SHERPA Rideshare Mission
The company Spaceflight Inc. of Seattle, WA, USA, founded in 2009, has the goal to revolutionize secondary payload flight services for fixed and deployable cargo and transport. In 2012, Spaceflight formally started its SHERPA in-space tug service project, which is dedicated to hosting and deploying small and secondary payloads.
Since the termination of the Falcon 1 program, the opportunities for access to space for small spacecraft have been limited. Spaceflight, Inc. (Spaceflight) is addressing this market need by providing commercial launch services for secondary and hosted payloads by using its SSPS (Spaceflight Secondary Payload System) and SHERPA in-space tug. The SSPS is a system designed to transport up to 1,500 kg of secondary and hosted payloads to space using the excess capacity on Medium and Intermediate class commercial launch vehicles. The SSPS can accommodate up to five 300 kg spacecraft, or many smaller spacecraft, on each of its five ports and operates independently from the primary launch vehicle to simplify payload and mission integration. SHERPA is an in-space tug that builds upon the capabilities of the SSPS by incorporating propulsion and power generation subsystems, which creates a free-flying tug dedicated to maneuvering to an optimal orbit to place secondary and hosted payloads. Spaceflight has manifested the SSPS on a Falcon 9 launch in 2013 and the SHERPA on the inaugural launch in 2015. 1)
Table 1: Some background on Spaceflight Inc. planned service provision scenario 2)
Inaugural SHERPA flight:
The 2015 SHERPA mission addresses the limitations of current launch configurations by maximizing the number of payloads integrated to the structure. The SHERPA concept is well suited to support small spacecraft that range from CubeSats to 300 kg minisatellites. By developing unique adapters for the standard ports, Spaceflight has enabled many different sizes of spacecraft to deploy from SHERPA, including the DARPA eXCITe microsatellite, built by NovaWurks Inc. of Alamitos, CA (NovaWurks is a wholly owned subsidiary of Spaceflight Industries, Inc., Seattle) and two BlackSky Global Pathfinder satellites. Future planned configurations involve multiple SHERPA rings on a single launch, as well as support for much larger spacecraft on the forward section of the SHERPA ring. 3) 4)
One of the innovative microsatellites included on the manifest is a 160 kg system, developed and flown by NovaWurks. Based on the current customer inputs, this 125 kg to 250 kg minisatellite class will be an area of high demand. Spaceflight has concluded, that this is one of the worst served markets in terms of access to space. Historically, this category was served by foreign launch service providers and the U.S. Government. As technology advances, this class of spacecraft has begun to catch the interest of commercial satellite providers. This new group of customers does not have access to the Government small launch vehicles, and, to date, commercial small launch vehicles are too expensive for the secondary market. While there are efforts underway to develop lower cost commercial small launch vehicles, the efficiencies in cost provided by the SHERPA concept will remain a competitive means to access popular orbits.
Three microsatellites on the 2015 SHERPA mission support the early phases of a constellation build out. With numerous spacecraft manifested on this mission Spaceflight has had to consider not only the mass to orbit, but also the volume consumed by any given spacecraft. By developing an adapter system that places two spacecraft on a single port, Spaceflight is able to maintain its Internet advertised pricing. Customers can recognize the benefit of this pricing by designing to industry and SHERPA published standards. This will increase the number of rideshare opportunities available and keep the cost per kg low.
Spaceflight procured the ESPA [EELV (Evolved Expendable Launch Vehicle) Secondary Payload Adapter] Grande ring from Moog CSA Engineering, has chosen Innovative Solutions in Space's QuadPack as the CubeSat deployer, Planetary Systems Corporation motorized LightBand as the separation system for non-containerized spacecraft, and a RUAG separation system to separate SHERPA from the launch vehicle. Additionally, Spaceflight will utilize Andrews Space for command and data handling subsystems to be installed on SHERPA (Figure 1).
Figure 1: Configuration of the SHERPA inaugural flight rideshare payload interfacing to the 61 cm bolt-hole pattern on the five ESPA Grande ports (image credit: Spaceflight)
For the 2015 SHERPA mission Spaceflight is utilizing the modularity to fly a number of 6U CubeSats. Not only do the QuadPacks provide modularity in the CubeSat configuration, the ability to integrate the system through multiple bolt hole patterns enables additional integration options. Spaceflight has designed an adapter plate that integrates the aft end to the plate. This design allows Spaceflight to place seven QuadPacks per SHERPA port.
The QuadPacks are compatible with CubeSats developed to the Cal Poly CubeSat Standards. This system also offers options to support some of the emerging trends in the CubeSat world; such as "tuna can" extrusion, and volumegrowth around the rail system.
Launch: The first SHERPA mission with 87 payloads on board is manifested on a SpaceX-F9 vehicle and is expected for Q1 2016. The launch site is VAFB (Vandenberg Air Force Base). The final configuration incorporates three microsatellites and 84 CubeSats of which the majority are 3Us. The microsatellites represent two different spectrums with regard to mass and interfaces. 5)
So far, the primary payload on this mission has not been disclosed.
SHERPA's maiden mission will deliver over 1200 kg of customer payloads to a sun synchronous orbit. SHERPA will test the its avionics, attitude determination and control system, as well as communications and other key subsystems to enable future payload delivery and hosted payload missions.
Customers on this mission include government, civil, and commercial customers from the United States, the Netherlands, Finland, France, Chile, Brazil, and South Korea. Missions range from new technology demonstrations, Earth imaging, to biological experiments.
Mission concept of operations:
A significant driver for the requirements of the 2015 SHERPA mission is the overriding concept of avoiding any additional risk to the primary spacecraft or the launch vehicle. Secondary payload mechanical and electrical designs have been addressed over the years at the Small Satellite Conference (Ref. 3).
The SHERPA is to be separated from the launch vehicle prior to any deployments. This operation enables Spaceflight to minimize the risk to the launch service provider because it is responsible for all deployments after SHERPA separation. This separation requirement drove Spaceflight to develop a robust avionics system at a competitive price. The avionics system had to be compatible with Spaceflight's advertised launch pricing. In the end, the company determined that developing their own system was the best solution.
The avionics incorporated into the 2015 SHERPA mission were kept as simple as possible and still meet mission requirements: battery, computer/sequencer, GPS and transmitter. The SHERPA system is powered off throughout the launch ascent phase (as a secondary payload itself) and activated via the separation event from the upper stage of the launch vehicle. Once initiated, the avionics perform the separation sequencing based on a timing script.
Timing the separation events was an important trade analysis. The first consideration was to avoid spacecraft re-contact. The second was to manage the forces imparted on the SHERPA because there is no system to maintain and control attitude. The third requirement was to deploy all the satellites and confirm the deployments prior to the end of battery life. The analyses and lessons learned are documented for use in future mission planning. Spaceflight have developed a separation plan that minimizes risk of contact and does so without control authority on the SHERPA.
Spaceflight also included a GPS for position knowledge and a radio to transmit telemetry as part of the SHERPA. Avionics will capture the GPS location of each separation event, and confirm deployment. The avionics provide the telemetry data to customers within 30 minutes of a successful ground station pass.
The overall mission timeline is as follows:
• Separation from the upper stage of the launch vehicle
• Avionic system initiated and mission sequence timing begins
• 30 minute coast – no SHERPA activity
• Enter payload deployment sequence
• 45 minute payload deployment time
• Continue to relay telemetry over selected ground sites until end of battery life approximately 10 hours post SHERPA separation.
Figure 2: Overview of the SHERPA event timeline (image credit: Spaceflight)
Flight hardware design trades:
Deployment Electronics: A common challenge in aerospace is defining a set of requirements that can be met through "off the shelf" solutions. In the case of the SHERPA, Spaceflight developed electronics capable of meeting mission requirements. Requirements included the electrical inhibits required by the launch service provider to ensure no inadvertent deployments.
Figure 3: Internal view of SHERPA showing electronics (image credit: Spaceflight)
Power is a considerable challenge when there are 87 spacecraft being deployed. Both the GPS and radio must be powered, for up to 10 hours. The design solution Spaceflight selected was the CORTEX Avionics Suite. The CORTEX battery is a Spaceflight COTS unit based on lithium-ion cells purchased from Yardney. Spaceflight also chose to keep the flight computer, or sequencer, in house. The CORTEX 160 flight computer performs the computing, command and storage functions needed for the mission.
Figure 4: Photo of the CORTEX 160 flight computer (image credit: Spaceflight)
Microsatellite adapters: Spaceflight developed two requirements that defined the final design of the adapter plates; modular and minimal mass. Overall mass is a significant concern in the business of purchasing excess launch capacity and then selling that capacity to satellite providers. The final designs are machined aluminum plates with the minimal mass required to support the flight configuration payloads under the anticipated launch load (Ref. 3).
For the 2015 SHERPA mission, Spaceflight is using 3 different plates (payload adaptors) were designed to interface to the Moog ESPA Grande ring, the core of the SHERPA spacecraft. These payload adaptors allow for 6 different satellite, separation or dispenser configurations. The plates are estimated to support 80% of the current customer base Spaceflight tracks. The radial port adapter is a flat plate for microsatellites greater than 50 kg. It bolts directly to the SHERPA 60.96 cm port interface and can provide separation diameter interfaces of either 29.8 cm or the 38.1 cm motorized LightBand. It is also capable of supporting a total mass of up to 150 kg.
The plate itself is machined from one piece of aluminum alloy. The diameter is just slightly wider than the 60.96 cm bolt pattern at 65.40 cm; it has a total mass of 13.3 kg. In addition to supporting the spacecraft, the backside of the plate is used to mount the various avionics components required for the 2015 SHERPA mission. The plate has accommodations for cable pass through.
The dual port adapter enables Spaceflight to fully utilize the volume available on a port and supports two spacecraft, each with a mass of up to 85 kg. This adapter is machined from a single piece of aluminum and has a mass of 21.7 kg. It attaches to the ESPA Grande ring via the 61 cm bolt hole pattern and has accommodations for both the 29.8 cm and 20.3 cm motorized LightBands.
Figure 5: Photo of the dual port adapter (image credit: Spaceflight)
CubeSat adapters: To date Spaceflight has flown over 76 CubeSats across a range of launch systems using a variety of different dispensers. With no slowdown in upcoming CubeSat deployments forecast, the QuadPack Plate was developed. It supports 7 QuadPacks per plate.
The initial SHERPA flight is based on the ESPA Grande ring, along with a number of adapters, which are paired with Spaceflight Systems electronics. The concept behind SHERPA is to make the configuration modular. With this set of adapters and electronics Spaceflight can mix and match components to suit most any set of payloads.
Figure 6: QuadPack Plate integrated with dispensers and electronics (image credit: Spaceflight)
Spaceflight's final piece of in-house capability is the newly developed Spaceflight Ground Station Network. Communications between the SHERPA and ground station are primarily accomplished using Spaceflight Networks. The system will be a global ground station network developed to support commercial and government satellites. It is available to Spaceflight customers on a pay per-minute basis. The network has a "bent-pipe" architecture: all ground stations are unmanned and controlled by the central network operations center located in Seattle, WA. The ground stations are connected via terrestrial fiber, and customers are given direct access (via TCP/IP) to communicate with their spacecraft during the designated communications window. Spaceflight Networks currently support communications in UHF, S, and X-bands. For commercial spacecraft, S-band uplink and X-band downlinks are used. For the 2015 SHERPA mission, Spaceflight will use three of the ground stations as well as support from NASA Wallops Flight Facility to downlink the mission telemetry (Ref. 3).
Future SHERPA capabilities:
Spaceflight intends to solve the SmallSat challenge of deploying customers into the optimal orbit by including a propulsion system on SHERPA, providing the capability to perform LEO altitude shifts, as well as SmallSat missions to a geosynchronous transfer orbit and trans-lunar injection orbits.
Spaceflight will offer 4 main variants of SHERPA with the following performance:
Table 2: Spaceflight SHERPA family variants
Figure 7: SHERPA performance (image credit: Spaceflight)
1) Jason Andrews, "Spaceflight Secondary Payload System (SSPS) and SHERPA Tug - A New Business Model for Secondary and Hosted Payloads," Proceedings of the 26th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, USA, August 13-16, 2012, paper: SSC12-V-6, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1049&context=smallsat
2) Phil Brzytwa, Curt Blake, "Spaceflight Sherpa mission Q3 2015; cost effective rideshare access to space for the growing smallsat industry," Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14 D2.2.7
3) Kaitlyn Kelley, Mitch Elson, Jason Andrews, "Deploying 87 Satellites in One Launch: Design trades completed for the 2015 SHERPA flight hardware," Proceedings of the 29th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, USA, August 8-13, 2014, paper: SSC15-II-2, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3171&context=smallsat
4) "General Payload Users Guide," Spaceflight Inc., SF‐2100‐PUG‐00001, Rev F 2015‐22‐15, URL: http://www.spaceflightindustries.com/wp-content/uploads/2015/05/SPUG-RevF.pdf
5) "Jason Andrews: 87 Small Sat launch via "SHERPA" on F9 from Vandenberg in Q1 2016 (date estimated)," Aug. 11, 2015, URL: https://www.reddit.com/r/spacex/comments/3gmpln/jason_andrews_87_small
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).