Just after midnight, 00:23 UTC on March 16th 2019, a Delta 4 medium rocket placed a US military network relay satellite into orbit. Launching from Space Launch Complex 37B at Cape Canaveral AFB in Florida, the 66 meter tall Delta 4 is nearing retirement, with this being its second to last launch. After several technical issues, the ground teams eventually got the rocket and the satellite tracking network functioning correctly. The hydrogen fueled RS-68A main engine ignited moments before liftoff for 5 seconds before the hold down bolts released at T-0, firing away with 1.8 million pounds of thrust. This mission has extended ULA’s streak of successful missions to 133 since its inception in 2006.
The rocket veered towards an easterly direction over the Atlantic Ocean, aiming to place the communications satellite to its final operating position 36,000 km (22,000 miles) above the equator in geostationary orbit. The solid rocket boosters burned out and were jettisoned in pairs roughly 1 minute and 40 seconds into flight. The main engine continued to fly on until 4 minutes in when the first stage was cut off, and then released shortly after. The first stage then fell back to Earth into the Atlantic Ocean. The upper stage was powered by a RL10B-2 engine, made by Aerojet Rocketdyne, the same manufacturers of the main engine. The upper stage engine ignited twice to push the satellite into an elliptical transfer orbit. The satellite separated from the second stage at T+36 minutes 50 seconds.
On board was the WGS 10 military communications satellite. It is a 6000kg (13,200 lb) broadband satellite, that is joining nine others that have been slowly placed in orbit since 2007. The idea is to form a globe spanning network that can relay video, data and other useful information between the battlefield and the headquarters, wherever they may be. The WGS fleet transmits both classified and unclassified information, and supports the US and its allies. On board is a digital channelizer that allows the satellite to relay signals using high data-rate X-band and Ka-band frequencies during its 14 year expected life. All of the WGS satellites were launched on ULA rockets, with the first two on Atlas V’s and all the rest on Delta 4’s. This mission had an estimated price tag of $400 million.
Marking the second to last flight of the Delta 4 Medium variant rocket, it is noticeable as only having a single first stage core, whereas the Delta 4 Heavy has three. ULA are retiring certain areas of their launch family as they plan to debut the new Vulcan booster soon which will apparently be cheaper than their current offering. The decision to halt selling of the Delta 4 medium flight was made in 2014, but this and the next launch were already on the books at that time. The Delta 4 medium is apparently more expensive than the Atlas V launcher, but with a similar launch capability, leading to the reason for retirement. ULA described it as it being cheaper to run a few launchers more frequently than many launchers sporadically. The bigger Delta 4 heavy will continue to launch heavier payloads well into the mid 2020’s. Another reason for keeping the Delta 4 Medium was to allow the US military to have two choices to launch their payloads, that and the Atlas V. Now that the Falcon 9 is cleared to fly military satellites there is less need for the Delta variant.
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