SLS Launch Vehicle specifications take shape as development continues

by Chris Gebhardt

Eight months after the conclusion of the final Space Shuttle mission, the iconic winged-vehicle’s successor continues to take shape at various NASA centers, as the U.S. space agency further defines the Space Launch System requirements – as seen in the expansive SLS Program Launch Vehicle Specifications document, which provides details on the latest design baseline requirements for SLS Block 1, Block 1A, and Block 2 configurations.

Design Baseline – The three SLS rocket variants:

As NASA refocuses its goals on exploration beyond Low Earth Orbit (LEO), one of the primary and important characteristics of the new SLS rocket is its heritage with the Space Shuttle and diversification in terms of payload requirements and Block variant requirements.

To this end, the SLS Element Performance and Mass Constraints for the SLS’s Core Stage and Upper Stage engines – including the Shuttle holdover RS-25D engines, the under-development RS-25E engines, and the J-2X engine – have all been defined.

Beginning service with the Shuttle Program holdover RS-25D engines (commonly known as Space Shuttle Main Engines, or SSMEs), the SLS will utilize these in-stock engines for operational loads as defined in the SLSP (Space Launch System Program) Launch Vehicle Specification Rev-A document (available for download on L2 – L2 Link).

For maximum power levels (percent RPL), the RS-25Ds will obtain 109 percent of rated thrust and have a standard throttle range of 65-109 percent – the same throttle range that could have been utilized by the engines with the Shuttle orbiters… though Shuttle typically defined a lower-limit 67 percent throttle and upper-limit 104.5 percent throttle range, with 109 percent reserved for abort contingencies.

The RS-25D engines themselves will each weigh 7,816 lbs.

The propellant mixture ratio range will be 5.85 to 6.10 +/- 1 percent with a Minimum Vacuum Thrust at Maximum power (lbf) of 511, 117 lbf and a Min Vac Isp (Specific Impulse) at maximum power (s) of 452.3 s.

Comparatively, the RS-25E engines, which will enter service once the RS-25D manifest is depleted due to the lack of recoverability of the Core Stage of SLS to which the RS-25Ds will be mounted, will operate with a maximum power level (percent RPL) of 111 percent, a throttle range of 65-111 percent, and a total mass that is Not To Exceed 8,156 lbs.

The RS-25Es will have a mixture ratio of 5.85 to 6.10 +/- 1 percent (identical to the RS-25Ds),  a min vac thrust at max power requirement of 515,700 lbf, and a min vac Isp at max power of 450.8 s.

The Upper Stage’s J-2X engine will operate at a maximum power level of 100 percent RPL and will not have a throttle range outside the 100 percent throttle setting.

The engine itself will weight 5,400 lbs.

The mixture ratio will the 5.5 with a min vac thrust at max power of 276,450 lbf and a min vac Isp at max power of 435 s.

In addition to the Core and Upper Stage engine specifications, the requirements for the Solid Rocket Boosters are also laid out in the SLSP Launch Vehicle Specification document.

The twin 5-segment SRBs will both use a polybutadiene acrylonitrile (PBAN) propellant with a total prop-loaded weight of 1,392,194 lbs. This is a drastic increase in weight over the SRB’s empty weight of 221,319 lbs each.

(Image taken from the amazing 220mb DM-2 Five Seg Motor Ground Test Video – available in L2).

This represents a total solid PBAN propellant weight in each SRB of 1,170,875 lbs – for a total of 2,341,750 lbs of solid propellant for the SLS vehicle.

Despite each SRB having one additional propellant segment over those used by the Space Shuttle launch vehicle, the SRBs for SLS will not be recoverable or reusable – meaning that no post-flight inspections of the SRB joints and O-rings will be possible.

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SLS Block 1 configuration, requirements, and constraints:

In its inaugural, Block 1 configuration, the SLS rocket will be capable of lofting 70 metric tons of payload off the surface of Earth.

As stated in the SLSP Launch Vehicle Specification document, “The Block 1 SLS configuration (~70 metric ton) is comprised of a 27.6-foot nominal outer wall diameter (cryogenic pressure vessels excluding thermal protection system foam and flanges) core stage, with propulsion provided by two five-segment PBAN solid rocket boosters and four RS-25D liquid hydrogen (LH2)/liquid oxygen (LO2) core stage engines.”

While the RS-25D engines are holdovers from the Space Shuttle Program, as are the SRB casings themselves, the 5-segment SRB is a direct heritage component of the now-defunct Constellation Program and the impressive work accomplished by ATK in developing and testing the new hardware elements required for the massive 5-segment solids.

Moreover, the Block 1 SLS configuration will include two possible additions depending on mission requirements: a Cargo Payload Adapter (CPA) for non-crewed payload flights or two possible payload adapters for Orion/MPCV missions with or without an ICPS (Interim Cryogenic Propulsion Stage).

Moreover, the basic design constraints for the Block 1 SLS include inclusion of an avionics unit and developmental flight instrumentation to verify vehicle performance during ascent.

The SLS’s Block 1 Core Stage engines will include pogo suppression as prominently seen during the Apollo Saturn V era, and all Core Stage and Booster designs will include implementation of imagery assets for “mission critical and safety related events.”

There will be no Reaction Control System (RCS) on the core stage.

Furthermore, the Mobile Launcher (ML) will need a stabilizer interface, Ares-style support posts for the vehicle, and booster forward thrust takeout.

The Core Stage of the SLS Block 1 will also need to be “structurally stable without pressurization during all phases of ground transportation and processing,” incorporate STS/Shuttle heritage booster separation hardware, and contain an ICPS for “Orion MPCV Early Test Flight missions.”

As with Shuttle, the twin SRBs will support the entire weight of the SLS stack on the ML and will use Shuttle heritage Thrust Vector Control (TVC). However, the SRBs will have a “7.22 expansion ratio nozzle with 155.8-inch exit plane outer diameter.” All SRB/booster hardware will “meet design-to launch reliability and maintainability.”

Additionally, the TVC heritage from Shuttle’s SRBs will also be used to control the four Core Stage RS-25D liquid engines. The RS-25D engines will have a gimbal limit of 8 degrees. All heritage RS-25D hardware will “meet demonstrated launch reliability and maintainability.”

In all, the SLS Block 1 will stand 320.9 feet tall with a Core Stage height of 212.2 feet – a height roughly equivalent to the elevation of the Gaseous Oxygen Vent arm used to vent gaseous oxygen away from the top of the Space Shuttle stack… a point that will now be 108 feet below the top of the SLS Block 1.

Block 1A configuration, requirements, and constraints:

In comparison to the Block 1 design, the Block 1A design for SLS will be capable of lofting 105 metric tons into Earth orbit.

As stated in the specification overview document, “The Block 1A configuration (~105 metric ton) introduces the new competitively-procured boosters (solid or liquid) in place of the five-segment PBAN boosters and utilizes four RS-25D or 25E core stage engines.”

For the cargo configuration of the SLS Block 1A vehicle, the payload fairing will be a two-piece fairing to “maximize the payload volume while still retaining the ability to process the payload and vehicle in existing Kennedy Space Center (KSC) facilities.”

Design limitations/requirements/constraints for the SLS Block 1A configuration include payload adapter(s), an avionics unit, and developmental flight instrumentation.

The Core Stage engines will continue to include pogo oscillation suppression technology – as will the boosters’ engines if the boosters are liquid instead of solid propellant boosters.

The Core Stage and Boosters will still provide all assets necessary for imagery of the launch vehicle through “mission critical and safety related events,” and no RCS will be including on the Core Stage.

The ML will need a stabilizer interface – like with the Block 1 design – as well as Booster forward thrust takeout.

Moreover, the Core Stage and Booster (if liquid) will need to be “structurally stable without pressurization during all phases of ground transportation and processing.”

Booster separation will be provided by “booster element” and will not necessarily be heritage from Shuttle. A booster support vehicle will also be needed on the ML for the competitively-procured Boosters.

Heritage TVC hardware will continue to be used for Core Stage RS-25D or RS-25E engines, and Core Stage engine gimbal will once again be limited to 8 degrees.

In all, the SLS Block 1A vehicle will stand 314.2 feet tell in its payload/cargo configuration and 320.9 feet tall in his crewed/Orion configuration.

Block 2 configuration, requirements, and constraints – The beast in its currently-thought-of final form:

The fully evolved Block 2 – or Block II – is the beast of the SLS range, capable of launching a massive 130 metric tons.

“The Block 2 configuration (~130 metric ton) will utilize the core stage developed for the Block 1 configuration, four or five RS-25E LH2/LO2 core stage engines, two competitively-procured (liquid or solid) boosters, and a new 27.6-foot-diameter upper stage,” notes the SLSP Launch Vehicle Specifications document.

“The upper stage will be powered by two J-2X LH2/LO2 engines, initially developed under the Constellation Program.”

Like the Block 1A design, the cargo-only version of the SLS Block 2 will feature a payload adapter and two-piece payload fairing.

As noted in the specifications document, this vehicle is capable of combined Orion/Cargo-style missions.

“For Orion MPCV missions, the SLS vehicle may be required to accommodate other payloads in addition to Orion MPCV,” notes the SLSP document.

This version of the SLS rocket will stand an impressive 384.3 feet in height in cargo configuration and 378.5 feet tall in crew/Orion configuration.

Height estimates for the combined Orion/Cargo flights were not provide in the documentation.

As with its previous configurations, the SLS Block 2 configuration will include payload adapters, an avionics unit, and developmental flight instrumentation.

Core Stage and booster (if liquid) engines will include pogo suppression technology – as will the upper stage J-2X engines. 

All Core Stage, Upper Stage, and booster designs will include assets for imagery of the SLS launch vehicle through “mission critical and safety related events.”

No Reaction Control System will be mounted to or included on the Core or Upper Stages of the SLS Block 2.

The ML will again need a stabilizer interface and booster forward thrust takeout. The Core Stage, Upper Stage, and booster (if liquid) will be required to be “structurally stable without pressurization during all phases of ground transportation and processing” at the Kennedy Space Center.

Booster separation will, like with Block 1A, be provided by booster element, and a booster support vehicle will be needed at the ML.

The Upper Stage will furthermore need a Core-Upper Stage Separation system.

Moreover, should the Core Stage utilize five RS-25E engines instead of four engines, the center engine “will be fixed (i.e. no TVC on the center engine)” while the other four Core Stage engines will have TVC and a gimbal limitation of 8 degrees with a “center engine keep out zone.”

(Images: Via L2 content from L2’s SLS specific L2 section, which includes, presentations, videos, graphics and internal updates on the SLS and HLV, available on no other site. Other images via ULA, NASA)

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