Day: March 18, 2024

Thursday, March 14th, 2024 saw SpaceX attempt the third Integrated Flight Test (IFT-3) of its massive Starship / Super Heavy launch system after the Federal Aviation Administration (FAA) granted a limited launch license to the company on March 13th.

Despite SpaceX and its followers hailing the first two launch attempts as “successes”, the short-order loss of both vehicles within 4 minutes of the launch of IFT-1 and which either vehicle achieving its core milestones in IFT-2, meant that both of those flights were extremely limited in their “success”. As a result of both, SpaceX spent considerable time reviewing the launch profile for the vehicles and making changes and improvement to both the Starship craft and Super Heavy. These resulted in IFT-3 being a broadly successful – although the loss of both vehicles at different points in the flight meant it was not an unqualified success.

Following lift-off at 13:25 UTC, with an initially perfect firing of all 33 Raptor engines on the booster, the stack of rocket and starship passed through Max-Q, the period where both experience maximum mechanical stresses as they ascend through the atmosphere, within the first minute of flight.

Even so, at 2:42 into the flight, the engines on the booster shut down and two seconds later, the starship upper stage ignited all six of its engines in a “hot staging” manoeuvre, separating from the booster after the engines had fired. This went a lot smoother than evidenced in the second launch attempt in November 2023, and the booster was this time able to change direction and execute an successful “boost back” burn – using the motors to kill its ascent velocity and push it back towards the launch site.

However, it was during the boost-back that possible hints of engine issues appeared: several of those recording and reporting on the launch noted that some of the engine exhaust plumes were tinged green, indicative of one or more engines consuming itself (green indicates the copper used in the engines is being consumed), a long-term issue with the Raptor 2. Nevertheless, the booster successfully re-oriented itself and started a planned engine-first descent towards the Gulf of Mexico and a splashdown.

For this to happen, the booster needed to slow itself by a further re-lighting several engines in a braking manoeuvre roughly a kilometres above the water. Whilst three engines did ignite, two immediately failed, and the vehicle was destroyed less than 500 metres above the Gulf – although it is not clear if the flight termination system was triggered or the booster blew itself apart. At the time of destruction, it was travelling with sufficient velocity to hit the water at 1,112 km/h.

Starship went on to achieve orbit, on course for a splashdown in the Indian Ocean. Travelling at around 240 km above the Earth, the vehicle carried out a test of the “Pez dispenser” payload bay door – a slot in the vehicle’s hull at the base of the payload bay and specifically designed to eject Starlink satellites (these being almost the only payload for Starship at present). Also tested was a so-called “propellant transfer” test, shunting a small amount of liquid oxygen between the vehicles’ main and header tanks.

However, SpaceX cancelled the vehicle’s planned de-orbit burn with one of its Raptor engines and instead allowed the vehicle to “go long”, continuing along its orbital track until gravity until drag caused it to re-enter the  denser part of the atmosphere for a hoped-for splashdown. In the event, and following an initially very successful re-entry, the vehicle broke apart at an altitude of around 65 km.

The orbital flight segment of the test was impressive whilst also raising questions as to Starship’s future orbital flight dynamics. Notably, throughout its half orbit of the Earth, the Starship was in a state of continuous “bbq roll”, that is, spinning around its longitudinal axis (and making it seem like the Earth was constantly looping around it on videos). Such rolls are not uncommon on space vehicles when in sunlight, as they help spread the thermal load of the Sun’s heat over the vehicle’s outer skin, preventing uneven heating (or overheating).

In this respect, Starship is especially vulnerable to such thermal stresses: it is completely reliant on cryogenic propellants which tend to revert to a gaseous state (and require venting to prevent tanks being over-stressed), and it is made of stainless steel, and extremely poor thermal insulator. This is compounded by the fact that the hull of the vehicle is also the the outer surface of the propellant tanks, so outside of the thermal protection system (TPS) tiles coating one side of the vehicle and designed to protect it during re-entry in to Earth’s atmosphere, there is next to no thermal insultation between the vehicle’s propellant reserved and the Sun, thus leaving rolling the vehicle as the simplest means of regulating internal temperatures.

Even so, the rate of roll, combined with its continuous does raise questions: was the rolling seen on this flight simply an overly precautious desire to limit thermal blooming inside the vehicle, or will it be part of starship SOP in the future. If the latter, then there are going to be some significant issues to address (how are to starships supposed to pump propellants being them in they have to roll like this once mated and the fuel to be transferred from one to the other is being exposed to a severe Coriolis effect as a result of the spin? Was the spin in this instance the cause of the planned de-orbit burn being cancelled because a smooth flow of propellants to the motor to be fired could not be guaranteed?

That said, the vehicle did perform its own mini “propellant transfer”, pumping a small amount of liquid oxygen between its own tanks. However, the overall value of this test is perhaps not as significant as some SpaceX fans have stated, given it is a long way short of the 100+ tonnes of propellants at a time that will need to be transferred between vehicles when it comes to sending the proposed Starship lunar lander to the Moon .

But leaving such thoughts aside, the one undoubted spectacular element in the flight were the initial phases of re-entry into the denser atmosphere, when cameras mounted on the vehicle’s control surfaces were able to video the build-up of super-heated plasma around the craft as it slammed into the atmosphere. While this has been filmed from within various space vehicles (Apollo, shuttle, etc.), this is the first time (I believe) it has ever been recorded from outside the vehicle going through re-entry.

Another unique element of the vehicle demonstrated prior to re-entry was the use of vented gas as a means of controlling the vehicle’s orientation. As noted above, cryogenic fuels tend to “boil off” and turn gaseous unless kept perfectly chilled. This gas must then be vented in order to prevent it becoming too voluminous and rupturing its containment tank (hence why rockets using cryogenic fuels are constantly venting gasses prior to launch following propellant loading & then having to be constantly “topped off”). However, rather than just letting go of this gas in space as they do on the ground, SpaceX channel it through a series of “cold thrusters” around the starship vehicle, enabling them to use the vented gas to “steer” the vehicle, avoiding the need for more traditional (and mass-using) thrusters systems requiring their own tanks of hypergolic propellants or gas.

While overall successful, the loss of both vehicles does mean a mishap investigation overseen by the FAA has been triggered, which may delay the planned launch of another test flight originally targeted for just a few weeks time. Even so, SpaceX are to be congratulated with the results overall, carrying the company as they do a modest step forward in the system’s development.

Continue reading “Space Sunday: starships, volcanoes and Voyagers” →