With SLS on track to make its debut launch in December 2017 from Launch Complex 39B at the Kennedy Space Center, NASA continues its efforts to refine the vehicle’s test flight necessities ahead of EM-1 – particularly, the removal of the requirement to human rate the rocket’s Interim Cryogenic Propulsion Stage.
SLS Block 1 and Exploration Mission 1:
The first mission of the SLS (Space Launch System) rocket, slated for 17 December 2017, will be the first use of LC-39B at the Kennedy Space Center since the October 2009 test flight of the never-fully-realized successor to the Space Shuttle: the Ares I crew launch rocket.
Since the highly successful test flight of Ares I-X, Pad-B has undergone a radical transformation into a 21st launch pad – including the removal of the Shuttle-era launch tower and associated hardware, and a complete below-pad-surface reconstruction of data lines and modernization of launch systems.
Additionally, pad surface hardware and structures have and are undergoing major upgrades to bring the pad up to and beyond specifications needed to support the launch of the SLS heavy-lift rocket.
Debuting in its Block I iteration on Exploration Mission 1 (EM-1), SLS will liftoff from Pad 39B under 8.87 million pounds of thrust generated from its twin 5-segment Solid Rocket Boosters (7.2 millions pounds of thrust) and 4 RS-25 engines (1.67 million pounds of thrust).
Following burnout and separation of the twin SRBs and separation of the core stage, SLS Block I will continue on to a preliminary Low Earth Orbit under the power of its second stage, a modified version of the Delta Cryogenic Second Stage (DCSS) called the Interim Cryogenic Propulsion Stage (ICPS) on SLS.
The ICPS, for EM-1, will consist of a single RL 10B-2 engine burning Liquid Hydrogen and Liquid Oxygen – resulting in a thrust capacity of 24,800 lbf.
After achieving Earth orbit, the uncrewed ICPS and Orion systems will undergo checkout procedures before flight controllers commit the ICPS to a trans-lunar injection burn to send the Orion (with its European Space Agency-provide ATV-derived service module) capsule on a circumlunar flight set to end with a high-velocity atmospheric reentry (at 11km/s) and parachute assisted descent into the Pacific Ocean.
Originally, NASA planned to fly EM-1 with the human-rated version of the ICPS.
But that has since changed.
The decision not to human rate ICPS for EM-1:
In short, EM-1 is strictly an uncrewed demonstration test flight to validate integrated vehicle performance, spacecraft systems, and high-speed reentry effects on Orion’s thermal protection system prior to the commencement of human flights on SLS/Orion on EM-2.
As referenced in the Exploration Systems Development Management Directive document from 27 February 2014 (available for download on L2), “Human rating of the ICPS is not required to accomplish the test objectives of EM-1.
“The SLS program manager is hereby directed to adjust planning and remove human rating requirements for the ICPS in support of the EM-1 flight.”
At the core of the decision to remove the human rating mandate of the ICPS for EM-1 is a three-fold developing understanding of the Orion/SLS test flight sequence.
This includes the use of the Delta IV rocket’s second stage for Orion mission EFT-1, the needs of the ICPS on EM-1, and the potential schedule impacts that human rating the ICPS might have for a mission that won’t carry humans.
Specifically, Exploration Flight Test 1 (EFT-1) later this year is set to test a functional Orion spacecraft during launch, near-Earth operations, and simulated lunar trajectory high-speed atmospheric entry.
That mission, set to launch on 4 December 2014 from the Cape Canaveral Air Force Station, FL, aboard a Delta IV Heavy rocket will use the Delta IV’s second stage, the DCSS, to accomplish several of the flight test’s objectives.
Use of the DCSS will allow NASA to place Orion into a high-apogee orbit and careen Orion back into Earth’s atmosphere at 32,000 km/h to simulate the forces and temperatures the capsule will encounter during lunar and beyond-lunar return trajectories.
As stated by the Management Directive document, “ICPS is intended to leverage the work Orion is already doing to integrate Orion with the commercial Delta IV Second Stage (DCSS) on the uncrewed Engineering Flight Test (EFT-1).”
This means that the bulk of the Orion-DCSS (the commercial version of the ICPS to be used on Block I SLS) integration work will already be complete for EFT-1 later this year.
This, coupled with an increasing understanding of the ICPS’s role during EM-1, contributed to the decision to remove the human rating component of ICPS’s requirements for EM-1.
As related by the Management Directive, “For EM-1, ICPS is intended to provide low-Earth orbit (LEO) parking and minimal Earth departure delta-V for early Orion test flights.”
Carrying forward the Orion-DCSS integration work that will already have been completed for EFT-1 to EM-1, with any tweaks that will inevitably come from EFT-1, makes the most sense when encapsulated in the mission objectives of EM-1.
Moreover, with SLS and Orion both currently maintaining schedule to achieve a December 2017 launch for EM-1, the deletion of the human rating mandate for the ICPS on EM-1 not only saves time, money, and resources on a requirement that is technically unnecessary to accomplish the EM-1 test flight objectives but also adds additional schedule protection for EM-1’s launch date.
Contextualizing the decision with its pros and potential cons:
As stated in the Management Directive document, “ICPS was always intended to eventually be replaced with a more capable upper stage (currently referred to as the Exploration Upper Stage, or EUS).”
Originally, the EUS was, and still officially is, planned to debut on the EM-3 flight of SLS/Orion.
However, it now appears possible that the EUS might be ready for EM-2 – the first planned crewed flight of SLS and Orion.
From the beginning, the EUS has always been a human-rated upper stage, and its development acceleration would go a long way toward increasing SLS’s capabilities in its early years of service (including potential service to the international launch community).
But, with all things in the field of space travel development, it is entirely possible that the EUS will not be ready for EM-2, meaning that the ICPS will have to be used, as is currently planned and baselined, as the upper stage for EM-2.
By this point, the ICPS would have to be human rated – as EM-2 is scheduled to carry crew.
This would mean that EM-2 would be the first flight of the human-rated ICPS and the ICPS’s technical second flight as part of SLS.
This distinction could be important given a stated preference from the Astronaut Office to not fly crew on an un-flown stage.
However, its just as likely that this could just be a distinction in terms, as ICPS will have flown in its non-human rated configuration on EM-1 and any human-rated ICPS configuration will have been thoroughly ground tested before the EM-2 flight and there would still be abort capability for Orion and its crew during ICPS-powered flight.
However, if NASA Administration and/or the Astronaut Office demand a test-flight of the human-rated ICPS before placing crew onboard Orion for EM-2 there would potentially be multiple opportunities to test fly the human-rated ICPS on any one of the intervening Delta IV rocket missions (as the ICPS is the government version/name of the the DCSS currently in use by the Delta IV fleet) from the time of that decision through 2021: the target launch year of the EM-2 SLS/Orion crewed mission.
Moreover, it might even be possible to test fly the human-rated ICPS on SLS itself if EM-2 turns out not to be the second flight of SLS.
Currently, EM-2, slated for 2021, is the second flight of SLS. However, there are some indications that this will not be the case.
Specifically, there are some indications that a uncrewed cargo flight of SLS will be inserted between EM-1 and EM-2, thereby making EM-2 the third flight of SLS and giving either an opportunity to test fly the human-rated ICPS on the newly-added second SLS flight or giving more time to bring EUS online for EM-2 (if EM-2 slips far beyond its currently targeted 2021 launch year).
Nonetheless, it should be made clear that the decision not to human rate the ICPS for EM-1 in 2017 in no way deliberately or automatically delays the start of crewed missions of SLS/Orion in 2021.
While there is noted risk – as there is with all decisions made in space flight – to EM-2 by not human-rating the ICPS for EM-1, it is important to note that work on human-rating the ICPS in general has not been stopped by this decisions.
If ICPS is needed for EM-2, it will be human-rated. If EUS is ready for EM-2, then unnecessary work to human-rate the ICPS will have been avoided, and time, money, and effort will have been saved.
(Images: Via L2 content from L2’s SLS specific L2 section, which includes, presentations, videos, graphics and internal – interactive with actual SLS engineers – updates on the SLS and HLV, available on no other site. Other images via NASA)