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SpaceX is attempting a huge feat in spacecraft engineering. It is seeking to land the first stage of its Falcon 9-R rocket on a floating platform at sea. Normally this would end up at the bottom of the ocean. If successful, SpaceX will shake the rocket launch market, by shaving millions of dollars off launch costs.
Today’s rockets are one shot wonders. They burn up fuel in a few minutes and splash down into terrestrial oceans, having put their payload on the right trajectory. This is wasteful and that is why scientists have dreamed of building reusable launch vehicles.
The holy grail of rocket launchers is a concept referred to as the single stage to orbit (SSTO) vehicle. The idea is to use a reusable launch vehicle (RLV) which has the capability to deliver a payload to orbit, re-enter the Earth’s atmosphere and land, where it can then be refuelled. The process can then be repeated with a short turnaround.
https://theconversation.com/explainer-why-reusable-rockets-are-so-hard-to-make-36036
(Score: 2) by subs on Tuesday January 13 2015, @03:28AM
Ahh, I did miss a critical part there. An empty rocket is much lighter than a fully fueled one, and since the second and third stages have broken off to fly away on their own, this reduces the mass greatly.
Just for perspective, a fully-fueled F9R with payload is about 510 tons on the pad. Of that, the second stage with propellant and payload is about 100 tons, while the dry mass of the first stage is only about 20 tons. So assuming you've still got around 20 tons of propellant in the first stage at separation, your vehicle mass suddenly drops from 140 tons to just 40 tons. That's also part of the reason why when they execute the turn-around burn they only burn 3 engines instead of the full 9 and only a single one for the final landing maneuver. Those things are immensely powerful and at such a low vessel mass could very well surpass the vehicle's structural g limits. F9 flight loading limit is around 5g, and considering each engine delivers a minimum of ~50 tons in vacuum (at 70% minimum thrust setting), firing all engines even at minimum thrust would easily give you 10g+ acceleration.
As for first stage purpose, you're partly right. While atmospheric resistance plays some role during launch, its effect diminishes rather quickly above ~15-20km altitude. Really, the main reason for staging in rockets is to get around the exponential nature of the rocket equation. To get to LEO, you need about 9000m/s of delta-V. Of that, only about 200-300m/s is expended on getting through the atmosphere. A good chunk (around 800m/s) is actually spent on working against gravity and lifting the vehicle so it doesn't come crashing down before you have time to accelerate up to orbital velocity - that's gravitational "drag". It's essentially the energy spent on "hovering" the vehicle over the ground while you're speeding it up sideways; this would be the same regardless if there were an atmosphere or not. That is not to say that you were wrong, just wanted to share my thoughts.