Reusable rockets
Almost all
rockets used to date are non reusable. A typical satellite launch would put
into orbit only perhaps 2% of the weight of the launcher. About 88% of the
weight will be fuel but the launcher about 10% of the total is expendable often
falling into the sea and sinking without trace, As Elon Musk( leader of Space X
) puts it “supposing you flew a jumbo jet across the Atlantic, threw it away
and built another to fly back”
Put this way it
seems obvious but the difficulty is that launching to space from earth requires
prodigious amounts of energy to over come the gravitational pull. This is so
difficult that virtually all rockets employ a staged configuration. In this the
whole is lifted by the first stage which then falls away leaving a lighter
second to continue on its way. Every ounce in weight carried to orbit
unnecessarily is an ounce less payload.
Early launchers
were very inefficient placing not much over 1% of launch weight into orbit.
Expendable rockets have gradually improved so that Saturn V ( the Apollo moon
mission ) achieved 4.1% to orbit
To reuse a
rocket it has to return to earth landing safely and this requires fuel to
deaccelerate. It also requires control equipment to guide to a landing. This
weight effectively reduces the payload capacity.
The only
practical reusable system today is the Falcon 9 by Space X. This is two stage
rocket where the first stage carries enough extra fuel to land back after stage
separation. The control is incredibly precise. Because the rocket cannot hover
it must land back precisely as the rocket engine shuts off. This manoeuvre is
known as a “hoverslam”.
The Falcon 9 has large landing legs which flip out just a right time before
landing. These legs also pose a weight penalty.
The Falcon 9
first stage is recovered but not the second. The fairing around the payload is
recovered by the expedient of parachuting into the sea where it floats.
Refurbishment is necessary but other recovery methods using big catcher nets
proved unsuitable being both expensive and prone to failure.
This reusable
technology enables Space X to offer launches much more cheaply than
competitors. The difference is so large that competitors are designing new reusable rockets. Space X
themselves for the next generation of large rockets are designing so they can
hover. This somewhat simplifies landing control but increases the fuel weight
penalty. The hovering rocket will not have the weight penalty of landing legs
but is designed to be caught in the hover by a ground tower.
This is new
technology largely pioneered by Space X but also learning from the past
problems. The most ambitious effort at reusability was the Space Shuttle. This
was designed to glide to landing so like an unpowered aeroplane. At launch the
shuttle was also propelled by 2 large solid fuel boosters and both were
attached to a massive fuel tank providing the fuel for the shuttle engines. The
solid fuel boosters were designed to be detached once exhausted and fall into
the sea ready for reuse. This proved to be a problem as refurbishment was
lengthy and expensive.
A bigger
problem were the heat resistant tiles protecting the shuttle from the fierce
heat of re-entry into the atmosphere. These tiles had be replaced after every
mission, every one was different and manual replacement was expensive and time
consuming. As a result the shuttle program was considered too expensive overall
and ended. Space X are seeking to solve the re-entry problem by using
standardised tiles robotically placed as far as possible. The Space X rocket is
in trials and it is yet to be seen if their approach will be successful.
It is clear
that reusability is key to Rocketry success and all major companies are
designing for future reusability. The sacrifice in terms of payload is
outweighed by the cost benefit.
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