SEP tugs that attach to the tanker to help raise the orbit, but stay in orbit after use, that might work.

There is a trade off between speed of chemical propulsion and efficiency of SEP. That trade works in favor of speed for reusable rockets, since you want to get them back quickly and reuse them the maximum number of times. Fuel cost is the least significant imput.

Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #27 on: 06/21/2017 01:33 PM »
Note:

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.

The tanker still needs to get to orbit before SEP can work, and needs to be able to return to earth, so the form of the "SEP tanker" may need to be very similar to a regular tanker with the addition of SEP engines. Actually SEP is  good idea to augment thrust during Mars transit as well, so it may be an integral part of ITS. If there were SEP tugs that attach to the tanker to help raise the orbit, but stay in orbit after use, that might work.

There is a trade off between speed of chemical propulsion and efficiency of SEP. That trade works in favor of speed for reusable rockets, since you want to get them back quickly and reuse them the maximum number of times. Fuel cost is the least significant imput.
Sorry, I meant an in-space tanker/depot to haul propellant from LEO to near escape. It'd go back to LEO, not to the surface. It'd be refueled by regular tankers (and Argon).
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Online AncientU

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #28 on: 06/21/2017 02:51 PM »
Note:

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.

The tanker still needs to get to orbit before SEP can work, and needs to be able to return to earth, so the form of the "SEP tanker" may need to be very similar to a regular tanker with the addition of SEP engines. Actually SEP is  good idea to augment thrust during Mars transit as well, so it may be an integral part of ITS. If there were SEP tugs that attach to the tanker to help raise the orbit, but stay in orbit after use, that might work.

There is a trade off between speed of chemical propulsion and efficiency of SEP. That trade works in favor of speed for reusable rockets, since you want to get them back quickly and reuse them the maximum number of times. Fuel cost is the least significant imput.

With a most efficient launch system launching chemical rockets, hauling chemical fuel to refuel chemical spaceships, why complicate things with SEP?  SEP only really makes a difference when mass to orbit is hugely expensive and trips are very long... Cis-Lunar space is all 'next door' for the envisioned fleet.  SEP fuel is also quite expensive and a rare commodity.

On the other hand, SEP could be used for fuel settling and boil-off minimization during interplanetary trips...
« Last Edit: 06/21/2017 02:52 PM by AncientU »
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Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #29 on: 06/21/2017 10:48 PM »
Note:

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.

The tanker still needs to get to orbit before SEP can work, and needs to be able to return to earth, so the form of the "SEP tanker" may need to be very similar to a regular tanker with the addition of SEP engines. Actually SEP is  good idea to augment thrust during Mars transit as well, so it may be an integral part of ITS. If there were SEP tugs that attach to the tanker to help raise the orbit, but stay in orbit after use, that might work.

There is a trade off between speed of chemical propulsion and efficiency of SEP. That trade works in favor of speed for reusable rockets, since you want to get them back quickly and reuse them the maximum number of times. Fuel cost is the least significant imput.

With a most efficient launch system launching chemical rockets, hauling chemical fuel to refuel chemical spaceships, why complicate things with SEP?  SEP only really makes a difference when mass to orbit is hugely expensive and trips are very long... Cis-Lunar space is all 'next door' for the envisioned fleet.  SEP fuel is also quite expensive and a rare commodity.

On the other hand, SEP could be used for fuel settling and boil-off minimization during interplanetary trips...
I already addressed all this. (As I mentioned, you wouldn't use Xenon, you'd use Argon which is plentiful on both Earth and Mars.)

It's not true that SEP propellant is necessarily expensive. Argon (one possible propellant) is just as cheap as liquid methane. Not true that it only makes sense when launch is expensive. I addressed a lot of these things in my previous post above: http://forum.nasaspaceflight.com/index.php?topic=43169.msg1692217#msg1692217
« Last Edit: 06/22/2017 01:59 AM by Robotbeat »
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Online Semmel

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #30 on: 06/22/2017 07:34 AM »
This is false, though perhaps non-intuitive. A very highly elliptical and a circular orbit are actually very close to each other delta-v wise if both are very close to escape. You can spiral out with SEP then do a very small burn to bring your perigee very low. (Although a spiral may not be the optimum.)

Additionally, I was being very general when I said near escape. You could also do this at a distant lunar orbit or EML1 or EML2 or highly elliptical Earth orbit or what have you. They're all fairly close to each other delta-v wise. So you can actually take advantage of the Moon if you want.

Ohh, good point Robotbeat. It might also be possible to shape the orbit a bit by using the moon to ones advantage. Not sure if that is practical though. Anyway, I didnt realize you are talking about the far future of hundrets of ITSes launching together. I tune out here, rather not interested in solving potential optimizations for stuff in 100 years or so.

Offline Robotbeat

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #31 on: 06/22/2017 12:54 PM »
Yeah, I think it'd probably be a mistake to shoot for SEP in the early days when you haven't yet saturated the benefit from just launching more often with the same hardware.

I'd like to think it wouldn't take 100 years to get to the point when you'd be launching hundreds or thousands of ITSes, though. :)
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Offline ZachF

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #32 on: 06/22/2017 03:45 PM »
8.4m is not huge when refueling a 12m vehicle.
Huge is when ITS launches a depot that can refuel several (3-5 or more?) spaceships -- something of order 10,000t capacity.
You could use the BFR booster itself, barrel-stretched to as tall as the full ITS. That'd be about 10,000 tons capacity. Maybe put end of life Raptors on it to save on cost. Without reusable bits on it, should cost about  same as an ITS booster, or ~$250 million.

Good Idea.

You could use a variation of the BFR booster, say one with a detachable 42 engine pod on the bottom with it's own landing tanks/gear, and has another smaller detachable second stage engine (say with 5 Raptors) with that can later be recovered by returning ITSs. The empty ~7,000 ton tank would be filled by tankers and pulled up to a higher inclination via SEP. When empty, the tug brings it back down.

Offline ZachF

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #33 on: 06/22/2017 03:54 PM »
Here's a crappy MS paint diagram.

Large engine pod would push entire stack up to roughly Stage 1 Staging point. It would use it's own landing tanks to land. Stage 2 pod would push it up to LEO where it would later be recovered (maybe many at once) by a returning ITS. Tankers would fill the tank then SEP tugs would pull it up to high orbit/moon/etc.

It would be based on ITS tech

Online envy887

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #34 on: 06/22/2017 05:09 PM »
While I like the idea of using the BFS as the main tankage for a prop depot, I doubt that staging away the Raptors is worth the major headaches it would take to make feasible since it probably only saves ~100 tonnes or 2% of the mass of the fully loaded tanker. I also doubt it's worth stretching the booster much.

By the time a depot is really needed, there will likely be some near end of life or outdated boosters that are still good for one last launch. Just stick a custom nosecone on top containing SEP fold-out solar arrays, thrusters, and fuel, and cryo-cooler/radiators as needed. Then retrofit to remove TPS and add MMOD shields + MLI.

Launch SSTO to a low orbit (it has about a 200 tonne payload), fill it with tankers using the SEP and/or boiloff to maintain the low orbit. Once full, SEP to high orbit, offload the fuel, return to LEO and repeat.

Offline Lemurion

Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #35 on: 06/25/2017 07:24 PM »
Tankers are on the critical path; depots aren't.

Initially, SpaceX is probably going with tankers over depots because you can pull off the mission with three units: one BFR, one ITS, and one Tanker.

SpaceX can benefit from depots, but they don't NEED depots, so they are focusing on using tankers to the greatest advantage. If things change in future so that SpaceX determines a need for depots, they will change horses and jump on the depot bandwagon. In the meantime, they are going to follow the existing pattern of getting the most out of system elements they already have rather than adding new ones.

Online Navier–Stokes

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #36 on: 06/28/2017 01:40 AM »
u/brickmack just posted a very nice concept art of an ITS propellant depot over on r/SpaceXLounge:
Quote from: brickmack
As demand for ITS grows, not only for Mars and lunar flights but also for satellite deployments, a point will be reached when sending tankers directly to every waiting Spaceship takes too long (especially during Mars launch windows, when demand will likely spike). Propellant depots are a natural expansion on the ITS architecture, allowing more flexibility in the time propellant is delivered, and more efficient utilization of launch capacity (you'll never waste a full tanker flight fueling a departing ship that only needs a quarter of the tanker's load). Here, 2 ITS ships are shown docked to a fictionalized depot, built from 10 meter diameter tanks (to easily fit within a notional 12 meter widebody cargo variant of ITS)

Bonus, preview of an updated Dragon model I'm working on after the recent redesign

Offline Lars-J

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #37 on: 06/28/2017 04:08 AM »
Very nice!

But one thing that should be considered, and is IMO a valid argument *against* a massive depot, is the risk for orbital  debris impact. You do NOT want to put all your eggs in one basket. So if you do want to use depots, it makes more sense for smaller depots instead of one massive one. IMO.

Offline rakaydos

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #38 on: 06/28/2017 04:21 AM »
Very nice!

But one thing that should be considered, and is IMO a valid argument *against* a massive depot, is the risk for orbital  debris impact. You do NOT want to put all your eggs in one basket. So if you do want to use depots, it makes more sense for smaller depots instead of one massive one. IMO.
What about many small tanks in one depot structure? So debris damage will take out 1-2 tanks, but the remaining ones will be intact, and the damaged ones can be changed out with replacements on the next cargo flight.

Offline brickmack

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Re: ITS Tanker Rendezvous vs Propellant Depot
« Reply #39 on: 06/28/2017 05:42 AM »
u/brickmack just posted a very nice concept art of an ITS propellant depot over on r/SpaceXLounge:

Hey thats me!

But one thing that should be considered, and is IMO a valid argument *against* a massive depot, is the risk for orbital  debris impact. You do NOT want to put all your eggs in one basket. So if you do want to use depots, it makes more sense for smaller depots instead of one massive one. IMO.

I disagree. The total volume of propellant needed will remain unchanged with either option. But larger depots have a higher volume to surface area ratio, and less overhead (docking hardware, independent propulsion, guidance, etc will be pretty much unchanged), which means that you should expect a smaller number of critical debris strikes per unit of propellant capacity across the entire depot constellation. And in modularly constructed depots like the design I presented, the tanks should be swappable, which massively reduces the cost of replacement. Plus, with the huge payload capacity offered by an ITS (assuming such a widebody cargo variant is ultimately built), you can just stick loads of MMOD shielding on there anyway, so debris impacts shouldn't be much of a risk except for large pieces (which would be tracked and avoided)

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 »

Offline 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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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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