Author Topic: ITS Tanker Rendezvous vs Propellant Depot  (Read 5375 times)

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.

Online 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.

Online 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.

Online 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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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.

Online 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.

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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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Online 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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Offline 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.

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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

Offline 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.

Offline 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.

Online 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)

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