SHERPA (space tug)
SHERPA is a commercial satellite-dispensing space tug (sometimes called a kick stage) designed to fly small secondary payloads to their desired location before deploying them.
Riding atop the launcher's final stage, SHERPA's release follows deployment of the primary mission payload for the dispensing of minisatellites, microsatellites, or nanosatellites such as CubeSats. SHERPA builds upon the capabilities of the Spaceflight Secondary Payload System (SSPS) by incorporating propulsion and power generation subsystems, which creates a propulsive tug dedicated to maneuvering to an optimal orbit to place secondary and hosted payloads.
SHERPA was developed by Andrews Space, a subsidiary of Spaceflight Industries, and was unveiled in 2012. The maiden flight was on 3 December 2018 on a Falcon 9 rocket, and it consisted of two separate unpropelled variants of the dispenser.
SHERPA is a three-axis stabilized platform capable of on-orbit maneuvering meant to deploy small satellites carried as secondary payloads on rideshare orbital launches. SHERPA is integrated to the rocket as a standard adapter that is designed to fit on the SpaceX Falcon 9, Orbital Sciences Corp.'s Antares, and United Launch Alliance's Atlas 5 and Delta rockets. SHERPA is to be separated from the launch vehicle prior to any deployments.
SHERPA is a commercial derivative of the ESPA Grande ring, and it was developed and manufactured by Andrews Space, a subsidiary of Spaceflight Industries since 2010 and was unveiled in May 2012. Spaceflight Industries fabricates SHERPA, and the SSPS, at its facility in Tukwila, Washington. 
Riding atop the launcher's final stage, SHERPA is to be separated from the launch vehicle prior to any deployments or dispensing of minisatellites, microsatellites, nanosatellites and CubeSats. SHERPA features an optional propulsion system to place its payloads in an orbit other than the primary payload's orbit. The powered variants are capable of large orbit change.
There are at least four SHERPA variants: SHERPA (non-propelled), SHERPA 400, 1000 and 2200. Each SHERPA is able to be launched in a stacked configuration with other SHERPA modules for later separation and independent free-flying.
The basic SHERPA is based on a commonly-used secondary payload adapter known as an ESPA ring and it is not propelled. It is used for low Earth orbit deployments, and can unfurl a dragsail to lower its orbit before payload deployment.
- SHERPA 400
The 400 variant is used for low Earth orbit deployments, and it features two tanks with mono-propellant.  SHERPA 400 has a fuelled mass of 1,000 kilograms and it has a maximum capacity of 1,500 kg (3,300 lb) to low Earth orbit. It is capable of accompanying a primary payload to 800 km and then lower its orbit to a more favorable altitude to drop off secondaries. Most small satellites are required to orbit at about 450 kilometers to deorbit or move to an unused orbit within 25 years of the mission's completion.
- SHERPA 1000
This variant features additional monopropellant volume stored in 4 tanks.
- SHERPA 2200
The 2200 variant has a fuelled mass of 2,000 kg and it features a more powerful bi-propellant fuel (stored in 4 tanks) for the delivery of small payloads to geostationary transfer orbit (GTO) as well as the lunar environs. GTO is a highly elliptical Earth orbit with an apogee of 42,164 km (26,199 mi).
SHERPA's first mission was to deploy 90 small payloads, during a 2015 launch on a Falcon 9 rocket, then it was rescheduled for 2017, but delays caused in part by a Falcon 9 rocket explosion on a launch pad in 2016, prompted Spaceflight to cancel the mission.
The first launch of SHERPA was on 3 December 2018 on a rideshare mission called SSO-A: SmallSat Express. The two SHERPA dispensers were originally planned to deploy more than 70 small satellites from 18 countries, which included 15 microsatellites and 56 CubeSats carried on two separate SHERPA dispensers. However, later changes reduced the number of satellites to 64. Both dispensers separated from the Falcon 9 rocket once it entered a polar Sun-synchronous orbit around 575 kilometers above Earth. Both dispensers in this mission lack propulsion, but unfurled dragsails to lower their altitude as needed for sequential payload release. The total payload mass riding on this Falcon 9 rocket was approximately 4 metric tons (4,000 kg).
- SHERPA News. Spaceflight Industries. Accessed: 17 November 2018.
- "Spaceflight Plans 2014 Demo Launch of Sherpa In-space Tug". SpaceNews. 2012-05-07.
- Spaceflight Unveils SHERPA In-Space Tug. Doug Messier, Parabolic Arc. May 7, 2012.
- Jason Andrews. "Spaceflight Secondary Payload System (SSPS) and SHERPA Tug - A New Business Model for Secondary and Hosted Payloads (2012)".
- Spaceflight Secondary Payload System Archived 2012-07-07 at Archive.today, retrieved 2012-05-10.
- SHERPA Rideshare Mission. eoPortal Directory. Accessed: 17 November 2018.
- Spaceflight preps for first launch of unique orbiting satellite deployers. Stephern Clarke, Spaceflight Now. 23 August 2018.
- Andrews, Jason (August 2012). "Spaceflight Secondary Payload System and SHERPA Space Tug". 26th Conference on Small Satellites.
- Sherpa Commercial Satellite Tug Aims For 2015 Debut. Marc Carreau, Aviation Week. 5 May 2014.
- Vallado, David A. (2007). Fundamentals of Astrodynamics and Applications. Hawthorne, CA: Microcosm Press. p. 31.
- "A Message from Spaceflight President Curt Blake on the FormaSat-5/SHERPA launch - Spaceflight". Spaceflight. March 2, 2017. Retrieved March 2, 2017.
- "SpaceX Will Lose Millions on Its Taiwanese Satellite Launch". Wired. 24 August 2017.
- Sorensen, Jodi (August 6, 2018). "Spaceflight prepares historic launch of more than 70 spacecraft aboard SpaceX Falcon9". Spaceflight Industries. Retrieved August 6, 2018.
- Introducing SSO-A: The Smallsat Express. Spaceflight Industries. Accessed: 17 November 2018.
- Pietrobon, Steven (December 8, 2018). "United States Commercial ELV Launch Manifest". Retrieved December 11, 2018.
- SpaceX Falcon 9 rideshare launch to send a commercial lander to the Moon in 2019. Eric Ralph, Teslarati News. 12 September 2018.