After a wait of nearly a decade, the International Space Station is just a matter of weeks away from welcoming another new visitor, in the form of the Japanese HTV (H-II Transfer Vehicle). NASA recently held a Flight Readiness Review (FRR) for their role in aiding the cargo vehicle’s debut stay at the orbital outpost, revealing many fascinating details about the mission.
HTV Quick Look:
The HTV is currently scheduled to launch on September 11 (local time – September 10 US time) from Tanegashima Space Center on an H-IIB vehicle – into an initial 200 km x 300 km orbit.
The 10 ton JAXA cargo vehicle is capable of supplying a total of six tons of pressurized and unpressurized cargo to the ISS at an altitude of 407 km. Pressurized cargo can be received at the rack level (an International Standard Payload Rack (ISPR)) or sub-rack level; such as Cargo Transfer Bags (CTBs).
Sub-rack level cargo is integrated into HTV resupply racks (HRRs). All HRRs and ISPR equivalents are integrated into the HTV Pressurized Logistics Carrier (PLC). Unpressurized cargo is integrated onto an exposed pallet and, subsequently, into the HTV Unpressurized Logistics Carrier (UPLC or ULC). It is also capable of carrying several hundred kilos of water.
After the HTV has delivered cargo to the ISS, waste cargo from the ISS is loaded into the HTV; and is destroyed upon reentry into the Earth’s atmosphere.
This cargo capability is crucial for the Station, as managers plan out the convoy of agency and commercial cargo vehicles that will need to pick up the massive payload capability of the soon-to-retire shuttle – as much as that retirement date remains unknown.
It total, the HTV debut flight will involve a mission duration of six days of rendezvous operations, 30 days of “attached” phase operations, followed by two to three days of departure and a disposal via re-entry.
HTV Operations:
A total of 12 demonstrations of HTV capability are planned for this mission, with reviews on its performance taking place at key stages of the flight – including an Integrated Mission Management Team (MMT) meeting on Flight Day 6, which will result in approval for the HTV to continue on to rendezvous with the Station.
“Free flight demonstrations are planned for all of safety critical functions before the flight phase where these functions are needed,” noted one of 16 MOD FRR presentations acquired by L2, with this article covering two of the presentations.
“Far Field Rendezvous demonstrations are planned for Flight Day 3 and must be successfully completed prior to integrated options – including active and passive aborts (5), free drift and absolute GPS. To be reviewed by IMMT on FD6.”
Following approval to proceed, real time monitoring and discussions on the status of the flight will be carried out as the vehicle approaches the ISS, with the option to abort at any stage.
“Any issues – or failures to meet established success criteria – will result in rendezvous termination,” noted the Agenda 2 (Mission Overview) FRR presentation.
As would be expected, the approach of the vehicle – for the first time – to the Station, will be closely monitored. This in itself will require new operations to debut for both HTV controllers on the ground, and the crew onboard the ISS.
“New Operations: HTV Crew Monitoring (HCM) is performed during R-Bar operations. Consists of dynamic and static overlays viewed on RWS (Robotic Workstation) monitors using truss and MSS cameras,” added the Agenda 3 Flight Director FRR presentation.
“Considered a hazard control for loss of a single Rendezvous Sensor (RVS). Requires crew to monitor PROX range and RVS range during approach. Requires use of Inner Capture Volume (ICV). Terminate HTV approach if HTV exceeds upper limit of ICV. Confirmation that HTV is controlling within the ICV prior to HTV free drift.”
Again, a safe termination of the approach, in the event of a problem – such as LOS (Loss Of Signal) – is a priority for the safety of the ISS.
“HCM is also used to monitor approach corridor. Dynamic abort corridor. Inside 300m, if LOS with MCC-H (Mission Control Center, Houston) and HTV exceeds abort corridor, crew prime to initiate HTV abort,” added the presentation.
“Rendezvous can be terminated via an Abort or a No-Go command (i.e. no burn). Flight rules define burn Go/No-Go criteria based on system functionality and trajectory limits.
“HTV will automatically abort for many system failures. Ground initiated abort is required for violation of trajectory limits or certain ISS failure (e.g. loss of attitude control). No-Go (upload of 0 delta V) is only allowed for a few burns that have a 24 hour safe coast trajectory without an abort. All HTV aborts are designed to ensure a minimum X-axis delta V of 1.2 m/sec.
“Abort ensures that the HTV drift trajectory will not enter the approach ellipsoid within 24 hours of burn completion. Abort prior to 300 meters is retrograde. Abort during 300 meter yaw around is passive (i.e. no burn). Abort after 300 meter yaw around is posigrade.”
Robotics and Transfers:
Among the mission priorities are items such as the successful use of the SSRMS (Space Station Remote Manipulator System) during HTV capture, and its dual use with the Japanese robotic arm.
The SSRMS will play a key role as the vehicle arrives at the ISS, given it will not dock like previous cargo ships. Instead, the SSRMS – or Canadarm2 – will grapple the HTV, before robotic operations will gently translate the new arrival on to the Harmony module.
“Capture sequence: Orbit night is prime plan. ISS thrusters are inhibited (CMG only control). SSRMS moves in from High Hover. HTV commanded to free drift (initiates 99 second capture clock). SSRMS final alignment and capture through first and second phase. Crew safing during capture sequence involves SSRMS back-away and for some cases HTV Retreat,” noted a robotics element on the Flight Director FRR presentation.
“Track and Capture: First free flyer capture for SSRMS. Enhanced on-board training to support maintenance of track and capture skills.
“Hot Backup: Provides capability to quickly switch SSRMS strings. ~90 seconds from failure recognition to completion of string swap. Standard string swaps required 20-30 minutes. Allows for continuation of capture or HTV release for partial capture scenarios.”
Any problems – resulting in delays to the operations – during the robotics to berth the HTV on Station will result in the vehicle being held in an overnight park position, while the crew prepare to attempt the docking the following day.
“HTV proximity operations, capture, berthing and critical activation are performed in one day. Crew workday is 7.5 hours (1 hour beyond standard workday), but in family with critical/complex activities.
“If delays are expected to exceed a 10 hour work day, HTV will be taken to an Overnight Park position. Overnight park would also be used for an issue identified during PCBM inspection.
“Beta dependent park positions (4 total) designed to optimize HTV power generation. Although optimized for power generation, overnight park will result in primary battery power draw if overnight park is required.”
Further robotics will be involved when the HTV’s Exposed Pallet (EP) is handed over to the Japanese arm (JEM-RMS), which will then locate the EP on to the Exposed Facility (JEM-EF), following its own recent installation during STS-127.
“EP removal from ULC via SSRMS. SSRMS to JEM RMS handoff of EP. JEM RMS installation of EP on JEM-EF,” noted Flight Day 10 activities in the FRR mission overview presentation.
“Exposed Pallet Removal and Insertion: Crew will command EP latch mechanisms for release and re-installation (single use only – parafin actuated mechanisms).
“SSRMS used to remove and install the EP. Installation is planned with Force Moment Accommodation (FMA). Provides closed loop feedback to alleviate loads.
“First time use outside of commissioning. Installation can be performed without FMA if issues arise.
“HTV Berthing Camera System (HBCS) on EP used for EP alignment and insertion. Video sent to RWS for use with HBCS overlay. Target inside HTV.”
The handoff operations between the SSRMS and JEM RMS will also be another first for the ISS.
“SSRMS to JEM RMS Handoff: EP is handed off between SSRMS and JEM RMS for transfer to/from HTV and the JEF. First handoff between SSRMS and JEM RMS. Very similar to SSRMS to SRMS handoff,” added the New Operations notes in the FRR presentation.
“EP Payloads are powered when on SSRMS, but not from JEM RMS. Failure of JEM RMS while EP is grappled would require SSRMS to double-grapple for dynamic load events.”
This will be followed by Flight Day 12’s: “JEM RMS removal of EP. JEM RMS to SSRMS handoff of EP. SSRMS installation of EP into HTV.”
The bulk of the HTV work on Station relates to the transfer of the vehicle’s internal cargo – involving 70 hours of soft stowage transfer and trash (from the ISS) being stowed back on the HTV – taking place between Flight Day 12 and 28, providing there are no issues with the vehicle during these procedures.
“NASA is responsible for cargo packing and transfer. JAXA is responsible for HTV mass property calculations,” added the FRR documentation.
“HTV has a fairly large acceptable region for c.g. (Center of Gravity) placement. Although this c.g. volume is considered acceptable from a GNC stand-point, the actual knowledge must be maintained in a tighter region with sufficient time for HTV to upload key GNC control parameters.
“For early end of mission (EOM), the tight nominal EOM (End Of Mission) c.g. will not be met. No specific scenario identified for early EOM.
“Could be needed for unexpected scenarios such as HTV power dropping below redlines, propellant leaks or an ISS emergency requiring HTV hatch closure.
“Gross cargo balancing constraints have been defined with and without EP to allow for gross c.g. placement within envelope. Allows JAXA to pre-design a contingency GNC parameter to protect for early HTV release.
“Nominal departure will utilize a much smaller c.g. envelope and refined GNC parameters. Early EOM response expected to take at least 24 hours.”
On Flight Day 29, the SSRMS will be translated back to the HTV for grappling, before the maneuver to release position. Flight Day 30 will see the SSRMS release, ahead of the HTV’s departure burns to gain distance from the ISS.
Around two days later, the HTV will end its mission – and its life – with a destructive re-entry.
“Departure Burns: 4 burns (IDM1, IDM2, DSM1, DSM2). Open loop burns preloaded before release. Trajectory is 24-hour safe and outside the approach ellipsoid after IDM2. End of integrated operations.”
Contingencies:
The mission can suffer several issues and still result in a successful operation. This is due to inbuilt contingency margins, as listed in the FRR documentation.
The majority of the contingency notes relate to the aforementioned rendezvous termination. However, it’s the recovery process that allows the mission to get back on track, as noted in the Flight Director FRR presentation.
“To simplify and minimize recovery maneuver options and to allow for troubleshooting, re-rendezvous is planned for 48 hours. First maneuver following breakout is 10-20 hours after the Abort/No-Go.
“All recovery maneuvers are performed outside of the approach ellipsoid (AE). Recovery targeting ensures HTV meets trajectory safety criteria (24 hour coast trajectory will not enter the AE).
“HTV goes to a point aft on the V-bar called V-bar Point Rear (VPR) where it will station-keep (90 km – 200 km+). AI departure and capture times are fixed with respect to GMT so the same crew/ground timeline is used.”
In fact, due to the power margins on the ISS, a total of four rendezvous are available – the initial and three re-rendezvous attempts – over a 48 hour period.
“Mission specific power analysis for launch shows sufficient primary battery power to support the following: Nominal rendezvous. 3 re-rendezvous attempts (48 hour turnaround). Overnight Park. ISS contingency allocation of 12 hours standalone power. HTV relocation to Node 2 zenith and back to nadir.
“Prop assessments predict sufficient margin to support 3 rerendezvous attempts. HTV consumables status will be reported periodically throughout the mission. During integrated operations, re-rendezvous and capture redlines are reported at AI, 300 meters, and 30 meters.”
Other contingencies are also available, with notes relating to the potential use of EVAs from ISS crewmembers, although strict rules are in place for such a requirement.
L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.