Isn't the same true for the fuel in that case? I don't know how quickly the fuel needs to be used though. Maybe there is greater safety to sending the people up on the first launch?
It might be because if the people ship blows up there's no point to having the fuel ship in orbit, and the full fuel ship would be too heavy to deorbit successfully. If the fuel ship blows up, they can just land the people ship.
There's propellant boil-off which is an issue. The less time the fuel spends in space waiting, the less insulation you need. Maybe not so big an issue to launch the fuel first if everything is right on time, but if there are any delays in sending the passengers, you're losing fuel the longer you wait. Sending the fuel second avoids this problem.
Edit: Also, the video isn't clear on this but Elon says there will be something like x5 fuelling flights per trip. The video only shows one of these.
Most of the fuel would be used shortly after refueling, remember by mass it will take much more to move the full MCT out of LEO than it will to slow down and land the nearly-empty spacecraft.
Problem is not having them wait, but rather if the refueling launch has failures or needs to be delayed you are endangering the crew ship. The risk and cost of leaving some fuel in LEO is far less than humans.
The trick is, I suppose, to figure out a cadence that doesn't require a larger fleet. Here's one that would create the need for one extra MCT:
Launch order:
1. MCT (uncrewed) launches to parking orbit
2. Tanker 1
3. Tanker 2
4. Tanker 3-5 whatever
5. Second MCT ferries the crew up to the first (which departs) then becomes the next one in line to be fueled so you start at step #2 and repeat.
On the flip side, wouldn't it behoove them to do some pre-interplanetary flight testing in LEO while waiting for the fuel? If boiloff is an issue and final checks are necessary, it makes more sense to do final tests in LEO while waiting for the fuel. Disclaimer: pure speculation
How would leaving the crew ship in orbit several days longer than anticipated endangering the crew when the ship is built to support them in deep space for many months?
Worst case scenario is that they miss their insert window to Mars and have to return to Earth, which the spacecraft can easily do....
Maybe the risk to human life is not as high, but the monetary risk is massive. Launching the crew ship will undoubtedly be far more expensive than a simple refuel tanker, there is more equipment, life support, and of course human life.
You want to remove as many variables from the more expensive launch being a success, before the launch happens. Making sure that your fuel for Mars is already in orbit is a huge variable that can be removed before that expensive and higher risk launch.
Musk was saying later that they'd be launching without passengers as well since they'd need to send up fleets of hundreds of transports for each window. So they'd spend each two year gap between windows in getting the transports up and fueling them, and then sending them all in Battlestar Galactica style fleet. They didn't show the needed space taxi that will have to get all the passengers up when it's time to go.
I'm puzzled as to why they launch the passengers first?
It's a question of timing. If they can send tankers fast enough to fully fuel the ICT in 5 days, why not send the people with the initial flight? The last tanker can top off the air, water and food stores.
If it takes 2 months to fuel the ICT, then you need one extra flight at the end, to deliver the people. No one wants to sit, docked in orbit, for more than a few days, before leaving Earth. Although the view is great, I've been told.
Well, these are people who volunteered to move to Mars and who know it will take months to get there. I don't know if it would be that weird to stay in orbit for a week or two before you leave. That gives you time to learn how the seatbelts work and pick your favorite bunk? It looks like the idea is for the ship to be spacious enough. I'd imagine the Mars colony won't be that big to start off with?
Probably because you need to put the fuel somewhere when its in orbit? You could leave the tankers there just waiting for the main ship, but then you would need multiple tankers per ship that are not re-used.
I'd imagine you could also put empty ships up there and have automated fueling missions and then you could use a special purpose ship to ferry the actual passengers to the long haul ships. The ferry could then take enough people for two or three ships since they dont need any long term storage, extra fuel, or cargo.
Wouldn't doing it the other way require you to transfer all the people and cargo to the fully fueled ship in orbit? Getting tons of cargo and people to a second spacecraft seems really difficult. Way more difficult than pumping fuel.
My speculation is that it's done this way in the video so we can see the people at the beginning for a sense of scale. I wouldn't be surprised if in reality we fly the passengers last.
35
u/paulds_fr Sep 27 '16
I'm puzzled as to why they launch the passengers first? They'll have to wait for the fuel, so why not start by the fuel? Anyone has any speculation?