James Webb: Find out how Ariane 5 was prepared and adapted for the largest telescope ever to go into space


Residual pressure inside the fairing, need to protect the telescope mirrors, upper stage: find out how the Ariane 5 launcher was prepared and adapted to launch the James-Webb observatory. Technical explanations from Daniel de Chambure, responsible at ESA for the development of Ariane 5 and adaptations for specific missions.

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At the end of the final mission analysis review, which is part of a work started over three years ago to prepare for the launch of the James-Webb Observatory (JWST) aboard a Ariane 5, the European Space Agency has confirmed «  the feasibility of the mission but also the qualification of JWST for the Ariane 5 atmosphere », Explains Daniel de Chambure, responsible forTHAT development of Ariane 5 and adaptations for specific missions. In other words, the Ariane 5-JWST set is good for flight!

This final mission analysis review was made  » necessary by the specificity of the mission and the JWST, the largest telescope never launched into space « . If this is not the first time thatAriane 5 launches a payload bound for Lagrange point 2 – it had launched the satellites Herschel et Planck in May 2009 – is the  » first time the launcher must take into account very restrictive parameters « . Of course, like all the payloads on board the Ariane 5, the  » JWST was forced to comply with the user manual ofAriane 5, but several critical points required to adapt Ariane 5 « . This is the case with the residual pressure inside the coping and the obligation to protect the mirrors telescope as well as some equipment sensitive to Soleil.

The separation of the JWST will take place at an altitude of approximately 1,400 kilometers

For this mission, the trajectory of Ariane 5 was adapted to the flight profile of the JWST. If the atmospheric flight phases are  » similar to a GTO launch, with the separation from the fairing to a slightly higher altitude, plus or minus 115 kilometers, the ballistic phase is very different « . After the engine of the upper stage has been switched off, the separation of the JWST will take place at an altitude of approximately 1,400 kilometers against generally an altitude of around 1,000 kilometers for GTO. The upper floor therefore goes  » provide a stronger acceleration, so as to give sufficient impetus for the observatory to rejoin its orbit around point L2, 1.5 million kilometers from Earth, which will take three weeks « . The uptime of the upper stage is practically the same as for a GTO but «  due to the lighter payload, 6.2 tons, the acceleration is much stronger « . Although the JWST hasergols, NASA does not want to use them to join L2. They are intended for attitude control of JWST in its orbit; the  » consumption of propellants for other tasks would reduce the life of the JWST », Estimated at more or less 10 years.

Oscillatory roll to protect the JWST from solar radiation

To avoid damaging certain equipment and also the telescope mirror divided into 18 hexagonal segments, NASA wanted it to be  » exposed as little as possible to light of the Sun after separation of the coping and in particular without fixed illumination, so as to avoid any risk of a hot spot inside certain external cavities of the satellite ”.

To overcome this unprecedented and strong constraint, the ESA and ArianeGroup have taken several measures, including a rather impressive one which consists of  » control the roll of Ariane 5 to avoid as much as possible directly exposing the segments of the JWST mirror facing the Sun! « To check the attitude of the launcher in relation to the Sun, the teams from ESA andAG have developed  » algorithms and control and roll and attitude laws so that the satellite always has the same face oriented towards the Sun with movements oscillatory to avoid hot spots « . In addition, if usually a tolerance of a few tens of degrees is applied,  » it is restricted to less than 10 small degrees for the JWST « . Another consequence,  » the launch time has been fixed at around noon, time of Kourou. Ariane 5 will take off around noon with the Sun on its nose, so that at the time of the separation of the James-Webb the Sun is found behind the launcher ».

22 meters long and 10 meters wide, the heat shield is almost the size of a tennis court!

To this strong constraint, another is added:  » the residual pressure inside the coping must be controlled « . This other request from NASA is explained by the complexity of the heat shield. 22 meters long and 10 meters wide, it is almost as big as a tennis court! Due to its size, the shield, made of five layers, will be launched folded and unfold into space. NASA fears  » that at the time of depressurization of the cover, before its separation, pockets ofair remain trapped inside the layers of the heat shield, which could damage it by causing tears during deployment « . To reduce this risk, the 28 vents have been  » modified all around the casing of the launcher « . Equipped with valves, they  » release pressure when the cuff is overpressure « . To verify the proper functioning of these vents, «  several tests were successfully carried out during previous Ariane 5 flights ».

Finally, to avoid contaminating the mirrors during the various preparation procedures for launching and boarding the launcher, a  » atmosphere ISO 7 is required at all stages of this preparation, from arrival to CSG when it was installed on board Ariane 5 « . And these pollution problems don’t stop with take-off! To avoid unnecessary clutter low orbit, he was  » decided that the upper stage will be sent beyond the Lagrange 2 point « . To avoid any risk of pollution and collision with the launcher, the maneuvers of «  separation and distance of the JWST from the upper floor will be more than two hundred meters « . A greater distance than usual. In particular, this will allow the tanks to be purged without risking damage to the JWST. This new maneuver was used during the last Ariane 5 flights. But instead of sending the stage to L2, it was used for GTO flights (more classic for Ariane 5) at  » decrease the perigee of its orbit so as to accelerate its fallout into theatmosphere and comply with the 25-year rule which stipulates that any object in low orbit enters theatmosphere before a quarter of a century ».

A launch date still uncertain

It seems certain that the launch date of October 31 will be difficult to keep and that we can expect an additional delay, probably of the order of a few weeks, or even two months at most. Arianespace refuses to give a timetable for the next Ariane 5 launches, indicating nevertheless that two flights are planned before the launch of the JWST. The first of these two flights is scheduled for the end of July. Usually, a period of three to four weeks is required between two flights.

However, unlike planetary missions, such as those to Mars, it is not necessary to launch the JWST within a window firing that would only open a few days or weeks a year (every 26 months for Mars for example). To launch the space observatory in the direction of its position, at Lagrange point number 2, 1.5 million kilometers from Earth, Arianespace has almost daily launch opportunities. Missing the date of October 31 is therefore not a problem for the program, especially since it was almost 15 years behind the initial schedule – the JWST was to be launched in 2007 -, we are no longer a few weeks and months.

Technical problems on Ariane 5 could postpone the launch of the James-Webb space telescope

Article by Remy Decourt published on 05/18/2021

While we thought the schedule for the James-Webb space observatory was frozen, with a launch scheduled for October 31, the Ariane 5 launcher might not be ready on that date! Because of anomalies encountered during the two previous missions which forced Arianespace to suspend flights.

During the February and August 2020 launches, the caps of the Ariane 5 launcher did not separate nominally, causing vibrations above allowable limits. If this fault was without incidence on the satellites deployed in orbit, ArianeGroup, Ruag (which builds the fairings) and Arianespace take this problem very seriously. Since this last launch, Ariane 5 has been immobilized on the ground for the duration of the investigations. To date, no return flight date has been communicated. Arianespace declined to give a timetable for the next Ariane 5 launches, nevertheless indicating that two flights were planned before the launch of theobservatoire spatial James-Webb (JWST).

In anticipation of the launch of the James-Webb, this « problem » is obviously a matter of concern and among the work underway at Ruag to understand and correct these anomalies, it is also being assessed what effects an abnormally high level of vibration could have on the body. beautiful mechanics of the space observatory and future missions. You should know that due to the size of the primary mirror of the JWST (6.5 meters), it could not be built in one piece because no launcher in service can carry such a large mirror. This is why it was decided to design a folding mirror in three sections that would be deployed once the telescope arrived in space! Excessive vibrations could damage the deployment mechanism or even render it inoperative.

The JWST is no longer a few weeks or months late

It therefore seems certain that the launch date of October 31 will be difficult to keep and that we can expect an additional delay, probably of the order of a few weeks to a few months maximum.

Unlike planetary missions, such as those to Mars, it is not necessary to launch the JWST inside a firing window that would open only a few days or weeks per year (every 26 months for Mars for example). To launch the space observatory in the direction of its position, at Lagrange point number 2, 1.5 million kilometers from Earth, Arianespace has almost daily launch opportunities. Missing the date of October 31 is therefore not a problem for the program, especially since it was almost 15 years behind the initial schedule – the JWST was to be launched in 2007 -, we are no longer a few weeks and months.

The current situation is ironic because, for much of the development of JWST, Ariane 5 was viewed as the least risky aspect of the program, which is nearing $ 10 billion in cost.

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