Thursday, January 30, 2014

Re-using Ariane’s first stage - The study

As spaceX is working hard to routinely reuse its Falcon 9 rocket stages, one can remember that 30 years ago, Ariane 1 could have been the very first liquid rocket to have its 1st stage safely recovered and refurbished after every launches.

 From 1979 to 1981, CNES, the french space agency, and ESA made a deep study to use a set of parachutes in order to slow down the first stage's fall into the Atlantic Ocean. Cutting cost on launch operation was seen as critical for Europe at that time. The Space Shuttle was about to make its first flight and this new spacecraft was to slash launch cost. The fear of ESA was that all commercial satellite operators would leave the Ariane order book in favor of the space shuttle one, leaving few governmental flight for the European rocket with pricey launches as a consequences.

Saving costs

Recovering the first stage is attractive from an economic point of view because it accounts for roughly 40 per cent of the cost of an Ariane rocket. The propellant tanks of the stage are made of stainless steel. In 1982, the recovery of the wreckage of the ill-fated fifth flight (L05) showed that those tanks, although bumped, remained corrosion-free after a short stay in sea-water. It was then foreseen that they could be reuse after cleaning with fresh water and refurbished.

The same principle would apply for other parts such as the engine turbopumps and the propulsion bay that might be re-use on later flights. However, engineers foresaw a necessary replacement of the engine nozzles, because they would distort when their hot surface touched the sea. Overall, the saving count indicated that an Ariane launch costs can be cut by roughly 10 to 15%.

Recovery plan

The conclusion of the 1981 studies pointed that a couple of critical phases had to be mastered for a successful recovery :

  • To point and slow down the stage to allow a gentle sea landing. The calculations indicated that a maximum speed of 12,5m/s was needed at landing in order to stay below structural strength of the empty stage and avoid any damages.
  • To find and lift that stage quickly enough to prevent any corrosion by seawater or damages from the waves. 
ESA crafted a detailed recovery plan to overcome those critical phases.

Braking sequence
The first stage of Ariane would separate at an altitude of about 53 km and a velocity of 2100 m/s (7500km/h). lt would continue its coasting flight up to an altitude of 87 km, after which it would fall back into the sea, some 340 kms from the launch site in the Atlantic ocean. As the stage would go through thicker parts of the atmosphere, its speed would decrease and reach 160m/s (ca 600km/h) at 5000 m high. At this height, a mortar would fired two drogue parachutes. These would stabilise the stage and pull out an intermediate parachute of 12,5m diameter. This parachute would first half opened at 2m diameter and its purpose would be to stabilize the oscillations of the falling stage and to make sure it is pointing with the engine bay downward.

The parachute would then fully open and further reduces the speed of the stage down to 70m/s (252 km/h). At 2000m high, the final braking sequence would start with the deployment of four main parachutes of 20m diameter. They would slowly open and lift the stage so that it enters the sea at the required maximum 12,5 m/s speed.

Once in the water, calculations show that the buoyancy of the stage would keep it near vertical with a maximum 5° tilting angle.
However, In some cases, the tank pressure could go down to as low as 0,9 bars and cause damages to the structural integrity of the stage, so it might be necessary to repressurise the tanks in order to ensure that the stage remains afloat after impact. A pressurization system was yet to be defined but could either be part of the rocket or provided by the recovery team at sea level.

Recovery sequence
The recovery ship would track optically the stage as it descends. Four radio beacons fitted in the front interstage will also ease the localization of the booster when floating at sea. As soon as the stage is in view, a tug boat would bring a dedicated recovery barge toward the impact zone. The barge would include a floating sledge nicknamed “the spoon” specially designed to recover the floating stage.

On site, the recovery crew would first secured the stage by checking it for any propellant leaks (Nitrogen tetroxide and UH 25 are highly toxic and should be carefully vented before any human activities) and disabling the flight termination system to prevent any explosion during operations. Divers would then release the spent parachutes and tilt the stage near horizontal with the help of buoyancy bags. The recovery sledge would be positioned under the stage and both would be strongly tied together. Next, a winch would tow the sledge safely onto the barge through a dedicated ramp. At last, the sledge would be safely tied to the barge for the journey back to Cayenne harbour, French Guiana. 

Once at land, the recovered stage would be carefully cleaned with fresh water and a neutralising agent would be sprayed onto it. The main components would be disassembled and dried. A first assessment would be done before transporting the parts back to the manufacturing plants scattered in Europe. There, deep analysis will be performed and the parts in good shape will go through the normal validation process before being assembled again on a new stage.

- To be continued -

References : 

  • ESA bulletin nr 25 pp33 - Feb 1981
  • ESA Bulletin nr 39 pp19 - Feb 1982
  • FLIGHT International - 17 April 1982 - Fokker makes Ariane a parachute 

Images are my personal thought of what could have been the recovery scene. Inspired from New Scientist - 6 May 1982 - Down to earth rocket