5. Spacecraft Mission Technologies

As emphasized elsewhere in this report, Delta Sprint uses existing off-the-shelf systems wherever possible to complete its mission.  The Descent Module will be constructed of an aluminum-copper alloy and will contain the most reliable systems currently available.  The Service Module contains the most hazardous systems, including the solid fueled deorbit motor and orbital maneuvering engines.  The Service Module has the ability to swing out to allow spacewalk or remote inspection of the thermal protection designed to protect you during the fiery return to Earth.  The prospect of total guidance failure is only barely survivable and would no doubt be terrifying.  To prevent this, the guidance system uses three computers, two of which need to fail before there is a major problem.

5.1 Standard and Spartan Descent Module

5.1.1 Primary Structure

The main structural elements will be made of 2024-T8 forged aluminum alloy in an isogrid configuration.  The seats will be arranged in two rows, with one seat in the front row.  This front row seat has access to the life support racks manual controls and its crew member will execute some of the preflight checklist from this seat.  This crew member has no critical role in the ascent or entry piloting of the vehicle.  The back row has two seats.  On the left is the commander, on the right is the pilot.  Each of them is provided with a single large LCD console to interface with observation cameras and automatic guidance systems.  Manual controls are contained in a caddy between them.  The large, oval shaped hatches on each side contain small (200mm) round windows.  The hatches themselves add strength to the spacecraft at axial accellerations above 1.25g.  These hatches open outward and have ordinance to rapidly jettison them in an emergency, as the load bearing seals will be relatively complex and potentially more failure prone.

The life support racks are contained to the left and right and forward of the front row seat and are each sealable and ventable to the vacuum of space in the event of a fire in one of these bays.  The landing gear (airbag) bay and parachute compartment are structural by the necessity of their function.  The forward docking port is placed using a bolted interface and double seal so that it can be replaced in processing on the ground (it may use a variant of the 1575mm diameter Evolved Expendable Launch Vehicle (EELV) payload interface.)

Qualification testing will be to a 1.50 safety factor with the structure heated to 80 degrees Centigrade to simulate the worst survivable structural thermal and mechanical loads.  The qualification article will be inspected and analyzed for engineering data, and will not be used in piloted flight.

Acceptance testing for each flight article will be to a 1.25 safety factor with the structure at appoximately 25 degrees Centigrade.

5.1.2 Descent Module Reaction Control System

The DMRCS uses hydrazine (N2H4) catalyzed by Shell 405 and parts compatible with the SMRCS, as well as gaseous nitrogen pressurant.  The supply is far smaller than on the SMRCS and does not have a deorbit capability.  The DMRCS is integrated with the SMRCS when the Service Module is present and in the flight position (i.e. not swung out for inspection of the base shield.)  The tanks are so small as to be more properly referred to as bottles and are located just outside the life support racks.  The pressurant bottles are positioned and packaged so that they do not pose a threat to the crafts structural integrity if they explode, and so that they compromise the less critical parts of the thermal protection system (even so, a pressurant explosion and resultant failure of the DMRCS would result in an erratic attitude if undocked, and no longer provide pressure to the DMRCS, resulting in a loss of control if the Service Module is not docked.  Thus, a pressurant explosion during entry would certainly be fatal, although one while docked to a space station allows for the possibilty of repair or rescue.)

5.1.3 Electrical Systems

Lithium-ion battery chemistry for primary power and Lithium-thionyl-chloride batteries for emergency power have been tentatively selected.  The electrical system has not yet been sized, but is intended to provide up to 4 days of normal mission power and two docking attempts, or up to 10 days of emergency endurance when fully charged.  Emergency power systems are intended to provide enough power for up to 24 hours of emergency lighting and circulation, and the ability to manually deorbit and descend the spacecraft.  Emergency power is not intended for guidance or cabin temperature control, but is intended to provide a backup in case of a total electrical and guidance failure.

The docking port and associated systems will have a charger power supply capable of charging the primary electrical system from the space station.  The main power circuit will use an appropriate direct current (DC) voltage for most systems and an alternating current (AC) circuit for some systems (such as the radar) which will have unusual power requirements.  Cabin lighting will use light emitting diodes (LEDs) similar to those being distributed by the Light Up The World ministry, for their low power consumption, high reliability and extreme life span (up to 11 years of continuous operation.)

5.1.4 Life Support

Lithium perchlorate and lithium hydroxide are the primary life support material for handling metabolic gasses (oxygen and carbon dioxide) and have no electrical requirements for their chemistry.  Off the shelf hardware should be available from other piloted spacecraft systems, mine collapse survival shelters, balloons, airliners and military aircraft.  A small pressurized nitrogen supply is needed in the event of a station depressurization emergency (see 4.1.2.4.7) Water is a material used not only in life support, but also for trimming the spacecraft for entry (see 4.1.3.4).  The base of the Descent Module contains two potable water tanks, and two grey water tanks.  The tanks have flexible bladders and links to DMRCS pressurization.  Water is put in normally by exceeding the tank pressure on the input, rather than venting the pressurant from the tank.  The tanks also have vents to the outside of the spacecraft, ducted to the Descent Module base and vented slightly forward to prevent contamination of the Service Module and base heat shield.  These vents are not intended to be used except in an emergency (overpressurization) or Deorbit Coast.  Human secondary metabolic functions are accomodated by a flexible curtain for privacy (the volume selected for use will be decided by system sensitivity to waste materials) and tentatively, the lightweight Gemini Bag or camping toilet for solid wastes, as internal volume and mass is probably too restricted for a commode, which is difficult to justify on Delta Sprints small crew and short solo endurance in any case (normal mission is 3 person-days, long mission is 12 person-days, and emergency endurance is 33 person days; STS-107 was 112 person-days, a typical ISS crew watch is 540 person-days and Mars Direct is about 3700 person-days.)  Liquid waste should be placed in the grey water tank if possible.  A supply of sodium polyacrylate contained by a Gemini Bag may be an alternative or backup option.

5.1.5 Guidance

Guidance will be provided by a redundant solid state system with no moving parts.  Attitude rates and accelleration values will be provided by twelve single axis laser gyros to three guidance computers which each contain digitally active stable elements (DASE) to provide an inertial reference.  Each computers DASE will be allowed to drift independently.  Each computer relays its decisions and DASE status to its own EEPROM log "drive".  Each computer possesses another EEPROM "drive" from which it can boot its operating system and control logic during the pre-launch sequence, prior to undocking from the space station, and after malfunctions.  Each guidance computer has an averages control system that averages and logs each DASE and each computers state vector data, and checks for significant disagreement.  One of the computers is a "primary" computer that takes up this role to check itself and the other two.  If a DASE data or state vector data set exceeds error limits, the primary computer takes it off line, although that computer will continue to operate, it will not be allowed to influence guidance decisions.  If the primary computer malfunctions, another computer takes over the primary role.  Note that ascent and entry malfunctions and the bugs that cause them need to be rigorously prevented and hunted down and corrected as they occur both in simulation and flight.

5.1.5.1 Ascent Operation

Each computer uses its own DASE to make ascent decisions.  The reasoning behind this is to prevent a circuit logic or program logic failure from happening to all three computers at once if the bug is dependent on a specific data value in the DASE (all computers should have slightly different DASE and state vector data from drift.) A number of virtual decisions are made during the ascent prior to second stage separation to verify operational reliability.  These do not have an actual effect on the vehicle, as it is being controlled by the Delta IIs Redundant Inertial Flight Control Assembly, or RIFCA built into the launch vehicle, and due to rendezvous alignment ascent logic, are likely to disagree with those being made by the RIFCA.  The computers reset their ascent commands to attitude hold prior to SECO before being activating the Delta Sprint and Fregat RCS.  The guidance system and guidance errors are handled similarly to the ascent Fregat burn when the Delta Sprint guidance system actually does have control authority.  If Delta Sprint guidance problems occuring prior to MECO require an abort, it is not executed until after MECO.  Delta Sprint guidance does not have the ability to command an abort. Abort can be commanded either by the crew, ground command system, or by the Destruct Signal Detection system, which detects the Range Safety Destruct command given to the launch vehicle when it exceeds its range safety corridor.  During the Fregat ascent burn, the Delta Sprint guidance works to align the insertion plane with the space station plane.  The guidance decisions they "think about" during second stage burn will appear less drastic than the ones they actually execute during the Fregat burn (yaws and pitches of up to 30 degrees are expected.)

If one DASE disagrees too much with the other two, its computer is taken off line and allowed to continue operation, but not influence guidance decisions.  The crew has the option of rebooting the computer or reloading an average of the other computers DASE and state vector data or data from ground uplink and bringing the computer back on line.  It is not expected that these features would be used unless crew safety is threatened by total guidance failure or entry into a manual command mode.

If all three DASE disagree by more than a certain amount, the system will notify the crew that guidance has become unreliable.  The crew can elect to continue guidance normal operation, which will use the two computers most in agreement and all three computers gain update data from ground uplink and/or sensor data on altitude and speed and position, and crew input (The crew can judge attitudes visually out the windows and cameras as long as a reference (horizon, sun, or moon) is provided and its location in the sky is understood.)  If this happens during ascent, an abort is declared.  Because guidance can continue to operate during these DASE failure modes, triple DASE failure, is not considered a total guidance failure by the definition of 4.1.3.2.  The crew also have the option of operating the spacecraft manually based on guidance data and crew estimates of DASE and state vector errors in the guidance data.  Guidance would continue to try to correct DASE and state vector data based on sensor readings.  An operational procedure of this might look like, for example, the commander flying the ship while the pilot looks out the cameras and the passenger looks out the windows reporting back on error estimates.  The safety value of these modes is highly dependent on crew and ground controller training.  It is probable that after a degree of guidance reliability has been proven, training for these flight modes can be relaxed.  Guidance bugs based on correct DASE and state vector data cause the affected computer to be taken off line.  If two computers malfunction in the same way at the same time, than the correctly operating computer will be taken off line, backup sensors detect an environment error as the craft leaves its predicted flight path.  If all three computers are affected by a common bug, the environment error alarm (attitude, altitude, speed, accellerations, external temperatures, etc. exceed error margins predicted for the mission) comes without warning a few seconds after the failure.  Aborts due to environmental errors are considered total guidance failures by the definition of 4.1.3.2.  If all three computers disagree, a guidance alarm equivalent to an environmental error is sounded.

To summarize guidance operations modes:

a) normal guidance operation

b) marginal guidance operation and automatic control with the crew helping correct errors

c) manual flight with guidance providing critical flight data

d) manual flight control without guidance help (Section 4.1.3.2 mode.)

5.1.5.2 Descent Operations

Guidance operation during the deorbit maneuver and the descent (possibly excluding Deorbit Coast and Landing) operates similar to the ascent mode.  Delta Sprint has no external control surfaces available until the parasol is deployed.  The deorbit burn is aimed at getting the entry trajectory periapsis and longitude correct.  The periapsis is affected by yaw and pitch angle and by timing, while longitude of periapsis is affected mostly by timing.  Obviously, the characteristics of the orbit prior to the deorbit maneuver have an influence.  After entry interface, guidance normally controls roll for crossrange and downrange to attain an accurate point of entry for the Peak Loads Period.  Guidance controls Delta Sprints Descent Module for load relief during the Peak Loads Period and then afterwards has no effective control until the Main Parasol opens (therefore the Peak Loads trajectory needs to be accurate to within 5km (3 miles) from about 800km (500 miles) uprange.)  The Shuttle, during its analogous Constant Drag Phase has a lot of crossrange control but little downrange control because of its need to relieve thermal and mechanical loads.  The Shuttle, however has a much greater downrange margin when it enters the Terminal Area Energy Management phase at Mach 5.  Delta Sprint has no equivalent.  Delta Sprint does have the advantage over the Shuttle during the HAC / Pre-Final / Final Approach phase or its Delta Sprint equivalent because it has more glide range and maneuverability once the parasol is deployed.  Delta Sprint is also more tolerant to landing on adverse surfaces than Shuttle, which probably cannot survive an ocean ditching or landing on a non-runway surface.

5.1.5.3 Guidance On Orbit

Orbital guidance is tricky to do by hand and normally expends far more propellant than ideal, especially during combined corrections.  Combined correction is the most efficient mode of doing a rendezvous manuver.  Guidances job on orbit will be mostly computing corrective combinations that achieve orbit phasing and alignment at the same time for rendezvous, and for ground trace control after undocking (mostly phasing.)  It is this desire to combine rendezvous maneuvers which motivates placing the transfer orbit periapsis and apoapsis at the crossing nodes of the target orbit.  Guidance also controls the actual thruster firing, with instrument monitoring and command override support provided by the crew.

The most common rendezvous is actually the standard geostationary satellite ascent.  These missions rendezvous with a point above a certain geodetic longitude on the equator with a period matching Earths sidereal rotation, becoming a phantom rendezvous target to put the satellite into.  In doing so, the satellite executes two major combined maneuvers, the perigee kick and the apogee kick.  The perigee kick typically happens from a circular orbit inclined to the geodetic equator at a crossing node with the geodetic equator, which defines the target orbit (this maneuver is traditionally done by the launch vehicle.)  By putting the periapsis on the equator, the apoapsis at the target altitude is also on the geodetic equator close to the rendezvous target.  The apogee kick does most of the plane alignment (bringing the geodetic inclination of the satellite orbit to 0 degrees) while speeding the satellite up to circularize the orbit.  The satellite is usually a few degress out of longitude after this initial insertion, and the last few small maneuvers to get it on station are analogous to the docking maneuvers for a station rendezvous...although there is probably nothing to collide with.

5.1.5.4 Mass Properties Determination

It should be possible to train the guidance system to compute the mass properties of the spacecraft by firing thrusters and seeing how they affect attitude rates and translational influence.  Small translations have little effect on the current orbit and can be corrected.  By firing each of the thrusters in turn for highly accurate individual firings, its should be possible for guidance to accurately and automatically determine its center of gravity and total mass very accurately.  This mode would be used between undocking and the deorbit maneuver to predict the trim properties of the spacecraft during the deorbit maneuver and descent.

5.1.5.5 Docking

Guidance controls docking with the help of docking systems described in 5.1.6.  The docking and proximity procedures are similar to those for Shuttle.  The spacecraft uses the Fregat upper stage for what can be described alternately as the last rendezvous maneuver or the first docking maneuver.  In Shuttle-speak, it is called Terminal Intercept or TI, and is targeted based on information from the radar.  The spacecraft then jettisons the Fregat upper stage, then uses its own thrusters to complete the docking maneuvers.

5.1.6 Docking Systems

The docking port and most of the docking systems are located in a forward compartment protected by doors in the nose.  The doors are made from 2024 aluminum as well and are protected by either AFRSI or a Teflon-based ablative (the intention being that this ablative would protect against emergency thermal loads and have little exposure during a normal entry.  The doors are opened prior to Terminal Intercept and the dominant system other than the docking port itself comes into play.  Inside the left door, designed to point forward when the doors are open, is the docking radar (which can hopefully be adapted from a general aviation navigational or weather radar.)  The docking radar is used to obtain location, range, and closing speed information from the target, hopefully with the help of a transponder on the station.  As the spacecraft closes, two cameras and the laser range finder come into play.  In a small package on the docking port centerline, the laser rangefinder and one of the cameras point directly toward the station docking port.  Docking port alignment is confirmed visually by putting the crosses together on the docking port centerline.  It is difficult to describe, but should be intuitively apparent if one studies photographs of the existing docking system.  Another camera, with pan and tilt capability, is located in the right door.  It can be panned around to verify the distance between the docking ports within about 6 feet and can also search for other docking hazards, which could included EVA related equipment (handholds, footholds, Strela cranes, CETA carts, etc.) and robotic arms.  The right door camera turret may need to include gyro wheels to counter the inertial moments of the pan/tilt mechanism.

5.1.7 Thermal Control

It is intended that the Delta Sprints natural thermal equilibrium with its thermal protection system, the normal heat output both of the crew and of the Descent Module electronics achieve a comfortable thermal equilibrium temperature for the cabin.  This can be adjusted by moving around. interior insulation panels to vary the insulating value of the craft.  This would be analogous to unzipping a winter coat when the sun comes out.  To preserve power, use of cabin heating and air conditioning will be minimized at the design level, perhaps with heat to be provided during orbital sunsets to keep the temperature from oscillating or forcing the crew to move insulation panels every few minutes.  The cabins environment would be primarily in the control of the space station or mission docked to during Delta Sprints operational role as a lifeboat.  Humidity needs to be addressed, has been left for now.

5.1.8 Communications

Communications hardware details are not currently available, however communications requirements are less than those on Shuttle.

5.1.8.1 Compatibilities

Space Data and Tracking Network (STDN).  The addition of STDN tracking stations in Nova Scotia and/or Newfoundland is recommended as the Delta Sprint Standard ascent sequence is not complete before the spacecraft is over these areas.  Tracking and Data Relay Satellite System (TDRSS) communications compatible with S-band, but not Ku-band.  Meritt Island Launch Area (MILA) C-band transponders and UHF compatible radio.  UHF radio will also be compatible with civil aviation control towers.  (It may be possible to provide a dynamic database that can select appropriate radio frequencies based on current position during an emergency.  The computer should be independent from the guidance system so that its reliability requirements can be relaxed.)

5.1.8.2 Capabilities

UHF communications are voice only, while STDN and TDRSS compatible systems have telemetry and video capability.  Telemetry will include a vital set of instrumentation, and a reserve for auxilliary instrumentation analogous to the Columbias Orbiter Experiments Package (which includes the Shuttle Entry Air Data System (SEADS), Shuttle Infrared Leeside Temperature Sensor (SILTS), Shuttle Upper Atmosphere Mass Spectrometer (SUMS), Aerodynamic Coefficient Identification Package (ACIP), and the Modular Auxilliary Data System (MADS).)  Delta Sprints auxilliary instrumentation will include an array of temperature sensors and strain gauges behind thermal protection on the exterior of the structure, as well as a number of system readings (temperatures, flow rates, hatch sensors, rack, tank, and battery pressures, voltmeters, ammeters, etc.)  When these sensors exceed certain limits for values and rate of change, they are telemetered down in the auxilliary reserve automatically.  Additionally, the crew can select auxilliary sensors which can be telemetered.  Data prerecorded in this system can be transmitted to the ground after the readings were recorded (i.e. trying to identify a suspicious sound or other crew detected anomaly, identifying potential damage from an ascent or orbital anomaly detected by some other means.)

Such a system on STS-107 would have started transmitting the anomalous MADS sensor data to Mission Control at about EI+340 seconds during the descent, about 3 and a half minutes before the actual first indicator that the MMACS officer noticed at EI+541 seconds.  Even better, such a system may have saved STS-107 as anomalous sensor readings from two sensors in the wing leading edge would have been flagged and telemetered during ascent.  If the readings of these sensors had been discussed by Mission Control during ascent, then perhaps the discovery of the foam strike the next day would have been treated differently enough that the crew of STS-107 would still be among us.

5.1.9 Service Life

The Descent Module can be maintained on station through the pressurized habitable environment for most systems.  Other systems, including entry thermal protection require extravehicular activity(EVA) and remote manipulation systems (RMS).  The Descent Module is indended to be able to stay in space for up to two years in its original design, with expansion capability to five years to allow for future planetary missions.

Further information on Descent Module hazardous sysstems is available in Chapter 4.1.4

5.2 Service Module

The Service Module is intended to house those systems which are hazardous to a pressurized cabin environment, along with most of the propulsion systems that Delta Sprint uses on docking approach, docking, undocking, clearance and recovery.

5.2.1 Service Module Construction

The Service Module is a truss structure topped by the wok-shaped impact shield designed to protect the Descent Modules critical RCC base heatshield.  Compressive loads are carried through the RCC, impact shield and into the truss members around a wide area.  Tensile and lateral loads are carried through twelve wraparound latches.  One or two of the latches contain or are near the electrical and data connections and these can pivot to extende the Service Module away from the base shield to allow inspection of the thermal protection system of the Descent Module.  There are also connections between the SMRCS and DMRCS systems, either flexible through or near the pivoting latches, or reconnectable fixed connections in or near the other latches.

5.2.2 Orbital Maneuvering System

The OMS of the Delta Sprint uses nitrogen tetroxide (N2O4) as an oxidizer and hydrazine (N2H4) as fuel.  The two main thrusters (not the same type as Shuttle) are located in the Service Module flanking the deorbit motor and aligned with the center of gravity as configured for Ballistic Recovery Mode (see 4.1.3.2.2)  The system is pressurized by gaseous nitrogen (GN2) under high pressure so that pumps are not needed.

5.2.3 Service Module Reaction Control System

The SMRCS uses hydrazine (N2H4) as a monopropellant catalyzed in the individual thrusters by Shell 405, resulting in a simpler and more robust system than the bipropellants used in larger spacecraft (for reference, Mercury used hydrogen peroxide (H2O2) as a monopropellant in its thrusters.)  This type of system has been used on planetary spacecraft and commercial satellites lasting up to 25 years, a testament to its reliability.  They are still being used and parts are available which can be used on Delta Sprint without modification.  The SMRCS propellant supply has crossfeeds to the OMS fuel supply and the DMRCS supply.  The SMRCS has thrusters which allow translation in all directions (lateral translation with the help of DMRCS) with aft translation thrusters pointed to minimize exhaust contamination to the docking target.  Because of alignment, DMRCS aft translation thrusters are more efficient, but their plumes would impinge upon the space station in a docking position.  If necessary, a pressurant only (GN2) set of thrusters can be provided for close proximity operations with space stations that would pose little to no contamination danger.  (For those who fly Orbiter, try to imagine what those Delta Glider aft translation 40km/s fusion plasma plumes would really do to the International Space Station!!)

5.2.4 Inspection Mode

It is possible to have the Service Module fail to relatch into the flight position.  In this case, it may be possible to maneuver to thermally condition the Service Module to attempt to get the latches to work, if not, as long as the Service Module is swung back, the Descent Module can be configured to Ballistic Recovery Mode, the deorbit motor fired, and the Service Module jettisoned.  Because not all of the latches would be jettisoned, the Service Module might come off with a tip-off rate that could lead to a collision between the Service Module and Descent Module after separation.  The Descent Module should be able to do a clearance maneuver for this contingency (perhaps a settling aft translation maneuver while the latches are separated followed by a forward translation for clearance; this was done to jettison the Service Module for Apollo 13, because the separation switch was in the command module and the translation controls were in the lunar excursion module, this maneuver was actually quite difficult.)

5.2.5 Service Module Service Life

Due to the highly hazardous nature of systems and consumables on the Service Module, the original design is intended to have an on orbit life of 6 months with little maintenance (similar to the entire Soyuz spacecraft, TM and TMA variants.)  The support for Delta Sprint on planetary misisons will be provided by the interplanetary spacecraft and the Service Module for Delta Sprint would be determined by the desired lifeboat capabilities.  After Columbia believes it is lunatic to try to turn Delta Sprint into an interplanetary lifeboat capable of say, flying and correcting a free return trajectory from Mars, bottling those poor astronauts into that tiny capsule for almost two years.  Delta Sprints lifeboat capabilities might be having a Service Module capable of doing entry corridor corrections on a returning mission if say, an Earth Return Vehicle (ERV) should have a propulsion system problem on route to Earth.

5.3 Changes required for other options

5.3.1 Ascent Guidance Modes

For Delta Sprint Spartan, Delta Sprints Guidance system may require direct or indirect control of the Delta AJ10-118K second stage in order to conduct the same kind of rendezvous maneuvers that Delta Sprint Standard is able to do with Fregat.

5.3.2 Spartan Service Module

There is a bit of irony in this chapter title.  The Service Module for Delta Sprint Spartan by necessity to do all the rendezvous maneuvers with little help from the upper stage, will be larger and more complex than that for Delta Sprint Standard.  As the mission mass anticipated for Delta Sprint doesnt differ between Delta Sprint Standard and Delta Sprint Spartan, the Spartan would wind up with a smaller Descent Module and larger Service Module.  After Columbia believes that the technical challenge of doing this is more difficult than integrating Fregat into the Delta II, but is not insurmountable, which is why both options are being thoroughly studied.