Monday, August 26, 2013

The Coming SSTO's: Page 2.

Copyright 2013 Robert Clark

 In the blog post, The Coming SSTO's I calculated some delta v's that suggested we already have the capability to do SSTO's with significant payload. However, here I'll provide some more accurate estimates by using Dr. John Schilling's Launch Vehicle Performance Calculator page. I'll go back to the Atlas rocket SLV-3 Atlas / Agena B. The specifications are given here:

SLV-3 Atlas / Agena B.
Family: Atlas. Country: USA. Status: Hardware. Department of
Defence Designation: SLV-3.
Standardized Atlas booster with Agena B upper stage.
Payload: 600 kg. to a: 19,500 x 103,000 km orbit at 77.5 deg
inclination trajectory.
Stage Number: 0. 1 x Atlas MA-3 Gross Mass: 3,174 kg. Empty Mass:
3,174 kg. Thrust (vac): 167,740 kgf. Isp: 290 sec. Burn time: 120 sec.
Isp(sl): 256 sec. Diameter: 4.9 m. Span: 4.9 m. Length: 0.0 m.
Propellants: Lox/Kerosene No Engines: 2. LR-89-5
Stage Number: 1. 1 x Atlas Agena SLV-3 Gross Mass: 117,026 kg.
Empty Mass: 2,326 kg.
 Thrust (vac): 39,400 kgf. Isp: 316 sec. Burn
time: 265 sec. Isp(sl): 220 sec. Diameter: 3.1 m. Span: 4.9 m. Length:
20.7 m. Propellants: Lox/Kerosene No Engines: 1. LR-105-5
Stage Number: 2. 1 x Agena B Gross Mass: 7,167 kg. Empty Mass: 867
kg. Thrust (vac): 7,257 kgf. Isp: 285 sec. Burn time: 240 sec. Isp(sl): 0
sec. Diameter: 1.5 m. Span: 1.5 m. Length: 7.1 m. Propellants: Nitric
acid/UDMH No Engines: 1. Bell 8081

 We see stage 1 called the sustainer stage has nearly a 50 to 1 mass ratio. However, the Atlas had an unusual "stage and a half" structure where engines needed to lift off from the pad were jettisoned later on in the flight, leaving only a smaller, lower thrust engine behind. This engine which is the one used in stage 1, did not have enough thrust to lift off from the pad. So as in The Coming SSTO's post,  I'll replace it with the NK-33 engine which has now flown successfully on the Orbital Sciences Antares. 
 The propellant load remains 114,700 kg as in the original Atlas but the dry mass increases to 3,086 because of the heavier engine. The vacuum Isp is 331 s for the NK-33, and the vacuum thrust is 1,638 kN. Now input these numbers into Schilling's calculator. Select "No" for the "Restartable Upper Stage?" option and Cape Canaveral for the launch site. For the orbital inclination choose 28.5 degrees to match the latitude of Cape Canaveral. Then the Calculator gives these results:

Mission Performance:
Launch Vehicle:   User-Defined Launch Vehicle
Launch Site:   Cape Canaveral / KSC
Destination Orbit:  185 x 185 km, 28 deg
Estimated Payload:   4113 kg
95% Confidence Interval: 2860 - 5625 kg


 This value of 4,113 kg is remarkable in being close to that of the payload capability of the full Antares at 5,000 kg, a rocket of twice the gross mass, using two stages and two of the NK-33 engines on the first stage.
 Based on this, this SSTO version could be significantly cheaper than the current Antares. Plus in being only liquid fueled, it could be used as a manned launcher. Note that Orbital already has the Cygnus capsule which with the addition of a heat shield and life support could be a manned capsule.

 The mass ratio of 50 to 1 for the original Atlas is so high it would be interesting to calculate the payload capacity if we used instead the lower Isp Merlin 1D engine. By the SpaceX page, nine Merlin 1D's have total vacuum thrust of 6,672 kN. So one is 741 kN. We will need two to lift off, at 1,482 kN vacuum thrust. The two Merlin 1D's together weigh about 330 kg less than the NK-33 case, so subtract that much from the dry mass of the NK-33 case. However, the Isp is also reduced to 311 s Isp for the Merlin:
 Then Schilling's calculator gives:


Mission Performance:
Launch Vehicle:   User-Defined Launch Vehicle
Launch Site:   Cape Canaveral / KSC
Destination Orbit:  185 x 185 km, 28 deg
Estimated Payload:   3025 kg
95% Confidence Interval: 1952 - 4331 kg

 It is the quite high mass ratio that leads to these rather high payload capabilities.

 SpaceX might not be inclined to support such an experiment, as they are deeply invested in keeping the Falcon 9 first stage and Merlin 1D engines. However, Orbital Sciences farms out its construction of the Antares first stage to a company in the Ukraine.  So they may be inclined to try a new stage that would at the same time prove to be a revolutionary step of creating an operational SSTO.                                                                                            

   Bob Clark

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