SHERPA is a commercial satellite-dispensing space tug 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 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.
Overview
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. 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.
Variants
There are at least four SHERPA variants: SHERPA, 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. ;SHERPA 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 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 for the delivery of small payloads to geostationary transfer orbit as well as the lunar environs. GTO is a highly elliptical Earth orbit with an apogee of.
Maiden flight
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.
Second flight
Spaceflight launched a large number of smallsats on 23 December 23 2018, but used a SpaceX Falcon 9 launch vehicle second stage to provide the propulsive delta-v to put the satellites into the desired orbits. A follow-on flight with a SHERPA tug was scheduled for February 2019 on a Falcon 9 to a geostationary transfer orbit, but as of June 2019 has not materialized.
