Author Topic: ITS Tanker Rendezvous vs Propellant Depot  (Read 6909 times)

Offline 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?

Online 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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Online 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 »

Online 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 »

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