I think I've solved SpaceX's engineering problem of how to reuse the first stage.
So SpaceX says that rocket fuel accounts for 0.3% of launch costs. In other words, saving the hardware for reuse is the difference between an airline buying a new 747 for each flight or having to buy a new on. As an innovator, SpaceX wants to reuse the first stage of their Falcon 9 rocket to dramatically reduce the cost of launches.
Musk's interest in a reusable rocket is well known, but when asked about its progress he responded: "So far it has sucked, really. It is extremely difficult, and there's a reason that nobody has invented a fully reusable rocket before: it is super damn hard."
To build a rocket with sufficient shielding to withstand the pressures and heat of falling into denser atmosphere, Musk said, would mean the rocket would be too heavy to carry significant payloads into orbit.
My solution to this problem is to separate the first stage a bit early so that it still has some fuel left. Obviously this will reduce payload some, but the cost benefit will be huge. Plus the payload isn't always maxed out either. The first stage will continue upward until its velocity reaches zero. At this point, relight the middle Merlin 1 engine and run it at minimum thrust (about 70% I believe) to slow the descent. Either gimbal the engine or use a RCS (reaction control system) to maintain attitude as it descends. The velocity would probably be 300-400 knots down or however fast the vehicle can stand from a heating/aero standpoint. Once reaching a suitable altitude (probably 30-40,000') you deploy a parachute and cut the engine. Aim for water (ocean) and at the last instant fire solid fuel retro rockets to slow the splashdown to a couple feet per second.
Potential issues:
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The single Merlin engine at 70% thrust might be too much thrust for the light first stage.
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-Solution: Use 8 uprated Merlin 1D engines with a low thrust engine in the middle for ascent/descent.
Thoughts:
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This would probably take 10-15,000lb of fuel (total guess) that couldn't be used for getting the payload to orbit.
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Parachute adds weight
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Retro rockets add weight
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The proposed LOX/LH2 engine ("Raptor") could be used for the upper stage which would probably make up for the reduced performance of the first stage.
Any thoughts on the feasibility of this?
Posted Aug 12, 2011, 3:12 AM UTC
Remember, the Falcon 9 1st stage has 9 of those Merlin engines. so using 1 as a retro rocket at 70% thrust only gives roughly 7% of the thrust available at lift off. That's probably just about right, for slowing the empty booster to subsonic speed, as it hits the dense atmosphere below 120,000 feet.
The weight of the parachute and solid rocket that fires just before splashdown, could probably be kept below 1000 lbs., I guess. The weight of the liquid fuel used for the retro burn, is about the typical reserve fuel, loaded over and above what would normally be needed to launch the mission.
If the reserve fuel is needed during launch, well then there could be no recovery. But if the launch is nominal, as they say, then that fuel would be available for aiding the recovery process.
My own take on this, is that the tanks will be too damaged to be reused, no matter what. The engines, pumps, and control electronics are what you want to save.
So my idea is, use explosive bolts to separate all the valuable components from the tanks. Build in an aeroshell, similar in shape to a heat shield, between the tanks and the turbo pumps. After burnout and second stage ignition, that is where the explosive bolts do the separation. The high value parts ride behind the aeroshell during supersonic reentry. Then a small drouge shute deploys to slow the assembly enough so a larger parachute can bring the assembly down to the water.
Air bags should be inflated before splashdown, like the MER rovers used, both to cushion the impact and to keep the assembly afloat until it is picked up.