# Minotaur IV

Minotaur IV, also known as Peacekeeper SLV and OSP-2 PK is an active expendable launch system derived from the LGM-118 Peacekeeper ICBM. It is operated by Northrop Grumman Space Systems, and made its maiden flight on 22 April 2010, carrying the HTV-2a Hypersonic Test Vehicle. The first orbital launch occurred on 26 September 2010 with the SBSS satellite for the United States Air Force.
The Minotaur IV vehicle consists of four stages and is capable of placing 1,591 kilograms (3,508 lb) of payload into a low Earth orbit (LEO). It uses the first three stages of the Peacekeeper missile, combined with a new upper stage. On the baseline version, the fourth stage is an Orion 38. However a higher performance variant, designated Minotaur IV+, uses a Star 48BV instead. A three-stage configuration (no Orion 38), designated the Minotaur IV Lite, is available for suborbital trajectories. The Minotaur IV has also been flown with multiple upper stages. A five-stage derivative, the Minotaur V, made its maiden flight on 7 September 2013.
Minotaur IV launches are conducted from SLC-8 at Vandenberg Space Force Base, LP-0B at the Mid-Atlantic Regional Spaceport, SLC-46 at Cape Canaveral Space Force Station and Pacific Spaceport Complex – Alaska Pad 1 of the Pacific Spaceport Complex – Alaska (PSCA).

## Description

The Minotaur IV (and the overall Minotaur rocket family) was developed by Orbital Sciences (now owned by Northrop Grumman) as part of the United States Air Force's Orbital Suborbital Program. There are three variants available: Minotaur IV, IV+, and IV Lite. Minotaur IV and IV+ are used for low Earth orbit missions, while Minotaur IV Lite is intended for suborbital launches, such as testing prototype hypersonic vehicles. The separate Minotaur V is also available, consisting of a Minotaur IV+ with an added fifth stage for high-energy trajectories such as geostationary transfer orbit or trans-lunar injection.
The Minotaur IV family is derived from the LGM-118 Peacekeeper intercontinental ballistic missile (ICBM), deployed from 1985 until 2005. The Minotaur IV family utilizes decommissioned Peacekeeper solid rocket motors, which compose the first three stages in all Minotaur IV rockets and derivatives. This relatively simple architecture allows Minotaur to be launched from essentially anywhere in the US through the use of mobile launch facilities - although this capability has never been needed. Because of its use of decommissioned ICBM components, Minotaur IV can only be used to launch US government missions.

### Minotaur IV

The standard Minotaur IV rocket is composed of four stages. The first stage SR118 motor provides 2,224 kilonewtons (500,000 lb$_f$) of thrust during its 56.6-second burn, followed immediately after by stage separation and second-stage ignition. The second stage, powered by an SR119 motor, burns for 61 seconds and provides an average thrust of 1,223 kilonewtons (275,000 lb$_f$). The third stage then burns for 72 seconds, with an average thrust of 289 kilonewtons (65,000 lb$_f$). The initial three stages all have thrust vector control, allowing them to steer the rocket downrange by gimballing the motor nozzles. The second and third stages also feature extendable nozzles, allowing for improved performance in the upper portions of Earth's atmosphere as well as the vacuum of space.
The fourth stage of the Minotaur IV is the Orion 38 motor, which is also used in the Minotaur-C, Minotaur I, Pegasus, and Ground-Based Interceptor rockets. This motor performs the final orbital insertion burn for the payload. Like the first three stages, the Orion 38 also features thrust vectoring, with a 5-degree range of motion.
On one occasion, for the ORS-5 mission, Minotaur IV was outfitted with a second Orion 38 motor to allow the payload to be inserted into an equatorial orbit. In addition, another Minotaur IV mission featured a Hydrazine Auxiliary Propulsion System (HAPS) for additional orbital maneuvering capability. The HAPS was developed for the Pegasus rocket to fine-tune the payload's orbit since solid motors are not capable of fine adjustments.

### Minotaur IV+

The Minotaur IV+ is a higher-performance variant of the Minotaur IV. The first three stages are identical, but the Orion 38 fourth stage is replaced with a Star 48BV motor. The Star motor features more propellant than the Orion motor, allowing the rocket to carry roughly 200 kg (440 lb) of extra payload to low-Earth orbit, or can allow for payloads to be sent to elliptical orbits. The Star 48BV burns for 85.2 seconds with an average thrust of 68.63 kilonewtons (15,430 lb$_f$) and also features thrust vectoring, which is uncommon for Star 48 motors. The Star 48 motor has also seen use on the Atlas V, Delta IV, and Space Shuttle, alongside over 70 missions on the Delta II.
Minotaur IV+ was further evolved to create the Minotaur V rocket, which adds an extra Star 37FM stage to the vehicle for improved high-energy performance. This configuration has only flown once as of 2024 and is not scheduled for any further launches. In addition, the more powerful Minotaur VI and Minotaur VI+ concepts were based on the Minotaur IV+, featuring an additional SR118 motor as the first stage to improve vehicle performance. However, neither Minotaur VI variant has flown and no flights are scheduled.

### Minotaur IV Lite

The Minotaur IV Lite is a suborbital configuration of Minotaur IV. It features the same first three stages as the standard variant but lacks a fourth stage. The IV Lite is intended for suborbital missions, allowing government customers to test new technologies like hypersonic aircraft or missile interception. As of May 2024, the Minotaur IV Lite has only flown twice, both times in support of the HTV-2 program.
This variant is almost identical to the unflown Minotaur III rocket, which was also intended to perform suborbital missions.

## Launch history

| Flight No. | Date/Time (UTC) | Variant | Launch Site | Payload | Trajectory | Outcome | Remarks |
| 1 | 22 April 2010 23:00 | Minotaur IV Lite | Vandenberg, SLC-8 | HTV-2a | Suborbital | Success | Successful launch, but payload failed |
| 2 | 26 September 2010 04:41 | Minotaur IV | Vandenberg, SLC-8 | SBSS | SSO | Success | |
| 3 | 20 November 2010 01:25 | Minotaur IV HAPS | Kodiak, LP-1 | STPSAT-2 FASTRAC-A FASTRAC-B FalconSat-5 FASTSAT O/OREOS RAX NanoSail-D2 | LEO | Success | STP-S26 launch. Included a Hydrazine Auxiliary Propulsion System (HAPS) to take the vehicle to a secondary orbit after placing payloads into the primary orbit. |
| 4 | 11 August 2011 14:45 | Minotaur IV Lite | Vandenberg, SLC-8 | HTV-2b | Suborbital | Success | Successful launch, but payload failed |
| 5 | 27 September 2011 15:49 | Minotaur IV+ | Kodiak, LP-1 | TacSat-4 | LEO | Success | First Minotaur IV+ launch |
| 6 | 26 August 2017 06:04 | Minotaur IV / Orion 38 | CCAFS, SLC-46 | ORS-5 | LEO | Success | Ran in a 5-stage configuration, using an extra Orion 38 motor to put ORS-5 in to an equatorial orbit. |
| 7 | 15 July 2020 13:46 | Minotaur IV / Orion 38 | MARS, LP-0B | NROL-129 | LEO | Success | Carried four payloads (USA-305 to USA-308). First NRO launch on a Minotaur IV and first from Virginia's Space Coast. |

### Planned launches

| Date/Time (UTC) | Variant | Launch Site | Payload | Trajectory | Remarks |
| 2024 | Minotaur IV | Vandenberg, SLC-8 | NROL-174 | LEO | |
| September 2024 | Minotaur IV | | ? | LEO | STP-S29A mission |
| May 2025 | Minotaur IV | Vandenberg, SLC-8 | EWS-I 1 | LEO | USSF-261S-A mission |
| TBD | Minotaur IV Lite | Vandenberg, SLC-8 | CSM | Suborbital |
| TBD | Minotaur IV | | ? | LEO | ORS mission |
| TBD | Minotaur IV | | ? | LEO | ORS mission |

## STP-S26

The third Minotaur IV launch, which was also known as STP-S26, deployed eight payloads. It was the 29th small launch vehicle mission in STP's 49-year history of flying DoD space experiments, STP-S26 was intended to extend previous standard interface development efforts, implementing a number of capabilities aimed at enabling responsive access to space for small experimental satellites and payloads. STP-S96 launched at 01:25 UTC on 20 November 2019 from the Kodiak Launch Complex. The launch facility contractor was Alaska Aerospace Corporation (AAC). The payloads were released in a 650 kilometres (400 mi) orbit, before the HAPS upper stage was demonstrated by deploying two ballast payloads into a 1,200 kilometres (750 mi) orbit.
The primary objective of the STP-S26 launch was to deploy STPSAT-2 (USA-287), whilst demonstrating the ability of the Minotaur IV to carry additional payloads, by deploying FASTSAT, FASTRAC, RAX, O/OREOS and FalconSat-5. A Hydrazine Auxiliary Propulsion System upper stage was flown aboard the Minotaur to demonstrate its ability to deploy payloads to multiple orbits, however only mass simulators were deployed after the HAPS burn.
The launch marked the first flight of an STP-SIV (Standard Interface Vehicle) satellite, the first use of the Multi Mission Satellite Operations Center Ground System Architecture (MMSOC GSA), the first flight of the Minotaur IV's Multi-payload Adapter (MPA), the first use of a HAPS to obtain multiple orbits on a Minotaur IV flight, the first Minotaur launch from Kodiak Launch Complex (KLC), and the first deployment of CubeSats from a Minotaur IV via Poly-PicoSatellite Orbital Deployers (P-Pods).