Space Sunday: SpaceX, NASA and interstellar visitors

SpaceX Starbase, Boca Chica, September 7th, 2021: to the left, Booster 4 stands on the launch table, the launch support tower standing over it. To the top right is Starship 20 sitting on sub-orbital pad B, with the lower half of Booster 3 (the upper tank section of which was cut off and removed in August. Credit: RGV Aerial Photography

SpaceX is continuing to move towards a first test flight of its Starship / Super Heavy launcher combination with the return of Booster 4 to the orbital launch facilities – although there is still some way still to go before an actual launch attempt will be made.

Following the test stacking of Booster 4 and Starship 20 on the launch table back in August (see Space Sunday: the Ups and Downs of Space Vehicle Development), Booster 4 was rolled back to the production facilities at the company’s Starbase centre at Boca Chica, Texas, to undergo a number of revisions.

Chief among these has been modification to the vent valve system, nominally used to allow excesses gaseous oxygen and methane to be vented from the rocket’s tanks as it naturally “boils off” due to temperature differentials the vehicle experiences when fuelled ahead of a launch. In particular, the vents for the booster’s lower tank now have covers that direct any gas downwards along the rocket’s body, and the vents for the upper tank focre the gas outwards and away from the rocket.

Booster 4 re-departs the production facilities at Starbase to drive the 1.5 km down the road to the launch facilities Credit: StarshipGazer.com

This suggests that SpaceX plan to use the release of gas from the tanks as a means to help control the orientation of the rocket during its descent back through the atmosphere in a manner similar to a more traditional reaction control system (RCS). If this proves to be successful, it means SpaceX have further reduced Super Heavy’s mass by avoiding the need for separate RCS systems and their tankage.

Another issue with rockets is that as the fuel tanks empty they lose internal pressure, and this can interrupt the steady flow of propellants to the engines. To prevent this, most launch systems utilise a reserve of helium that can be fed into the tanks as the propellants are burnt, maintaining the necessary tank pressure. To remove the mass created by a helium system, SpaceX have opted to use the rarer option of autogenous pressurisation. This draws a small flow of heated propellants before they reach the engines, and feeds this flow – in gaseous form – back up the outside of the rocket via dedicated pipes to be returned to the fuel tanks to re-pressure them.

The new vent systems and the piping of the autogenous pressurisation feeds where clearly visible as Booster 4 was rolled back to the orbital launch facilities on Tuesday, September 7th, and hoisted back onto the launch table, with the speculation iit may remain there until the actual launch attempt.

Two views of Booster 4 showing the revised excess gas vents from the top of the lower tank tank and the autogenous pressurisation feed pipes, Also visible is the black mass of the QD Arm. Credit: What About It

When this will be is unclear; the operation to hoist the booster into position showed the launch table itself is still being completed, being wrapped in scaffolding. It’s also not clear how much of the necessary propellant and electrical feeds have been installed in the launch support tower – although the Quick Disconnect (QD) arm that actually feeds propellants into the Starship vehicle and provide it and the booster with electrical power has been installed (with further additions to come). Similarly, the actual tank farm that will supply consumables – water, propellants, etc., – to the pad to enable launches.

Even so, SpaceX CEO Elon Musk has suggested an initial static fire test with Booster 4 could come within the next week. Even if the majority of the required plumbing, etc., is in place, this seems possibly ambitious,  given that such a test will likely only come after at least one each of cryogenic propellant loading / pressurisation tests which will take pre-and post test checks.

How many static fire tests might be run is unclear; its unlikely that SpaceX will want to fire all 29 engines in the first test but will likely build up to it – perhaps starting with the three motors at the centre of vehicle, followed by a firing of all nine of the middle engines before progressing to firing all 29 engines. And it should be remembered any of these tests, from pressurisation through the engine firings, could result in the rocket sustaining damage or even being completely destroyed.

Booster 4 being gently lowered into the launch table ring mount at the Starbase orbital launch pad. Notes the amount of construction scaffolding still in place. Credit: Nic Ansuini / NASASpaceflight.com

After the August stack test, Starship 20 was moved from the the orbital launch pad to sub-orbital launch pad B, where it has been undergoing an extensive examination of its thermal protection system (TPS) designed to protect it during entry into the atmosphere. The tiles on this system appear to have suffered more than the anticipated amount of stress / damage due to it being lifted up onto the booster by its snout in order to be stacked on the booster, requiring a lot of them to be replaced and others refitted / re-aligned. This work is now drawing to a close, but does point to a need for the tile system to be more robust if rapid turnarounds of Starships is to be achieved in the future.

Most recently, the vehicle has been receiving the six Raptor motors that will power it. This has sparked speculation that once this work is complete, Starship 20 could be ready to start its cryogenic and fuel pressurisations tests ahead of static firing test – again, possibly the inner three first, then all six.

How it started and how it is going: two shots indicating the number of Starship 20 heat shield tiles that needed to be completely replaced (red tags) or which required refitting / realigning (green tags) following the operation to stack and remove the vehicle on its booster in August. Credit: NASASpaceflight.com

A final element key to any launch attempt (and the full booster static fire test) is the granting of permission and a licence by the Federal Aviation Administration, which appears to be rightly determined not to be rushed into giving the OK whilst it is still conducting an extensive review of the Starbase facilities and their overall suitability for Super Heavy / Starship launches  in the event of an accident (particularly after the airborne explosion of Starship SN11in march 2021 resulted in debris falling to earth 8 km from the SpaceX facilities and close to a populated area).

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