SLS searching for missions to solve flight rate dilemma
Space Launch System (SLS) managers are continuing to look at potential – and as yet unfunded – science missions to provide their monster rocket with a viable number of flights. The flight rate dilemma was recently highlighted when it was admitted NASA had looked into “how slow” a launch rate could be viable for the rocket that is expected to make her debut in 2018.
SLS Flight Rate:
As SLS continues to make very good progress during her transition from the design phase to the actual bending of metal at the Michoud Assembly Facility (MAF), the rocket’s projected flight rate remains a huge challenge that needs to be fixed to ensure her immense capability is fully utilized.
As has been the case for a number of years now, only two missions are currently manifested, while her projected schedule in the 2020s remains threadbare.
The issue of the flight rate for SLS was known even before the rocket was officially announced, with a stuttered start mainly caused by political language.
The requirement for debuting SLS in 2017 had little to do with the rocket’s Beyond Earth Orbit (BEO) exploration aspirations. It was the 2010 Authorization Act that called on SLS and Orion to provide a back up role to the Commercial Crew Program – actually calling for a 2016 operational debut, per the Act – in the event of a severe delay to what is now known as the USCV-1 (US Crew Vehicle -1) mission.
Ironically, SLS’ multi-billion dollar strain on the NASA budget has indirectly starved Commercial Crew funding, resulting in USCV-1 slipping to at least the end of 2017.
Also adding to the irony was the fact SLS and Orion were never likely to provide such a back up role, due to the obvious requirement the first SLS mission having to be uncrewed for safety reasons. The last time a crew rode on the debut of a NASA vehicle was in 1981, during Columbia’s STS-1 launch.
Notably, such a SLS and Orion mission to the ISS would have likely cost more money than it would to accelerate the Commercial Crew USCV-1 launch date.
The fallout of this political requirement for SLS to launch in 2017 was never solved, even when NASA made it clear Orion had been fully refocused on BEO missions. The result was a four year gap between the debut flight of SLS and the second mission, a flight that will involve a crew for the first time.
Although this schedule remains the case via public statements by NASA, internally Exploration Mission -1 (EM-1) has slipped to at least the middle of 2018.
Notably, the slip is specific to Orion’s schedule issues, with some sources noting SLS may still launch in 2017 on a test flight with either a dummy payload or a boilerplate Orion. However, this is just one of several options under consideration.
What remains the most likely scenario is a 2018 launch of EM-1, with Exploration Mission -2 (EM-2) accelerated to the end of 2020, as continues to be portrayed by internal documentation.
As if to convolute matters further, the second flight may change call signs, allowing for another test flight, this time with the powerful Exploration Upper Stage (EUS). Officials have noted it is unlikely they will allow a crew to ride on the new stage until it has been flight tested.
Also, changing the current EM-2 mission – a crewed asteroid sample mission – into the third flight of SLS would make some sense, due to the initial requirement of a robotic mission – called the Asteroid Redirect Robotic Mission (ARRM) – to capture the asteroid and place it into an orbit near to the Moon.
It is highly unlikely that the ARRM objectives could be achieved in time for the second SLS launch in 2020. However, SLS-2 could be called upon to launch the ARRM spacecraft, as previously suggested by NASA managers.
Once SLS is into the 2020s, the launch rate should see the rocket launching at least once per year, ramping up to a projected three times per year for the eventual Mars missions. However, the latter won’t be until the 2030s.
With no missions manifested past the EM-2 flight, the undesirable question of just how “slow” a launch rate would be viable for SLS and her workforce has now been asked.
“Everyone recognizes that safety issues can be induced with a fast launch rate; however, if the launch rate is too slow, people skills, processes, and equipment can degrade,” as was noted at the latest Aerospace Safety Advisory Panel (ASAP) meeting. “Generally, there is an optimum launch rate with some limits on ‘too fast’ or ‘too slow.'”
It was then revealed to the meeting that “The (SLS) Program has considered this conceptually and has some ideas on how slow is too slow.”
It is understood that NASA believes one launch every two years is “too slow” and over two launches per year is “too fast” for missions in the 2020s, based on current SLS production limitations.
In response, the ASAP encouraged the SLS Program to pursue this question and determine what the boundaries are, before references were once again noted about potential science missions.
“Budget limits will put pressure on launch rates,” the meeting notes continued. “One thing that may help the launch rate is the growing interest by the science community in using the SLS for science missions.
“One possibility that has arisen is the Europa mission. Missions like this can help fill the gaps and provide a safe and cost-efficient launch rate beyond a human mission to Mars.”
Such a mission with a spacecraft bound for the Jovian moon Europa would be an even larger challenge to enable, mainly from a funding standpoint.
However, it has been on the radar of SLS managers as one of the more steadfast attempts to attract science missions to their manifest, with previous L2 notes showing they have been working alongside the Jet Propulsion Laboratory (JPL) on a notional science mission to Europa. However, this was an exercise, as opposed to the opening plans for an action mission.
Since then, a Europa mission has continued to be cited in both L2 and public documentation, including the more recent NASA Advisory Council (NAC) overview on SLS’ status.
“Europa exploration was identified as a high priority in the ‘Visions and Voyages’ planetary science decadal survey. SLS can provide direct injection to Jupiter, eliminating several years of planetary gravity assists to reduce flight time to Europa from 6.3 years to 2.7,” noted the latest NAC presentations.
“Additional benefits of SLS for Europa Clipper include reduced operational costs, reduced mission risk, and greater mass margin.”
Internal evaluations have also placed the Europa mission as a potential addition to the manifest for the SLS Block 1B – the 105mT version of the Heavy Lift Launch Vehicle (HLV) – a configuration that is set to dominate the 2020’s SLS manifest and potentially beyond.
Known as the “Three Degrees Of Freedom (3-DOF) Reference and Loads Trajectories” evaluations, a Design Analysis Cycle (DAC) noted “Design Reference Missions (DRMs) addressed include the Europa mission configuration (27500),” per L2 notes.
Incidentally, it also involved two “Lunar mission configurations (27002 cargo and 28002 crew),” which sources note is related to any potential change to the current political direction, one that is currently shunning a return to the Lunar surface.
“Two loads trajectories (maximum dynamic pressure and maximum acceleration) were also generated for each DRM. The 3 reference and 6 loads trajectories will be presented to the Structures and Environments (StE) Ascent Flight Systems Integration Task Team (AFSITT).”
Also previously shown to be under evaluation is SLS’ involvement in a Mars Sample Return (MSR) mission.
The evaluations depicted, the “2024+ Single Shot MSR on SLS is based on “launch cadence and availability providing a single-shot Mars Sample Return opportunity.”
This would not be a primary mission for SLS, with the “Mars SEP (Solar Electric Power/Propulsion) Orbiter” riding as a secondary payload. The cited alternatives of using a Falcon Heavy or a Delta IV-Heavy would see it become their primary payload, if selected.
Based on such a projected timescale for the mission, the opportunity falls into the domain of SLS-5 or SLS-6, should the current manifest remain unchanged.
Such notional overviews involve Orion being used on the second leg of the mission, picking up the samples in space.
The idea was again touted in the recent NAC documentation, this time claiming the alternative vehicles would require multiple launches to achieve.
“Mars Sample Return was identified as a high priority in the ‘Visions and Voyages’ planetary science decadal survey. SLS offers single-launch option for Mars Sample Return, versus three launches with EELVs,” the NAC presentation claimed.
“Additional benefits of SLS for Mars Sample Return include reduced mission time, increased sample mass, and reduced mission cost, complexity and risk.”
Such claims are repeated for various other notional missions, including large probes to various deep space destinations. However, no such mission will become a reality without funding.
Officials have yet to note how factoring in the cost of the SLS program impacts on the likelihood of such science missions ever becoming a reality.
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