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.
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.
Quote from: AncientU on 06/19/2017 05:33 PM8.4m 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.
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.
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? ...
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.
Quote from: Lars-J on 06/20/2017 05:09 PMWhat 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.
Quote from: TrevorMonty on 06/20/2017 06:31 PMQuote from: Lars-J on 06/20/2017 05:09 PMWhat 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.
Quote from: Robotbeat on 06/20/2017 05:31 AMI 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.
Quote from: Robotbeat on 06/20/2017 05:31 AMI 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.
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.
Quote from: Robotbeat on 06/21/2017 03:23 AMNote: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.
Quote from: Jcc on 06/21/2017 12:05 PMQuote from: Robotbeat on 06/21/2017 03:23 AMNote: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...
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.
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
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.
u/brickmack just posted a very nice concept art of an ITS propellant depot over on r/SpaceXLounge:
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.
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.
Quote from: Robotbeat on 06/21/2017 03:23 AMOne 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.
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 EarthCons:- requires large infrastructure upfront, including capturing asteroids to orbit, in-space building, depots, cargo/crew transfer stations, etc- maintenance of transit shipsPros:- 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)
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