NASA FRR tentatively sets April 30 for Dragon’s first flight to fully prepared ISS
For the first time in history, NASA’s extensive Flight Readiness Review (FRR) process has concluded that a privately-owned spacecraft is tentatively cleared to visit the International Space Station (ISS). With a launch date still tracking April 30, the SpaceX Dragon spacecraft will lift off toward a space station that is now fully prepared to extend a robotic handshake to the commercial space industry, pending the closure of a few open items.
Flight Readiness Review:
NASA’s FRR process is a full review of every aspect of an upcoming mission, including safety reviews, technical readiness reviews, flight hardware reviews, and many other hardware, software, system and procedural related reviews. As was proven during the Space Shuttle Program, FRRs are a very effective tool to ensure that every system is as ready as it possibly can be to ensure safe and successful completion of an upcoming mission.
As was typical of the conclusion of Shuttle-era FRRs, a formal launch date is usually posted. However, the launch of SpaceX’s cargo-laden Dragon capsule – atop a Falcon 9 booster from Cape Canaveral Air Force Station (CCAFS) Launch Complex-40 (LC-40) – is tracking April 30, but that is pending the resolution of a couple of open items that require additional testing relating to software on the ISS side.
As of this time, April 30 is still the target for what will become a date of historical significance with the occurrence of the first ever launch of a privately-owned rocket and spacecraft to the ISS.
Monday’s FRR also carries some historical significance, as the first launch of a private spacecraft – widely touted as the “successor” to the Space Shuttle – has been approved on the very same day that the first Space Shuttle orbiter to head off into retirement spent her last full day at the Kennedy Space Center, with the responsibility for ISS support now reluctantly handed off from the seasoned Space Shuttle orbiters to their eager young commercial siblings.
Dragon mission overview:
The long-anticipated combined COTS-2/COTS-3 (C2/C3), also known as Demo-2/Demo-3 (D2/D3) and officially noted as the SpX-D mission, will be the first visit of SpaceX’s Dragon spacecraft to the ISS, with major vehicle modifications over the only previous Dragon flight in December 2010 being rendezvous and berthing systems, and spacecraft trunk complete with solar arrays and radiators.
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Following launch on April 30, Flight Day-1 (FD-1) and FD-2 will see Dragon perform the far field phasing portion of its mission, during which time checkouts of Dragon’s Absolute GPS (AGPS), free drift and abort capabilities will be performed. FD-3 will see Dragon perform a 2.5km fly-under of the ISS to test Dragon’s Ultra High Frequency (UHF) communication via the COTS UHF Communication Unit (CUCU) on ISS.
FD-3 will also see other tests being performed on Dragon, such as Dragon command ability by the ISS crew (strobe light switch on) via the Crew Command Panel (CCP) connected to the CUCU, and a Relative GPS (RGPS) checkout between Dragon and ISS. At the end of FD-3, the ISS Mission Management Team (IMMT) will meet with SpaceX managers to determine whether C2 mission objectives have been successfully satisfied.
Should C2 objectives be met, the IMMT will give a go-ahead for FD-4’s attempt at C3 objectives, which include a full Dragon rendezvous with the ISS, flying to within 30m of the station in order to be grappled by the Space Station Remote Manipulator System (SSRMS) and berthed (not docked) to the Node 2 Nadir port by Expedition 31 astronauts Don Pettit of NASA and André Kuipers of ESA, working from the ISS’ Cupola observation/robotics station.
Although free-flyer captures have been performed previously on ISS during the Japanese HTV-1 and HTV-2 missions, the Dragon capture may be slightly more challenging.
“There’s unknowns with these new vehicles [because] they haven’t been there [ISS] before, and from the past experience we know HTV has been very very solid, which made it relatively easy to capture,” explained NASA astronaut Joe Acaba – who will arrive at the ISS aboard Soyuz TMA-04M roughly two weeks after Dragon’s arrival – to NASASpaceflight.com’s Philip Sloss during a pre-flight interview.
” These others [commercial vehicles], they’re a little bit lighter with some bigger thrusters, so there may be more motion that we might see, so they may be a little bit more challenging.”
Following the opening of the ISS Node 2 Nadir and Dragon Common Berthing Mechanism (CBM) hatches and mating of all power/air hoses and ventilation between ISS and Dragon on FD-5, Dragon will enjoy a roughly two and a half week stay at the ISS, during which time Dragon’s non-essential cargo will be transferred to the station and trash/return items loaded in its place.
After the completion of the cargo transfers, Dragon will be unberthed from the ISS and released by the SSRMS for a re-entry and splashdown off the coast of California – thus regaining some marginal down-mass capability for the ISS Program that retired along with the Space Shuttles last year.
(A more in-depth overview of the C2/C3 mission will be covered in a separate article on NASASpaceflight.com.)
ISS ready to receive Dragon:
Dragon’s destination, the ISS, is now fully ready to receive its first commercial guest, following a SpX-D Stage Operations Readiness Review (SORR) preformed on 5th April, the presentation for which, along with other SORR presentations and an impressive set of exclusive ISS on-orbit status notes and images, are available to view and download on L2 (L2 Link).
With hardware and software transitions successfully completed earlier this year, and ISS robotic assets prepared last week for Dragon’s arrival, only a few issues relevant to the Dragon mission were noted in the SORR presentations, all of them minor and classed as low risk, and with all bar one being cleared as having no impact to SpX-D flight operations.
The only issue listed as having a “potential” issue to SpX-D flight ops was the recent 2A Beta Gimbal Assembly (BGA) pointing command issue and subsequent transaction errors. BGAs provide beta rotation capability to allow the Solar Array Wings (SAWs) to track the sun.
“Controllers saw a suppressed caution for an invalid BGA 2A Pointing Control Command. At the same time transaction errors were seen on the UB-PVA-24-1 bus and a channel swap from A to B. Data shows the transaction errors were due to no response from RTs (Relay Terminals) outboard of the BMRRM (Bearing Motor Roll Ring Module) which points to a potential issue with the signal ring in the BMRRM.”
The SORR presentation noted: “Transaction errors are consistent with hypothetical short due to FOD (Foreign Object Debris) in the signal roll ring of the BMRMM. There is one spare BMRMM stowed on-orbit. EVA procedures to replace a faulty BMMRMM are mature”.
Over the next few weeks, numerous similar transaction errors continued to occur on Channel B, causing swap overs to Channel A and subsequent swap backs to Channel B. From reviewing data, flight controllers were able to determine that: “All 5 of the most recent sets of transaction errors have now occurred between BGA angles of 27 degrees – 30 degrees”.
After remaining on Channel A for a few days in order to prevent further transaction errors on Channel B, a meeting of the Failure Investigation Team (FIT) determined that the forward plan, which is currently in effect, was to switch back to Channel B for 40 days to gather data on bus performance. “The automated response to swap bus channels in response to the loss of data connectivity will be inhibited to gather information on how long the data dropouts last”, continued the notes.
While the 2A BGA transaction errors was the only issue noted as having a potential impact on the SpX-D mission – although the issue is still classed as low risk and acceptable to flight, despite being ongoing – there were other notable issues that relate to the SpX-D mission that were cleared as being no impact.
The Cupola, the seven-windowed observation module from where the SSRMS will be controlled during Dragon’s capture, offers increased situational awareness over simple video monitors inside the ISS. However, the means by which this is achieved – 360 degree windows – have been having some minor issues of their own of late.
“Cupola Window 6 Temperature Divergence – Specialists have been tracking a divergence between the two temperature sensors on Cupola Window 6. This signature has been seen in the past on Window 5 when a laptop was placed near one of the temp sensors,” stated L2 ISS on-orbit status notes. “The crew was asked about Cupola configuration, but there were no laptops or other heat generating equipment near Window 6”.
Using a temperature probe, the crew took readings near the window temperature sensors. Based on those readings, no noteworthy divergence between the windows was detected, however there was a noticeable difference between the temperature sensor reading and manual reading for Window 6 RTD (Resistance Temperature Device) 2.
“During Passive Thermal Control System’s team Cupola Window temperature scans they noticed that Cupola Window 6 exceeded its upper temperature limit of 100 degrees F (37.3 degrees C) at 38.3 deg C for 13 minutes,” added the notes. “Per Flight Rules, if any Cupola window exceeds 36.7 degrees C for more than 5 minutes, the shutter must be closed.
“Exceeding the upper limit is a concern for the Cupola window seal. Engineering is assessing the temperature violation and its duration. The window shutter has been requested to remain closed until this assessment is complete. Upper Cupola Window temperatures can be managed by closing the window shutters when not in use.”
Notes the following day mentioned that temperature exceedance issues had also occurred on Cupola Window 1: “The Cupola Window 1 (Port) temperature exceeded the 36.7°C watch limit and maxed out at 39.3°C. It was above the limit for approximately 7 minutes. The flight rule only allows for <5 minutes above the 36.7°C limit.”
“Per the Flight Rules, the ground requested the crew to close the Window 1 shutter and the temperature began to drop. The concern is the integrity of the seals around the window. This small exceedance is not considered a hazard, but the elevated temperatures are under investigation for potential causes and prevention.”
As for probable causes, the SORR presentation noted: “On 25/03/12, window scratch panes (with heaters) were R&R for ATV-3 antenna photography. Leading theory is partial delamination of RTD 2 is possible; may have exacerbated during scratch pane removal (not easily accessible for inspection)”.
While the investigation into the window temperature issues is still ongoing, since the exceedance values are low and an adequate mitigation (closing the shutters) is available, this allowed the issue to be classed as being no risk to the SpX-D flight.
Another minor issue was noted which related to the Space Integrated GPS/Inertial Navigation System (SIGI), which will be utilised by Dragon during Relative GPS (RGPS) communication with the ISS. There are two SIGIs aboard the ISS, although one had been showing signs of ageing.
“SIGI-1 is near end of its 10 year life and an unused internal gyroscope has caused multiple failures recently. All were recovered by power cycles. The SpaceX LCC (Launch Commit Criteria) requires 2 SIGIs, after that only 1 is required for rendezvous and capture,” stated the SORR presentation.
“Dragon will abort during rendezvous if data is not available from an active SIGI within 5 or 10 minutes (depending on proximity to ISS). Following a power cycle, an ISS SIGI takes a minimum of 30 minutes to recover functionality”.
“SpaceX and ON prefer to R&R (Remove & Replace) SIGI-1 to increase the likelihood of mission success. There are 2 spares on orbit and 4 on the ground. R&R requires ~7 hours crew time, requires repeat of the 12 channel checkout, and 3 weeks run time prior to LCC. ON recommended R&R of SIGI-1 to minimize the likelihood that degraded internal gyroscope will cause it to fail during Dragon rendezvous.”
As continued by L2 ISS on-orbit status notes: “On April 4, the SIGI-1 was R&Rd. It was placed in first priority for both ISS State and Attitude on April 6. Yesterday, ground controllers completed the final checkout for SpaceX operations – a 12 channel test. The data dump for the 12-channel test is still under evaluation, however the test was nominal”.
As such, the SIGI issue is now considered resolved and is classed as no impact to the SpX-D mission.
Only one other minor issue pertains to the SpX-D mission, which is a slightly open protective petal on the Node 2 Nadir (N2N) Active Common Berthing Mechanism (ACBM) port to which Dragon will attach. In addition to internally removable Center Disk Covers (CDCs), ACBM petals provide protection for the ACBM – specifically the berthing ring – that would otherwise be exposed to space.
The four petals (one for each quadrant of the berthing ring) on each vacuum-exposed ACBM on the ISS are designed to open and close (like a flower petal – hence their name) remotely when commanded in order to expose the berthing ring. If a petal were to become stuck open, it would leave the ACBM berthing ring open to MMOD (Micro Meteoroid Orbital Debris) strikes, and if a petal were to become stuck closed, it would preclude a berthing from occurring.
As stated by the SORR presentation: “During an SSRMS pre-motion survey, the N2N ACBM Petal #1 (Starboard-Aft) appeared to be slightly open, although mostly closed. This condition was not noticed during a recent N2N CBM survey and checkout, due to the camera angles used at that time”.
“Per photo review, the petal #1 position may have been in this configuration since STS-123/Flight 1J/A (March 2008), when the Petal #1 launch restraints were released. Petal #1 is not believed to be warped and both sides appear to be the same displacement above adjacent petal covers. Petal #1 has been through approximately 15 successful open/close cycles with no issues.”
“Status: A prep for mate (exercising the latches and opening the petals) will be performed approximately 40 hours prior to berthing. After the SpaceX Dragon demo is berthed, OSO (Operations Support Officer) will ask if the crew could inspect Latch #1 to see if it is different from the others.”
Thus, with all ISS issues determined to be low risk and of little or no impact to the SpX-D flight, the SORR concluded that: “The ISS MER (Mission Evaluation Room) is Go for SpX Capture, Attached Ops, SpX Release and Stage operations”.
(Images: L2’s SpaceX Dragon C2/C3 Mission Special Section – Containing presentations, videos, images, interactive high level updates and more, with additional images via NASA and SpaceX).
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