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4 novembre 2011 5 04 /11 /novembre /2011 01:41

 

Risk contributors

An example of technical risk analysis for a STS mission is SPRA iteration 3.1 top risk contributors for STS-133:

  1. Micro-Meteoroid Orbital Debris (MMOD) strikes
  2. Space Shuttle Main Engine (SSME)-induced or SSME catastrophic failure Sony VAIO VGN-CR590EBT Battery
  3. Ascent debris strikes to TPS leading to LOCV on orbit or entry
  4. Crew error during entry
  5. RSRM-induced RSRM catastrophic failure (RSRM are the rocket motors of the Solid Rocket Boosters)
  6. COPV failure (COPV are tanks inside the orbiter that hold gas at high pressure) Sony VAIO VGN-CR590EBW Battery

An internal NASA risk assessment study (conducted by the Shuttle Program Safety and Mission Assurance Office at Johnson Space Center) released in late 2010 or early 2011 concluded that the agency had seriously underestimated the level of risk involved in operating the shuttle. Sony VAIO VGN-CR60B/L Battery

The report assessed that there was a 1 in 9 chance of a catastrophic disaster during the first nine flights of the shuttle but that safety improvements had later improved the risk ratio to 1 in 100.

Shuttle disasters

On January 28, 1986, Challenger disintegrated 73 seconds after launch due to the failure of the right SRB, killing all seven astronauts on board. Sony VAIO VGN-CR60B/P Battery

The disaster was caused by low-temperature impairment of an O-ring, a mission critical seal used between segments of the SRB casing. The failure of a lower O-ring seal allowed hot combustion gases to escape from between the booster sections and burn through the adjacent external tank, causing it to disintegrate.Sony VAIO VGN-CR60B/R Battery

Repeated warnings from design engineers voicing concerns about the lack of evidence of the O-rings' safety when the temperature was below 53 °F (12 °C) had been ignored by NASA managers.

On February 1, 2003, Columbia disintegrated during re-entry, killing its crew of seven, because of damage to the carbon-carbon leading edge of the wing caused during launch. Sony VAIO VGN-CR61B/L Battery

Ground control engineers had made three separate requests for high-resolution images taken by the Department of Defense that would have provided an understanding of the extent of the damage, while NASA's chief thermal protection system (TPS) engineer requested that astronauts on board Columbia be allowed to leave the vehicle to inspect the damage. Sony VAIO VGN-CR61B/N Battery

NASA managers intervened to stop the Department of Defense's assistance and refused the request for the spacewalk,and thus the feasibility of scenarios for astronaut repair or rescue by Atlantis were not considered by NASA management at the time. Sony VAIO VGN-CR61B/P Battery

Retirement

NASA retired the Space Shuttle in 2011, after 30 years of service. Discovery was the first of NASA's three remaining operational Space Shuttles to be retired. Michael Suffredini of the ISS program said that one additional trip was needed in 2011 to deliver parts to the International Space Station.Sony VAIO VGN-CR61B/R Battery

The final Space Shuttle mission was originally scheduled for late 2010, but the program was later extended to July 2011. Its final mission consisted of just four astronauts—Christopher Ferguson (Commander), Douglas Hurley (Pilot), Sandra Magnus (Mission Specialist 1), and Rex Walheim (Mission Specialist 2);Sony VAIO VGN-CR62B/L Battery

they conducted the 135th and last space shuttle mission on board Atlantis, which launched on July 8, 2011 and landed safely at the Kennedy Space Center on July 21, 2011 at 5:57 AM EDT (09:57 UTC).

Distribution of orbiters and other hardware

NASA announced it would transfer space-worthy orbiters to education institutions or museums at the conclusion of the space shuttle program. Sony VAIO VGN-CR62B/N Battery

Each museum or institution is responsible for covering theUS$28.8 million cost of preparing and transporting each vehicle for display. Twenty museums from across the country submitted proposals for receiving one of the retired orbiters. NASA also made Space Shuttle thermal protection system tiles available to schools and universities for less than US$25 each.Sony VAIO VGN-CR62B/P Battery

About 7,000 tiles were available on a first-come, first-served basis, limited to one per institution.

On April 12, 2011, NASA announced selection of locations for the remaining Shuttle orbiters:

  • Discovery will be delivered to the Udvar-Hazy Center of the Smithsonian Institution's National Air and Space Museum in Chantilly, Virginia, near Washington, D.C.
  • Endeavour will be delivered to the California Science Center in Los Angeles, California.
  • Enterprise (atmospheric test orbiter), currently on display at the National Air and Space Museum's Udvar-Hazy Center, will be moved to the Intrepid Sea-Air-Space Museum in New York City, New York. Sony VAIO VGN-CR70B/W Battery

Flight and mid-deck training hardware will be taken from the Johnson Space Center and will go to the National Air and Space Museum and theNational Museum of the U.S. Air Force. The full fuselage mockup, which includes the payload bay and aft section but no wings, is to go to the Museum of Flight in Seattle. Sony VAIO VGN-CR71B/W Battery

Mission Simulation and Training Facility's fixed simulator will go to the Adler Planetarium in Chicago, and the motion simulator will go to the Texas A&M Aerospace Engineering Department in College Station, Texas. Other simulators used in shuttle astronaut training will go to the Wings of Dreams Aviation Museum in Starke, Florida and the Virginia Air and Space Center in Hampton, Virginia.Sony VAIO VGN-CR72B/W Battery

In August 2011, the NASA Office of Inspector General (OIG) published a "Review of NASA's Selection of Display Locations for the Space Shuttle Orbiters"; the review had four main findings:

  • "NASA's decisions regarding Orbiter placement were the result of an Agency-created process that emphasized above all other considerations locating the Orbiters in places where the most people would have the opportunity to view them";Sony VAIO VGN-CR90HS Battery
  • "the Team made several errors during its evaluation process, including one that would have resulted in a numerical 'tie' among the Intrepid, the Kennedy Visitor Complex, and the National Museum of the U.S. Air Force (Air Force Museum) in Dayton, Ohio";Sony VAIO VGN-CR90NS Battery
  • there is "no evidence that the Team’s recommendation or the Administrator's decision were tainted by political influence or any other improper consideration";
  • "some of the choices NASA made during the selection process – specifically, its decision to manage aspects of the selection as if it were a competitive procurement and to delay announcement of its placement decisions until April 2011 Sony VAIO VGN-CR90S Battery
  • (more than 2 years after it first solicited information from interested entities)—may intensify challenges to the Agency and the selectees as they work to complete the process of placing the Orbiters in their new homes."

The NASA OIG had three recommendations, saying NASA should: Sony VAIO VGN-CR92HS Battery

  • "expeditiously review recipients' financial, logistical, and curatorial display plans to ensure they are feasible and consistent with the Agency's educational goals and processing and delivery schedules";
  • "ensure that recipient payments are closely coordinated with processing schedules, do not impede NASA's ability to efficiently prepare the Orbiters for museum display, and provide sufficient funds in advance of the work to be performed;Sony VAIO VGN-CR92NS Battery
  • and""work closely with the recipient organizations to minimize the possibility of delays in the delivery schedule that could increase the Agency's costs or impact other NASA missions and priorities."

In September 2011, the CEO and two board members of Seattle's Museum of Flight met with NASA Administrator Charles Bolden,Sony VAIO VGN-CR92S Battery

pointing out "significant errors in deciding where to put its four retiring space shuttles"; the errors alleged include inaccurate information on Museum of Flight's attendance and international visitor statistics, as well as the readiness of the Intrepid Sea-Air-Space Museum's exhibit site.Sony VAIO VGN-CS118E/Q Battery

Space Shuttle successors and legacy

Until another U.S. launch vehicle is ready, crews will travel to and from the International Space Station aboard Russian Soyuz spacecraft or possibly a future American commercial spacecraft. A planned successor to STS was the "Shuttle II" during the 1980s and 1990s, and later the Constellation program during the 2004–2010 period. Sony VAIO VGN-CS118E/R Battery

CSTS was a proposal to continue to operate STS commercially, after NASA.On September 14, 2011, NASA announced that it had selected the design of a newSpace Launch System that it said would take the agency's astronauts farther into space than ever before and provide the cornerstone for future human space exploration efforts by the U.S.Sony VAIO VGN-CS118E/W Battery

In culture

Space Shuttles have been featured in numerous works of fiction and nonfiction, from movies for kids to documentaries. Early examples include the 1979 James Bond film, Moonraker, where shuttles played a major role well before any were actually launched, Activision videogame Space Shuttle: Sony VAIO VGN-CS11S/P Battery

A Journey into Space (1982) and G. Harry Stine's novel Shuttle Down (1981). In the 1986 film SpaceCampAtlantis accidentally launched into space with a group of U.S. Space Campparticipants as its crew. The 1998 film Armageddon portrayed a combined crew of offshore oil rig workers and US military staff who pilot two modified shuttles to avert the destruction of Earth by an asteroid.Sony VAIO VGN-CS11S/Q Battery

Retired American test pilots visited a Russian satellite in the Clint Eastwood adventure film Space Cowboys (2000). This was followed by the 2004 Bollywood movie Swades, where a space shuttle was used to launch a special rainfall monitoring satellite.Sony VAIO VGN-CS11S/W Battery

The movie was filmed at Kennedy Space Center in the year following the Columbia disaster that had taken the life of KC Chawla, the first woman of Indian origin in space. On television, the 1996 drama The Cape portrayed the lives of a group of NASA astronauts as they prepared for and flew shuttle missions. Sony VAIO VGN-CS11Z/R Battery

Odyssey 5 was a short lived sci-fi series that featured the crew of a space shuttle as the last survivors of a disaster that destroyed Earth.

The Space Shuttle has also been the subject of toys and models; for example, a large Lego Space Shuttle model was constructed by visitors at Kennedy Space Center,  and smaller models have been sold commercially as a standard "LegoLand" set.Sony VAIO VGN-CS120J/P Battery

reusable launch system (or reusable launch vehicle, RLV) is a launch system which is capable of launching a launch vehicle into space more than once. This contrasts with expendable launch systems, where each launch vehicle is launched once and then discarded.Sony VAIO VGN-CS120J/Q Battery

No true orbital reusable launch system is currently in use. The closest example is the partially reusable Space Shuttle. The orbiter, which includes the main engines, and the two solid rocket boosters, are reused after several months of refitting work for each launch. The external fuel drop tank is typically discarded, but it is possible for it be re-used in space for various applications.Sony VAIO VGN-CS120J/R Battery

Orbital RLVs are thought to provide the possibility of low cost and highly reliable access to space. However, reusability implies weight penalties such as non-ablative reentry shielding and possibly a stronger structure to survive multiple uses, and given the lack of experience with these vehicles, the actual costs and reliability are yet to be seen.Sony VAIO VGN-CS190JTB Battery

In the early 1950s popular science fiction often depicted space launch vehicles as either single-stage reusable rocketshipswhich could launch and land vertically (SSTO VTVL), or single-stage reusable rocketplanes which could launch and land horizontally (SSTO HTHL).Sony VAIO VGN-CS190JTP Battery

The realities of early engine technology with low specific impulse or insufficient thrust-to-weight ratio to escape ourgravity well, compounded by construction materials without adequate performance (strength, stiffness, heat resistance) and low weight seemingly rendered that original single-stage reusable vehicle vision impossible.Sony VAIO VGN-CS190JTQ Battery

However advances in materials and engine technology have rendered this concept potentially feasible.

Before VTVL SSTO designs came the partially reusable multi-stage NEXUS launcher by Krafft Ehricke. The pioneer in the field of VTVL SSTO, Philip Bono, worked at Douglas. Bono proposed several launch vehicles including:Sony VAIO VGN-CS190JTR Battery

ROOST, ROMBUS, Ithacus,Pegasus and SASSTO. Most of his vehicles combined similar innovations to achieve SSTO capability. Bono proposed:

  • Plug nozzle engines to retain high specific impulse at all altitudes.
  • Base first reentry which allowed the reuse of the engine as a heat shield, lowering required heat shield mass.Sony VAIO VGN-CS190JTW Battery
  • Use of spherical tanks and stubby shape to reduce vehicle structural mass further.
  • Use of drop tanks to increase range.
  • Use of in-orbit refueling to increase range.  Sony VAIO VGN-CS190NAB Battery

Bono also proposed the use of his vehicles for space launch, rapid intercontinental military transport (Ithacus), rapid intercontinental civilian transport (Pegasus), even Moon and Mars missions (Project Selena, Project Deimos).

In Europe, Dietrich Koelle, inspired by Bono's SASSTO design, proposed his own VTVL vehicle named BETA.Sony VAIO VGN-CS190NAC Battery

Before HTHL SSTO designs came Eugen Sänger and his Silbervogel ("Silverbird") suborbital skip bomber. HTHL vehicles which can reach orbital velocity are harder to design than VTVL due to their higher vehicle structural weight. This led to several multi-stage prototypes such as a suborbital X-15. Sony VAIO VGN-CS190NAD Battery
Aerospaceplane being one of the first HTHL SSTO concepts. Proposals have been made to make such a vehicle more viable including:

  • Rail boost (e.g. 270 m/s at 3000 m on a mountain allowing 35% less SSTO takeoff mass for a given payload in one NASA study)
  • Use of lifting body designs to reduce vehicle structural mass.
  • Use of in-flight refueling. Sony VAIO VGN-CS190NBB Battery

Other launch system configuration designs are possible such as horizontal launch with vertical landing (HTVL) and vertical launch with horizontal landing (VTHL). One of the few HTVL vehicles is the 1960s concept spacecraft Hyperion SSTO, designed by by Philip Bono.Sony VAIO VGN-CS190NCA Battery

X-20 Dyna-Soar is an early example of a VTHL design,  while the HL-20 and X-34 are examples from the 1990s.As of February 2010, the VTHL X-37 has completed initial development and flown an initial classified orbital mission of over seven months duration.Sony VAIO VGN-CS190NCB Battery

Currently proposed VTHL manned spaceplanes include the Dream Chaser andPrometheus, both circa 2010 concept spaceplanes proposed to NASA under the CCDev program.

The late 1960s saw the start of the Space Shuttle design process. From an initial multitude of ideas a two-stage reusable VTHL design was pushed forward. Sony VAIO VGN-CS190NCC Battery

That eventually ended up as a reusable orbiter with an expendable drop tank and reusable solid rocket boosters to reduce design expenses.

During the 1970s further VTVL and HTHL SSTO designs were proposed for solar power satellite and military applications.Sony VAIO VGN-CS50B/W Battery

There was a VTVL SSTO study by Boeing. HTHL SSTO designs included the Rockwell Star-Raker and the Boeing HTHL SSTO study. However the focus of all space launch funding in the United States on the Shuttle killed off these prospects. The Soviet Union followed suit with Buran. Others preferred expendables for their lower design risk, and lower design cost.Sony VAIO VGN-CS51B/W Battery

Eventually the Shuttle was found to be expensive to maintain, even more expensive than an expendable launch system would have been. The cancellation of a Shuttle-Centaur rocket after the loss of Challenger also caused an hiatus that would make it necessary for the United States military to scramble back towards expendables to launch their payloads. Sony VAIO VGN-CS52JB/W Battery

Many commercial satellite customers had switched to expendables even before that, due to unresponsiveness to customer concerns by the Shuttle launch system.

In 1986 President Ronald Reagan called for an airbreathing scramjet plane to be built by the year 2000, called NASP/X-30 that would be capable of SSTO. Sony VAIO VGN-CS60B/P Battery

Based on the research project copper canyon the project failed due to severe technical issues and was cancelled in 1993.

This research may have inspired the British HOTOL program, which rather than airbreathing to high hypersonic speeds as with NASP, proposed to use a precooler up to Mach 5.5. Sony VAIO VGN-CS60B/Q Battery

The program's funding was canceled by the British government when the research identified some technical risks as well as indicating that that particular vehicle architecture would only be able to deliver a relatively small payload size to orbit.

When the Soviet Union imploded in the early nineties, the cost of Buran became untenable. Russia has only used pure expendables for space launch since.Sony VAIO VGN-CS60B/R Battery

The 1990s saw interest in developing new reusable vehicles. The military Strategic Defense Initiative ("Star Wars") program "Brilliant Pebbles" required low cost, rapid turnaround space launch. From this requirement came the McDonnell Douglas Delta Clipper VTVL SSTO proposal. Sony VAIO VGN-CS61B/P Battery

The DC-X prototype for Delta Clipper demonstrated rapid turnaround time and that automatic computer control of such a vehicle was possible. It also demonstrated it was possible to make a reusable space launch vehicle which did not require a large standing army to maintain like the Shuttle.Sony VAIO VGN-CS61B/Q Battery

In mid-1990, further British research and major reengineering to avoid deficiencies of the HOTOL design led to the far more promising Skylon design, with much greater payload.

From the commercial side, large satellite constellations such as Iridium satellite constellation were proposed which also had low cost space access demands. Sony VAIO VGN-CS61B/R Battery

This fueled a private launch industry, including partially reusable vehicle players, such as Kistler, and reusable vehicle players such as Rotary Rocket.

The end of that decade saw the implosion of the satellite constellation market with the bankruptcy of Iridium. In turn the nascent private launch industry collapsed. Sony VAIO VGN-CS62JB/P Battery

The fall of the Soviet Union eventually had political ripples which led to a scaling down of ballistic missile defense, including the demise of the "Brilliant Pebbles" program. The military decided to replace their aging expendable launcher workhorses, evolved from ballistic missile technology, with the EELV program. Sony VAIO VGN-CS62JB/Q Battery

NASA proposed riskier reusable concepts to replace Shuttle, to be demonstrated under the X-33 and X-34 programs.

The 21st century saw rising costs and teething problems lead to the cancellation of both X-33 and X-34. Then the Space Shuttle Columbia disaster and another grounding of the fleet. Sony VAIO VGN-CS62JB/R Battery

The Shuttle design was now over 20 years old and in need of replacement. Meanwhile the military EELV program churned out a new generation of better expendables. The commercial satellite market is depressed due to a glut of cheap expendable rockets and there is a dearth of satellite payloads.Sony VAIO VGN-CS71B/W Battery

Against this dire backdrop came the Ansari X Prize contest, inspired by the aviation contests made in the early 20th century. Many private companies competed for the Ansari X Prize, the winner being Scaled Composites with their reusable HTHL SpaceShipOne.Sony VAIO VGN-CS72JB/W Battery

It won the ten million dollars, by reaching 100 kilometers in altitude twice in a two week period with the equivalent of three people on board, with no more than ten percent of the non-fuel weight of the spacecraft replaced between flights. While SpaceShipOne is suborbital like the X-15, Sony VAIO VGN-CS90HS Battery

some hope the private sector can eventually develop reusable orbital vehicles given enough incentive. SpaceXis a recent player in the private launch market which has partially reusable vehicles.

Single stage

There are two approaches to Single stage to orbit or SSTO. The rocket equation says that an SSTO vehicle needs a high mass ratio.Sony VAIO VGN-CS90NS Battery

Mass ratio is defined as the mass of the fully fueled vehicle divided by the mass of the vehicle when empty (zero fuel weight, ZFW).

One way to increase the mass ratio is to reduce the mass of the empty vehicle by using very lightweight structures and high efficiency engines. Sony VAIO VGN-CS90S Battery

This tends to push up maintenance costs as component reliability can be impaired, and makes reuse more expensive to achieve. The margins are so small with this approach that there is uncertainty whether such a vehicle would be able to carry any payload into orbit. Also, lightweight implies small vehicles, which in turn implies small payloads, increasing the cost per kilogram of the payload.Sony VAIO VGN-CS91HS Battery

Two or more stages to orbit

Two stage to orbit requires designing and building two independent vehicles and dealing with the interactions between them at launch. Usually the second stage in launch vehicle is 5-10 times smaller than the first stage, although in biamese and triameseapproaches each vehicle is the same size.Sony VAIO VGN-CS91NS Battery

In addition, the first stage needs to be returned to the launch site for it to be reused. This is usually proposed to be done by flying a compromise trajectory that keeps the first stage above or close to the launch site at all times, or by using small airbreathing engines to fly the vehicle back, Sony VAIO VGN-CS91S Battery

or by recovering the first stage downrange and returning it some other way (often landing in the sea, and returning it by ship.) Most techniques involve some performance penalty; these can require the first stage to be several times larger for the same payload, although for recovery from downrange these penalties may be small.Sony VAIO VGN-CS92DS Battery

The second stage is normally returned after flying one or more orbits and reentering.

Horizontal landing

In this case the vehicle requires wings and undercarriage (unless landing at sea). This typically requires about 9-12% of the landing vehicle to be wings; which in turn implies that the takeoff weight is higher and/or the payload smaller.Sony VAIO VGN-CS92JS Battery

Concepts such as lifting bodies attempt to deal with the somewhat conflicting issues of reentry, hypersonic and subsonic flight; as does the delta wing shape of the Space Shuttle.

Vertical landing

Parachutes could be used to land vertically, either at sea, or with the use of small landing rockets, on land (as with Soyuz).Sony VAIO VGN-CS92XS Battery

Alternatively rockets could be used to softland the vehicle on the ground from the subsonic speeds reached at low altitude (seeDC-X). This typically requires about 10% of the landing weight of the vehicle to be propellant.

A slightly different approach to vertical landing is to use an autogyro or helicopter rotor. This requires perhaps 2-3% of the landing weight for the rotor.Sony VAIO VGN-FW11E Battery

Horizontal takeoff

The vehicle needs wings to take off. For reaching orbit, a 'wet wing' would often need to be used where the wing contains propellant. Around 9-12% of the vehicle takeoff weight is perhaps tied up in the wings.

Vertical takeoff

This is the traditional takeoff regime for pure rocket vehicles. Rockets are good for this regime, since they have a very high thrust/weight ratio (~100).Sony VAIO VGN-FW11L Battery

[edit]Airbreathing

Airbreathing approaches use the air for propulsion during ascent. The most commonly proposed approach is the scramjet, butturborocket, Liquid Air Cycle Engine (LACE) and precooled jet engines are also proposed to be used.

In all cases the highest speed that airbreathing can reach is far short of orbital speed (about Mach 15 for Scramjets and Mach 5-6 for the other engine designs) and rockets would be used for the remaining 10-20 Mach for orbit.Sony VAIO VGN-FW11M Battery

The thermal situation for airbreathers (particularly scramjets) can be awkward; normal rockets fly steep initial trajectories to avoid drag, whereas scramjets would deliberately fly through relatively thick atmosphere at high speed generating enormous heating of the airframe. The thermal situation for the other airbreathing approaches is much more benign, although is not without its challenges.Sony VAIO VGN-FW11S Battery

Hydrogen fuel

Hydrogen is often proposed since it has the highest exhaust velocity. However tankage and pump weights are high due to insulation and low propellant density; and this wipes out much of the advantage.

Still, the 'wet mass' of a hydrogen fuelled stage is lighter than an equivalent dense stage with the same payload and this can permit usage of wings, and is good for second stages.Sony VAIO VGN-FW11ZU Battery

Dense fuel

Dense fuel is sometimes proposed since, although it implies a heavier vehicle, the specific tankage and pump mass is much improved over hydrogen. Dense fuel is usually suggested for vertical takeoff vehicles, and is compatible with horizontal landing vehicles, since the vehicle is lighter than an equivalent hydrogen vehicle when empty of propellant. Sony VAIO VGN-FW139E/H Battery

Non-cryogenic dense fuels also permit the storage of fuel in wing structures. Projects have been underway to densify existing fuel types through various techniques. These include slush technologies for cryogenics like hydrogen and propane. Another densifying method has been studied that would also increase the specific impulse of fuels. Sony VAIO VGN-FW140AE Battery

Adding finely powdered carbon, aluminum, titanium, and boron to hydrogen and kerosene have been studied. These additives increase the specific impulse (Isp) but also the density of the fuel. For instance, the French ONERA missile program tested boron with kerosene in gelled slurries, as well as embedded in paraffin, and demonstrated increases in volumetric specific impulse of between 20-100%.Sony VAIO VGN-FW140D Battery

Tripropellant

Dense fuel is optimal early on in a flight, since the thrust to weight of the engines is better due to higher density; this means the vehicle accelerates more quickly and reaches orbit sooner, reducing gravity losses.

However, for reaching orbital speed, hydrogen is a better fuel, since the high exhaust velocity and hence lower propellant mass reduces the take off weight.Sony VAIO VGN-FW140E Battery

Therefore tripropellant vehicles  start off burning with dense fuel and transition to hydrogen. (In a sense the Space Shuttle does this with its combination of solid rockets and main engines, but tripropellant vehicles usually carry their engines to orbit. Sony VAIO VGN-FW140E/H Battery

Propellant costs

As with all current launch vehicles propellant costs for a rocket are much lower than the costs of the hardware. However, for reusable vehicles if the vehicles are successful, then the hardware is reused many times and this would bring the costs of the hardware down.Sony VAIO VGN-FW140E/W Battery

In addition, reusable vehicles are frequently heavier and hence less propellant efficient, so the propellant costs could start to multiply up to the point where they become significant.

Launch assistance

Since rocket delta-v has a non linear relationship to mass fraction due to the rocket equation, any small reduction in delta-v gives a relatively large reduction in the required mass fraction; and starting a mission at higher altitude also helps.

Many systems have proposed the use of aircraft to gain some initial velocity and altitude; either by towing, carrying or even simply refueling a vehicle at altitude.Sony VAIO VGN-FW140FE Battery

Various other launch assists have been proposed, such as ground based sleds, or maglev systems, high altitude (80 km) maglev systems such as launch loops, to more exotic systems such as tether propulsion systems to catch the vehicle at high altitude, or even Space Elevators.Sony VAIO VGN-FW140N Battery

Reentry heat shields

Robert Zubrin has said that as a rough rule of thumb, 15% of the landed weight of a vehicle needs to be aerobraking reentry shielding.

Reentry heat shields on these vehicles are often proposed to be some sort of ceramic and/or carbon-carbon heat shields, or occasionally metallic heat shields (possibly using water cooling or some sort of relatively exotic rare earth metal.)Sony VAIO VGN-FW140N/W Battery

Some shields would be single use ablatives and would be discarded after reentry.

A newer Thermal Protection System (TPS) technology was first developed for use in steering fins on ICBM MIRVs. Given the need for such warheads to reenter the atmosphere swiftly and retain hypersonic velocities to sea level,Sony VAIO VGN-FW145E Battery

researchers developed what are known as SHARP materials, typically hafnium diboride and zirconium diboride, whose thermal tolerance exceeds 3600 C. SHARP equipped vehicles can fly at Mach 11 at 30 km altitude and Mach 7 at sea level. The sharp-edged geometries permitted with these materials also eliminates plasma shock wave interference in radio communications during reentry.Sony VAIO VGN-FW145E/W Battery

SHARP materials are very robust and would not require constant maintenance, as is the case with technologies like silica tiles, used on the Space Shuttle, which account for over half of that vehicles maintenance costs and turnaround time. The maintenance savings alone are thus a major factor in favor of using these materials for a reusable launch vehicle, whose raison d'etre is high flight rates for economical launch costs.Sony VAIO VGN-FW160AE Battery

Weight penalty

The weight of a reusable vehicle is almost invariably higher than an expendable that was made with the same materials, for a given payload.

R&D

The R&D costs of reusable vehicle are expected to be higher, because making a vehicle reusable implies making it robust enough to survive more than one use, which adds to the testing required. Sony VAIO VGN-FW160D Battery

Increasing robustness is most easily done by adding weight; but this reduces performance and puts further pressure on the R&D to recoup this in some other way.

These extra costs must be recouped; and this pushes up the average cost of the vehicle.Sony VAIO VGN-FW160E Battery

Maintenance

Reusable launch systems require maintenance, which is often substantial. The Space Shuttle system requires extensive refurbishing between flights, primarily dealing with the silica tile TPS and the high performance LH2/LOX burning main engines. Sony VAIO VGN-FW160E/H Battery

Both systems require a significant amount of detailed inspection, rebuilding and parts replacement between flights, and account for over 75% of the maintenance costs of the Shuttle system. These costs, far in excess of what had been anticipated when the system was constructed, have cut the maximum flight rate of Shuttle to 1/4 of that planned. Sony VAIO VGN-FW160F/E Battery

This has also quadrupled the cost per pound of payload to orbit, making Shuttle economically infeasible in today's launch market for any but the largest payloads, for which there is no competition.

For any RLV technology to be successful, it must learn from the failings of Shuttle and overcome those failings with new technologies in the TPS and propulsion areas.Sony VAIO VGN-FW170J/H Battery

Manpower & Logistics

The Space Shuttle program requires a standing army of over 9,000 employees to maintain, refurbish, and relaunch the shuttle fleet, irrespective of flight rates. That manpower budget must be divided by the total number of flights per year. The fewer flights means the cost per flight goes up significantly. Sony VAIO VGN-FW180AE Battery

Streamlining the manpower requirements of any launch system is an essential part of making an RLV economical. Projects that have attempted to develop this ethic include the DC-X Delta Clipper project, as well as the current SpaceX Falcon 1 and Falcon 9 programs.Sony VAIO VGN-FW180D Battery

One issue mitigating against this drive for labor savings is government regulation. Given that NASA and USAF (as well as government programs in other countries) are the primary customers and sources of development capital,Sony VAIO VGN-FW180E Battery

government regulatory requirements for oversight, parwork, quality, safety, and other documentation tend to inflate the operational costs of any such system.Sony VAIO VGN-FW180E/H Battery,Sony VAIO VGN-FW180FU Battery,Sony VAIO VGN-FW190EBH Battery

 

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