Safety and Abort


      The early days of vertical rocket launch were plagued by rockets falling over on the launch pad.  While advances in technology solved that problem, the system is complicated and thus unreliable.  This problem is magnified when spacecraft are reused dozens of times. The Shuttle is so complex that NASA employs thousands of experts to ensure safety and reliability of launch components.  As a result, it costs less to launch payloads with new expendable rockets than recovering and reusing spacecraft like the Shuttle.  Nevertheless, vertically launched rockets succeed only around 97% of the time, even after 50 years of development.

Simplicity and Cost 

      An advantage of rail launch is the mechanically constrained first stage.  No complicated control system is required because the track is doing the guidance.  Once the vehicle leaves the track, it has plenty of air speed to use control surfaces, such as fins, to guide the vehicle out of the atmosphere.  Once out of the atmosphere, the vehicle needs thrust guidance but the problem is much reduced because the environment is so much simpler.  Space has no cross winds or weather.  The vehicle is also much lighter, so the necessary control authority  is reduced.  With rail launch, you only have to build a spacecraft - not an aircraft that must fly into space.  All that adds up to less equipment which means lower weight, less money, and higher reliability.  There is no such thing as a cheap "dumb" booster, unless it uses a dumb sled on a track.

     A second advantage of rail launch is the ability to push a sled with an RLV aboard a rail car around a spaceport just like an aircraft.   When ready for launch, it can be pushed to a fueling area, and then pushed toward the launch point away from spaceport facilities.  This allows the preparation for other launches to continue like an assembly line.  On the other hand, vertical launched rockets must be assembled and fueled at fixed pads, and every support item moved to the pad and then far away for launch.  Because of the danger a catastrophic tip-over during launch, the safety clearance area is enormous.   Since vertical launches are often delayed awaiting better weather, spaceports become shut down until a single launch can occur.  Can you imagine evacuating an airport prior to the launch of each aircraft?  Vertical launch also has the danger of cooking in its own rocket blast for a couple seconds at "blast off".

Abort Option

       With rail launches, sled rockets will reach full thrust near the bottom of the ramp, so engineers have several seconds to monitor the thrust and watch for fuel leaks.  If a problem is detected, they can shut down all engines before the RLV is halfway up the ramp. The steep incline will quickly slow the RLV, then it can be eased back down the sled.  The problem can be fixed and another launch attempted within hours.

     Problems with vertical take-off boosters are often detected at launch, but the launch cannot be halted.  The European space agency spent 10 years and $7 billion dollars to develop the Ariane-5 rocket. On June 4, 1996, the first Ariane-5 was launched. At 39 seconds after liftoff it exploded, destroying the rocket and cargo valued at half a billion dollars. So what happened? It turns out  the explosion was caused by activation of the self-destruct mechanism built into the rocket. The self-destruct was triggered by unusually large aerodynamic forces ripping off the boosters. These forces were due to an abrupt course correction made by the on-board steering computer, which was in compensation for a wrong turn off course that in fact never took place. The inertial guidance computer had told the steering computer that the rocket had gone way off course, when it was not off course at all.

     What caused this turn of events? The computations done by the inertial guidance computer was converting a 64-bit floating point number into a 16-bit signed integer number. At about 36 seconds into the flight, a number was encountered that was larger than 32768, the largest possible 16-bit signed integer, so the conversation failed. Thus, erroneous numbers were sent to the steering computer, causing it to think the missile was off course and leading to the explosion at 39 seconds into the flight. Again, a very costly disaster due to bad computer arithmetic with the complex vertical launch method.

     A more recent example was the first Delta 3 launch.  It went out of control about a minute into flight because it ran out of hydraulic fluid and the gymbols on the nozzles stuck.  Normally there is plenty of fluid - even though it doesn't recycle back to the reservoir - because control movements are small.  In this situation, the controls engineers had inadvertently used the mass and CG of the Delta 2 in their calculations and simulations.  Even though the control loop was closed, it was very unstable and caused great gymbol oscillations which consumed the fluid more rapidly than normal.  Human mistakes like that can happen at any time, adding to the other mechanical failure modes that go into the failure rate (remember the Mars polar lander metric vs English measurement fiasco?) 

     On October 15, 2002, a massive Russian Soyuz/Foton rocket launch failed at Plesetsk. One of the four side boosters attached to the Soyuz central stage showed an ignition delay of two seconds in reaching full thrust.  It was then working with pulsations for 1.5 seconds and at L+4 seconds showed a total rapid drop in thrust.  In accordance with the design of the nominal separation mechanism of the side boosters, a side booster not producing thrust falls off the launcher.  With only three side boosters left, the flight program of the Soyuz launcher generated a signal for automatic shutdown of all engines, which was permitted to be actuated at L+20 seconds. The launcher then fell down to Earth and exploded, killing a soldier and injuring others.  (Russian rockets are used for Boeing's Sea Launch and Lockheed-Martin's Atlas V).  It was later determined that a clogged fuel line was responsible.  The complexity of vertical launch make failures so common that Boeing is proud of its 98% success rate, and these are always new components.  Can you imagine Boeing bragging that its 747 aircraft take-off successfully 98% of the time?  

Escape Options

      With vertical launch, crew members are located on the top of the rockets and fuel over a hundred of feet from the surface.  If a fire breaks out or a rocket explodes during the launch sequence, climbing downward is fatal.  An inclined rail  launch keeps the crew much closer to the ground.  No massive "blast off" occurs and there is no danger of a rocket tipping over, so rescue crews can remain close by. 

      Any engine explosion during the first few seconds of a vertical take-off provide the crew with no chance of escape as they fall from over a hundred feet off the ground.  If the booster is having trouble, an RLV cannot release itself because it has no velocity to maneuver away, and the launch cannot be halted by shutting down the booster engines.  Ejection capsules are difficult to employ because they must fired upward in front of the malfunctioning rocket (a Delta 3 launch failure is shown at right), and then may descend into the inferno below.

       A rail system provides three escape options: 1) if a problem occurs during the first six seconds of launch, all rockets can be shut down.  The sled will continue upward until halted by gravity, and then descend back down the rail; 2) if a problem occurs after the first six seconds, the RLV has enough velocity so it can release prematurely and fly away to land elsewhere;  3) if a critical problem occurs which does not allow the first two options, the crew can eject from an escape capsule, similar to aircraft ejection seats.  Since the sled and RLV will continue moving upward along the track, a parachute can safely keep the ejected crew capsule away.  The rail keeps the rockets safely pointed skyward to eliminate the vertical launch danger that they may shoot off into a local community.

Inclined Rail Launch is Superior

     Rail launches are much simpler, safer, and cheaper than the complex vertical launch method.  Once a Sky Ramp system is perfected, it will not require an army of safety engineers like the Space Shuttle.  A 3% failure rate for today's vertically launched multi-stage rockets has pushed Insurance costs up to 17% of total launch costs.  Sky Ramps can reduce this failure rate, which becomes more important with the reusability challenge of launching "used" spacecraft.  Finally, only major winds can delay the launch of a supersonic RLV coming off a rail, so launches will not be frequently delayed by weather as is common today.

2008 Sky Ramp Technology