Sunday, 12 August 2012

Simple is best - Right?

Simple is best - Right?

As the Curiosity rover completes one year on the surface of Mars the idea that 'simple is best' seems to be confounded.

NASA’s latest rover on the surface of Mars used an elaborate ‘sky crane’ system to deliver the heaviest scientific payload ever sent to that fascinating planet. In addition it has a nuclear power source and wheels that will keep it trundling across the red planet for years. The method used for landing the Mars Science Laboratory (MSL) was ambitiously complex, so what happened to the adage ‘simple is best’? Does the legendary story of NASA's efforts to develop a zero gravity pen for use in space, only to find out that the Russian Cosmonauts were using a pencil, have any wider significance? It is tempting to assume that NASA engineers have always reached for complex solutions to problems when simple designs would suffice, but sometimes only sophistication will meet a challenge. What do we mean by simplicity or complexity anyway?

Three stages good - two stages bad.

Unlike returning to Earth, the atmosphere of Mars does not allow either the use of wings or a simple sequence of aero dynamic braking followed by the deployment of a parachute system. The thin atmosphere will allow a parachute to slow a spacecraft down, but not as much as on earth. Historically the heaviest payloads returned to Earth, usually manned spacecraft, have used a three stage sequence.

1)     Aerodynamic braking with a heat shield made from materials designed to dissipate high temperatures produced by high velocity friction with the atmosphere.

2)     Deployment of one or more parachutes

3)     A final touchdown cushioned by a landing on water or by a brief firing of rockets.

Attempts at eliminating one or more of these three stages have not succeeded. In the 1960s NASA toyed with the idea of equipping the Gemini spacecraft with Rogallo wings.  These special parachutes assume a kite like triangular shape that could have enabled an airfield landing by gliding down at such an angle that vertical speed of descent would be partly converted to forward motion. Time constraints lead to the abandonment of the project. Lifting-body gliders with no wings but deriving lift from an aerofoil shaped fuselage were never developed beyond experimental aircraft such as the HL10.

The Space Shuttle was a flawed design which revealed that the wings of a spacecraft could more vulnerable to damage than a conventional aircraft. The safety systems put in place after the loss of the Columbia shuttle would have detected damage to a wing, but would also have lead to the abandonment of the craft and the return of the crew by 1960s style Soyuz capsules. Perhaps the next generation of small scale and privately built spacecraft will have more workable designs. 

Four stages complicated - three stages problematic.

On Mars there are no oceans to cushion the final touchdown as on our planet. Just as the Russian Soyuz capsules end their parachute return to Earth with a very brief retro-rocket firing, successful landings on the red planet have used aerobraking, parachutes and then retro rockets or air bags, or a combination of both. In a sense, air bags are the equivalent of a sea landing on Earth, except that their deployment has to be timed very precisely. The decision to use rockets and air bags is a choice of a four stage entry, descent and landing (EDL) system. Although the Viking landers in 1976 successfully used a three stage EDL, finally landing with rocket engines, several attempts at landing on Mars using a three stage EDL have been failures.

The Russian Mars 2 and Mars 3 Landers used aero-dynamic braking, parachute and rocket deceleration, but in 1971 airbags were not available. Mars 2 reached the surface but returned no signal and Mars 3 transmitted for 20 seconds. It was reported that the Russian landers had some internal cushioning material, which is half way to having air bags, but this seems not to have helped. The Beagle 2 Mars lander used aerodynamic braking, a parachute and airbags but no rocket power, and failed. The Deep Space 2 hard-lander mission delivered two impactors to the surface of Mars with aerobraking but no parachutes in 1999. The impactors hit the surface at a speed of 179m/s and were supposed to release a penetrator to probe for water. No signals were returned and water ice was eventually detected by the Phoenix Lander in 2007 using the by now conventional aerobraking-parachute-rocket EDL. 

Old tricks and new gadgets

The Viking project delivered two working landers to the surface of Mars in 1976 using a simple three stage EDL with a five minute rocket powered descent. The design was simple because there was some uncertainty over the Martian atmosphere and it was safer to create what was essentially a 1960s style lunar Surveyor type spacecraft with an added heat shield and parachute. The amount of fuel required to sustain a five minute powered descent reduced the payload delivered to the surface of Mars.

The more complicated Curiosity rover followed the familiar pattern of atmospheric entry with a heat resistant aeroshell, followed by the release of the largest supersonic parachute ever built, and then separation from the parachute and the ignition of rocket engines capable of slowing the rover to a hover. Before landing occurred, the descent stage detached the Curiosity rover and lowered it to the surface on nylon chords. As soon as the rover touched down the chords and an umbilical line were detached and the rocket powered descent stage flew away and crashed.

There is a trade off between the risks involved in this more complicated EDL technique and the increased capacity of the scientific instruments that can be brought to the surface of Mars. The earlier rovers, Sojourner, Destiny and Spirit used air bags for the final contact with the Martian surface, but were tiny in comparison with the 899kg Curiosity rover.  The ‘sky crane’ concept used for the Curiosity rover is a solution to the problem of how to land a larger payload on Mars without the use of airbags. The sky crane EDL has evolved from the system used to land the Pathfinder lander/Sojourner rover combination in 1997.

After the parachute deployment the Pathfinder lander was lowered on a bridle and after airbag inflation rockets, between the parachute and the lander, fired for two seconds before the chord was cut. Still firing, the rocket carried the parachute away to avoid interfering with the lander. (The earlier Russian Mars probes had a sideways-firing rocket to carry the parachute away from the line of the spacecraft descent.) The sky crane EDL system involves the substitution of the pathfinder style retrorocket stage by a larger liquid fuelled stage with more able radar attitude and altitude sensing systems. With MSL/Curiosity the powered stage of EDL is around 50 seconds versus the 2 seconds for Pathfinder.

A blast from the past.

Although the sky crane system seems to be complicated it does obviate the need for legs and ramps. The wheels of the curiosity rover doubled as the landing gear. At a stroke the design removes the need for substantial landing legs, which would be used only once, and ramps to deliver the rover to the surface. Although ramps were used on the three previous Martian rovers, the Sojourner rover was only able to descend from the Pathfinder lander because it had two ramps. The exit from one ramp was found to be unsafe.

The logical thought process behind the evolution of the skycrane EDL system is reminiscent of the origin of the Lunar Module used by Apollo astronauts. Initial concepts for a lunar expedition, such as the British Interplanetary Society design from 1947, envisaged a spaceship landing and the moon and then taking off again. Even in 1961 when John F. Kennedy committed the U.S.A. to a manned lunar mission it was assumed that a rocket much larger than the Saturn 5 would be needed.

The genius of the Lunar Module-Command/Service Module (CSM) combination arose from the realisation that is was not necessary to land with all of the fuel needed to return to Earth. The concept of leaving fuel in orbit around the moon, landing, and then re-joining the CSM made the difference between needing an impossibly huge rocket to leave Earth and a ‘merely’ large launch vehicle. This is because the proportion of mass delivered to space is so small relative to the starting mass of the launch vehicle. Any small change to the size of the payload has a large effect on the size of the launch vehicle. The Apollo system also benefited from eliminating the transportation of parachutes to the surface of the Moon and back, every bit of weight saving matters in spaceflight.

Let’s be careful out there!

The ‘flying bedstead’ VTOL aircraft built to train astronauts for moon landings famously crashed when being flown by Neil Armstrong. His decision to eject safely from the falling vehicle may have helped in his selection to command the first Moon landing, along with his cool handling of a brief but dangerous malfunctioning thruster problem on Gemini 8. VTOL aircraft are notoriously difficult to control and another characteristic of the sky crane EDL system is that having the centre of mass below the rocket engines improves stability. The upper stage becomes a kind of ‘rocket parachute’ with the payload hanging beneath it. The first Russian manned Vostok space capsules were spherical with an off-set centre of mass so that the heat shield would be presented to the direction of travel without the need for active attitude control with thrusters. This system was never used by NASA as it was always the stated intention to fly to the moon and back. Returning to Earth requires a degree of steerability that is afforded by a conical capsule that is in effect a very blunt wing that can manoeuvre itself across a re-entry corridor. Again simplicity is lost but an operational advantage is gained. The Soyuz capsules adopted this system so that the Soviet Union could attempt a flight to the moon. Paradoxically, these capsules are restricted to Earth orbital missions and this feature is not needed.