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aftercolumbia
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« on: June 25, 2006, 07:56:45 PM »

Ascent: The First Step Into Space is a book under development by After Columbia (authored by Terry Wilson) with the Greenstar cost-optimized booster as its centrepiece.

Greenstar currently has the following technical decisions and assumptions behind it.  “Front runner” means that an item is possibly subject to change because it is not known for certain if it is optimal.  “Firm” means that it is certain that the item is optimal.  “Dogma” is an item not certain to be optimal, or perhaps known not to be, but is held firmly for perspective reasons (i.e. one of Greenstar’s goals is to find the pressure-fed cost-optimal configuration for a 5m diameter booster.  It is possible that a 5m turbopump-fed booster may be economical, but it is not Greenstar’s goal to determine that.)

- Pressure fed; no turbopumps (dogma)

- Pressure vessel safety factor 2.5, Structures safety factor 2.0; the rationale is to allow shipyard class tolerances rather than aerospace ones, and to reduce the need for and chances of delay during integrated ground testing. (firm)

- Material is currently implemented with a density of 7.8 g/mL and yield strength of 575MPa; assumed to be a quench and temper hardened steel.  No assumption is made on whether it is brittle at liquid oxygen temperatures.  The material has an allowable stress of 250MPa when used in tanks, and 287MPa when used in structures (front runner)

- First and Second Stages use Propulsion System, Lower (PS-L), currently operating at a tank pressure of 2500kPa (362psia) (front runner), with a motor chamber pressure of 2000kPa (290psia) (front runner); it uses segmented construction and tang and clevis joints equipped with inner/outer dual O-rings, LOX tank internal insulation (a simpler modification of the original SRB joint which would be unsuitable for SRB use for the same reasons that caused the STS-33 Challenger mishap on 28 January 1986; Greenstar provides much gentler operating conditions other than temperature.) (front runner)

- PS-L stages use a combination of angled verniers and differential throttling of both main and vernier motors for control in all three axes using moving components no bigger than globe or iris throttling valves. (firm)

- PS-L tankage takes the axial accelleration compression loads directly in the tank wall structure (firm)

- The first stage uses recovery parachutes to allow reuse (firm); at Urwumpe’s (Dennis Krenz) recommendation, they are now located in the forward skirt instead of in a piggy back pod (firm).

- The second stage is not recovered (front runner) but does possess enough thermal protection that it will impact the sea in one piece (firm).

- The third stage uses 1200kPa (174psia) tank pressure and 750kPa (109psia) motor chamber pressure (tentative) in an all-welded, post weld heat treated main structure and tanks (front runner)  This is called Propulsion System, Upper (PS-U)

- The first three stages use liquid oxygen and kerosene propellants (firm), mixed at 2.3:1 for PS-L stages (firm) and 2.2:1 for PS-U stages (front runner)

- PS-U stages use N2 cold gas RCS (firm; performance of monopropellant hydrazine sacrificed to eliminate the toxic hazard from ground safety consideration), plus assured liquid acquisition verniers using the main propellants to provide stage separation and settling thrust (front runner).  The main engine is position balanced and uses electromechanical gimbal actuation (front runner).

- The high energy top stage uses the Dunn Engineering self-pressurized hydrogen peroxide/propane propulsion system with 95% hydrogen peroxide mixed at 6.6:1 with a motor chamber pressure regulated at 750kPa (109psia) (front runner)  See www.dunnspace.com for details on self-pressurized propulsion system.  In Greenstar application, the pressure vessel will be a sphere.

- The top stage is designed with sufficient endurance to complete delivery to Geosynchronous Equatorial Orbit in anticipation of the demand for that service from lower cost satellites designed for the cheaper and friendlier launch market environment provided by Greenstar (firm, intermediate performances will still be available.)

- PS-U and PS-T both use structures (compression ring/interstage and fairing, respectively) which allow axial accelleration and aerodynamic compression forces to bypass the propellant tanks (front runner for PS-U, firm for PS-T)

- PS-L and PS-U vacuum optimized nozzles will be two part extendable (the lengths of the second stage skirting required to accomodate fixed nozzles created very unfavorable mass fractions and doubts as to whether the vehicle could handle the lateral loadings from wind sheer.)  PS-T and PS-L surface optimized nozzles are one piece. (firm)

- All stages use helium pressurization to provide a reserve pressure above saturation vapor pressure for vapor pressurized propellants (liquid oxygen and propane) to prevent line and manifold cavitation and as the primary pressurant for kerosene. (firm)

- The payload fairing integrates with the highest PS-U stage in the booster. (firm)  It is jettisoned at a deterministic thermal rate of 1000W/m^2 for trajectory/simulation studies, and with the second stage for delta-v optimizations.

- Errant Flight Termination is provided by intact abort, the FTS signal commands the Flight Termination System to do many things, only a few of which are necessary to ensure the booster is no longer capable of propelling itself to a course that will endanger the public.  It withdraws power from all valve power busses, disabling the guidance system’s ability to control the vehicle and forcing all valves, including pressure-venting valves, into a power-off configuration.  It commands closed all motor valves and a redundant set of FTS butterfly valves, which isolates all main and vernier motors from their propellant supplies.  The guidance system will continue to operate and telemetry transmitters will continue to transmit, as the information they are generating may be conducive to the failure investigation. (firm) An autonomous mode may be implemented for attitude limit and attitude rate (front runner; it may not be a good idea if launching from the Cape because of the possibility of an AFTS cut off while the vehicle’s Instantaneous Impact Point is in or too close to Africa.)  An autonomous mode which is activated by stage separation sensors and breakwires intended to detect vehicle breakup will be implemented (firm).

- As many pyrotechnic devices as possible will be eliminated from Greenstar for ground safety (and associated expense) reasons. (firm) Perhaps all of them can be eliminated (unlikely)

The Greenstar uses a blockable first stage in two lengths, the shorter of which is used only as a second stage.  The motors are common among all PS-L stages, and a PS-L stage can have one, three, or four motors, each version being equipped with an appropriate length nozzle.

The PS-U (third stage, fourth in very large configurations) stage comes in three sizes: four metre, five metre short, and five metre long.

The PS-T (fourth stage, fifth in one configuration) comes in two sizes, the smaller of which will fit under the four metre fairing.

The configuration numbering system has four digits, which are associated with the four stages of a high energy vehicle.  The first digit is the number of first stage full length blocks used (1, 3, or 5); the second digit is the size of the second stage (1 is half-length with one high expansion motor, 2 is full-length with three moderate expansion motors.)  The third digit represents the PS-U stage (’4′ is the four metre stage, ‘5′ is the short five metre stage, while ‘6′ is the long five metre stage.)  The fourth digit designates the fourth/fifth stage(s) (’0′ is no stage, and denotes a three stage low energy version of Greenstar, ‘1′ is the small PS-T stage, ‘2′ is the large PS-T stage, ‘3′ is the five metre short PS-U stage, and ‘4′ denotes a two stage stack consisting of the five metre short PS-U stage as the fourth, and the larger PS-T stage as the fifth.)

The first stage full length block is at a gross weight of 400,000kg, with the remaining stages sized around that.  It has shrunk somewhat because of the first stage areal thrust density issues of pressure fed vehicles of this size.  The rest of the stages will be sized on that assumption and these design point configurations:

1140 to a 60deg support orbit (The half length PS-L block actually contains about 1/3 the propellants of the full length block; both second and third stages respond to optimization)

3250 to a 60deg support orbit (here only the third stage responds to the optimization)

5260 to a 60deg support orbit (here only the third stage responds to the optimization)

1141 to 1500m/s GTO from a 60deg support orbit with a three maneuver mission plan

3252 to full GEO delivery from a 60deg support orbit with a four maneuver mission plan

5263 to a GTO with somewhere around 1500 to 2100m/s remaining delivery delta-v from a 60deg support orbit with a three maneuver mission plan

5264 to full GEO delivery with a four maneuver mission plan from a 60deg orbit

 These are the remaining off design point configurations, some of which will be eliminated from further consideration as development continues:

Low Energy: 1150, 3140, 3150, 3260, 5250

High Energy: 1151, 1152, 3141, 3151, 3152, 3251, 3261, 3262, 5251, 5252, 5253, 5254, 5261, 5262, 5263

Potentially Successful Wierdos: 1640, 1641, 1650, 1651, 3640, 3641, 3650, 3651, 3652 (These use the five metre long ‘6′ PS-U stage as a second stage.)

Payload envelope depth may be affected by the selection between the 5m short and 5m long PS-U stages because the compression ring for the PS-L forward skirt/interstage must begin at the top of the PS-U aft bulkhead and no further forward.  “May be” because the long PS-U stage may use a longer compression ring, but this hasn’t been determined yet.

The existing Delta II second stage provides an excellent example of a compression load bypass structure.  The compression ring interfaces forward with the fairing and aft with the first stage forward skirt.  This results in the trailing edge of the long second stage nozzle sneaking out of a very long tunnel reminiscient of the romantic undocking of the Starship Enterprise from its refit yard, only a heck of a lot faster.  Greenstar does not use this technique for 1/2 staging because there is still enough dynamic pressure to cause the first stage to rotate quickly after staging, a phenomenon counted upon to save the recovery parachutes from second stage exhaust.

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