Re: Why only 2 oz ?
alain williams,
To give you some numbers on the two above posts about fuel. I looked up the weight of the rocket that lifted the Mercury mission BepiColumbo. That's one big and one little orbiter to be taken to Mercury over 7 years. It's all rather silly/mad/impressive. See link to wiki here:
Arianne 5 first stage is 189 tonnes of which 175t is fuel.
2 x solid rocket boosters 277 tonnes each = 554t
1 x second stage 16.1t - 14t fuel (this can get about 10t to GTO)
Payload weight 4.1 tonnes - including 1.4t of fuel for an ion drive
All to deliver 1,230kg Mercury Planetary Orbiter and the 255kg Mercury Magnetosphere Orbiter
So that's a launch mass of about 763 tonnes
Of which a bit under 700t is fuel of varying types.
The solid rocket boosters last 30 seconds and then fall into the sea. The first stage of the rocket does a few minutes more, then does likewise. The upper stage of the rocket lights and takes the payload I'd imagine past geostationery transfer orbit and gives it as much velocity as it can - then drops off.
Then the Mercury Transfer Module fires up with its special electric thrusters, powered by solar cells. That use much less fuel than chemical rockets. They're much slower, but are using tiny amounts of fuel so can burn continuously for years.
But even that isn't nearly enough. It takes stupid amounts of power to decelerate enough to get close to the Sun, so it also has to do 1 Earth fly-by, after 1.5 years, to scrub off some speed. Followed by 2 Venus fly-bys and 6 Mercury fly-bys for a total flight of 7 years chucking away over 60 tonnes of spacecraft and about 700 tonnes of fuel in the process of getting those 2 craft massing a bit over 1.5 tonnes to Mercury orbit for a 1 year mission - though I'm sure they'll last longer than that.
I admit that this is a special case, as Mercury is a right bastard to get to. But it's like going on holiday to the seaside in an articulated lorry, carrying a camper van to stay in and only getting home on a bike because you've had to throw the others away due to not being able to refuel them.
Biting the hand that feeds IT
