Space Sunday: SpaceX – balloons, bouncy castles and rockets

The Falcon 9 carrying TESS lifts-off from Launch Complex 40, Canaveral Air Force Station, Florida, on Wednesday, April 18th

After a two-day delay, NASA’s Transiting Exoplanet Survey Satellite (TESS) launched from Cape Canaveral Air Force  Station atop a SpaceX Falcon 9 booster on Wednesday, April 18th.

As I previewed in my previous Space Sunday report, TESS is designed to seek out exoplanets using the transit method of observation – looking for dips in the brightness of stars which might indicate the passage of an orbiting planet between the star and the telescope. Once in its assigned orbit and operational, TESS will work alongside the Kepler space observatory – now sadly nearing the end of its operational life, and eventually the James Webb Space Telescope – in seeking worlds beyond our own solar system.

It will be another 56 days before TESS has reached its unique orbit, a “2:1 lunar resonant orbit“, which will allow the craft to remain balanced within the gravitational effects of the Moon and Earth, thus providing a stable orbital regime which should last for decades. However, the launch was perfect after issues with the Falcon 9’s navigation systems prompted the initial launch attempt on Monday, April 16th. Once it had lifted the upper stage and its tiny payload – TESS is just 365 kg in mass and about the size of an upright fridge / freezer combination – the Falcon 9’s first stage completed a successful burn back manoeuvre and made a successful at-sea landing on the SpaceX Autonomous Drone Ship Of Course I Still Love You, waiting some 300 kilometres off the Florida coast.

The Block 4 Falcon 9 first stage captures an image of the autonomous Drone Ship Of Course I Still Love You just 3 seconds from touch-down. Credit: SpaceX live stream

The second stage of the rocket placed TESS into an initial 250 km circular orbit about the Earth before shutting its motor down for a 35-minute cruise period which correctly positioned the vehicle to allow the engine to be re-lit and send TESS on its way towards a 273,000 km apogee orbit. Over  the next several weeks, the instruments aboard TESS will be powered-up and calibrated, including the four cameras it will use to imaged the stars around us in an attempt to locate planets orbiting them.

The first exoplanet – the ” hot Jupiter” 51 Pegasi B, unofficially dubbed Bellerophon, later named Dimidium and some 50 light years away –  was discovered in 1995. In the 23 years since that event, some 3,708 confirmed planets (at the time of writing) have been found, with a list of several thousand more awaiting verification. Most of these have been discovered by using the transit method, with the vast majority by the Kepler space observatory. Such are the capabilities of TESS, it could double this count during its whole-sky survey, the first phase of which will last two years.

The count of confirmed exoplanets over the past 23 years. The sharp rise in 2016 is as a result of extensive follow-ups to observations made by the Kepler observatory in the K2 phase of its mission. Credit: NASA

TESS’s primary mission is scheduled to last two years – but it orbit means it could study the skies around us for decades, seeking out planets amount the 200,000 stars that are the nearest to us.

SpaceX: Party Balloons and Bouncy Castles?

Elon Musk loves to tease. He’s also generally in earnest when discussion space flight. Sometimes the two things combine in unusual ways. Take a trio of tweets he sent on April 16th, 2018, for example:

This is gonna sound crazy, but … SpaceX will try to bring rocket upper stage back from orbital velocity using a giant party balloon. And then land on a bouncy house.

Elon Musk’s trio of tweets, April 16th, 2018

Precisely what he meant has been the subject of much Twitter debate and theorising in various space-related blogs, but the CEO of SpaceX is now keeping mum on the subject; most likely enjoying the feedback and making plans.

SpaceX has serious ambitions to make their launch vehicles pretty much fully reusable. As we already know, the company has pretty much perfected the successful landing, refurbishment and re-use of Falcon 9 first stages (also used in triplicate on their Falcon Heavy booster), and plan to use the same approach with their upcoming BFR – standing for Big Falcon (or at least, a word that sounds close to “Falcon” but with a cruder meaning) Rocket – formerly, the Interplanetary Transport System.

To date, SpaceX has successfully recovered 24 Falcon 9 first stages, with almost half of those recovered now refurbished and either re-flown, or awaiting re-use. But the first stage – which does all the heavy lifting, is perhaps the “easiest” element of the vehicle to recover. It does not achieve orbital velocity (around 7,820 metres per second, or 17,500 mph), but instead tends to reach a peak velocity of around 1,716 metres per second (roughly 3,800 mph or Mach 5).

While this is still enough to generate a significant amount of heat and cause a first stage to break-up / burn-up in an uncontrolled descent, it is “slow” enough to avoid the need for extensive (and heavy) shielding to protect against the friction heat of passage back into the denser part of Earth’s atmosphere, providing the stage can be oriented correctly so three out of its set of nine motors can be re-lit. The exhaust plume from these forces the atmospheric compression generated by the rocket’s penetration of the upper layers of the denser part of the atmosphere (and which actually generates the associated re-entry heat), to occur away from the rocket, so the need for additional heat shielding is avoided.

However. recovering the upper stage of the rocket is altogether a different proposition. This does reach orbital velocity, and so finding a way in which it can be safely recovered without relying on expensive and heavy heat shielding which would both increase launch costs and reduce the payload carrying capabilities of both the Falcon 9 and the Falcon Heavy is a doozy of a problem. So much so, that SpaceX have twice cancelled attempts to make the rocket’s upper stage recoverable – and as recently as late 2017, it was believed further attempts at trying to get the stage to a point where it could be recoverable had been abandoned in favour of focusing on the BFR’s massive upper space ship stage – which as a crew / passenger carrying vehicle needs to be able to make safe landings.

So what do Musk’s tweets mean? how could a balloon be used to slow a vehicle and help it through the searing heat of orbital re-entry (where the heat load is around 27 times hotter than the heat experienced by the first stage)? The most likely explanation is that SpaceX are exploring the potential of using a ballute – a portmanteau of balloon and parachute – with the upper stage.

First developed in 1958, ballutes have most commonly been employed as a speed retarding device to slow bombs released by aircraft at supersonic speeds to slow their passage through air, allowing the delivery aircraft to safely clear the potential blast area created by multiple ground impacts. However, ballutes also have a use in space flight: they offer a means to both massively increase the surface area of a re-entry vehicle, helping to slow it during entry into an atmosphere, and can be stored relatively compactly and with minimal mass overheads until they are required.

The Gemini personal escape system used a ballute, as shown in the training documentation (left) and seen is testing (right). Credits: NASA

NASA has in fact worked with ballutes for decades. During the Gemini Programme in the 1960s, for example, a ballute formed a part of the astronaut escape system, allowing an astronaut to be ejected from the vehicle at extremely high altitude, an allow him to descend to lower altitudes where a conventional parachute could be used.  More recently, a ballute system called the Low-Density Supersonic Decelerator (LDSD) was experimented with to see it if could help decelerate lander vehicles in their descent to the surface of Mars.

In this latter regard, it is the size of the ballute that is important. Attached to a Falcon 9 upper stage, it could be inflated via an inert gas at a point well above the denser part of the Earth’s atmosphere. It would present a relatively flat surface area large enough to interact with the very tenuous upper reaches of the atmosphere to act as an aerobrake, slowing the upper stage’s decent relatively rapidly, whilst it broad surface area would easily allow any generated heat to dissipate. Thus, the upper stage would enter the much denser parts of the atmosphere at a much lower velocity than otherwise would be the case; in turn this would generate less heat, which again could be dissipated over the large surface are of the balloon. An analogy might be that using a ballute converts a returning upper stage from an anvil falling into the denser atmosphere in to blanket of feathers.

Just how big a surface area might this be? Some rough calculations made by Dave Akin, an aerospace engineer at the University of Maryland, who is overseeing undergraduate research into the use of balloon-like systems for re-entry vehicles, estimates a Falcon upper stage would require a ballute roughly  37 metres (120 ft) across – a not impossible size, if admittedly challenging; hence the party balloon reference(?).

There are problems, however. Slow a vehicle too much as it drops into the Earth’s atmosphere, and its centre of mass could become a problem, destabilising its descent. this is the kind of problem Quinn Kupec, the undergraduate being supervised by Dave Akin at the University of Maryland, is already investigating, along with other options for using balloon-base deceleration. He’s offered his research to Musk and SpaceX – which drew an enthusiastic response from Musk.

And the bouncy castle – or bouncy house, as Musk called it? That remains a mystery, but likely refers to some exotic landing pad for the returning stage. Unlike a returning first stage of a Falcon rocket, the second stage is unlikely to have the fuel reserves needed to make a “soft” landing – and adding any kind of landing gear would again decrease the payload carrying capability. And even though a parachute system could be used to slow the descent through the denser atmosphere post re-entry, things slung under parachutes still tend to hit the ground at a fair speed, potentially damaging an upper stage beyond the point of repair and re-use. Thus, some kind of mobile, inflatable “landing pad” might be the answer, one which can be transported to a landing site and inflated to await the arrival of a returning upper stage.

And if that sounds crazy, just remember – SpaceX is already using a converted high-speed passenger boat called Mr Steven, equipped with a huge net over its stern deck to try to “catch” the other important elements used a in rocket launch: the payload fairings.

Sea trials: the 62 m (205 ft) long Mr. Steven, leased by SpaceX and converted to “catch” payload fairings in the huge net suspended over the stern deck. Credit: Teslarati / SpaceX / Sea Tran

So Just How Big Will the BFR Be?

When it was conceived, the SpaceX BFR was set to be the largest vehicle ever built for space flight. As the Interplanetary Transport System, it would have been some 122 metres tall (compared to the Saturn V’s 111) and have a 12 or 15 metre diameter core (compared to the Saturn V’s 10 metres) with an all-up lift-off mass of 10,500 tonnes (compared to the Saturn V’s 3,040). Since then, the system has been revised, reducing its overall height and diameter to 106 metres and 9 metres respectively.

But even though it has been reduced in scale, a move that both permits it to be more economical and attractive to potential users outside of Musk’s ambitions to colonise Mars and which means it will not require dedicated launch facilities to be constructed for it, BFR is still impressively big. But just how big?

 

The BFR tool rig which will be used to produce segments of the BFR core – note the Tesla car to give a sense of scale. Credit: SpaceX

On April 9th, 2018, Musk issued an image that give a clue. This shows the core rig tool, a huge cylinder that will be used to create the outer segments of the rocket’s hull. It’s essentially an inside-out oven. Flexible resin sheets of carbon fibre will be layered on the tool, which will then be heated, curing them and bonding them together to form a solid ring of rocket fuselage. These sections can then be stacked together to form the main body of the rocket. 

BFR will be a two-stage vehicle the core first stage and the combined upper stage / space ship. This has also been scaled down slightly in terms of diameter, but is still an impressive vehicle, as shown in the to-scale image below, comparing it to a space shuttle orbiter vehicle.

The original ITS upper stage space ship (l), the current BFR variant (c) and a space shuttle orbiter for scale (r). Credit: unknown

Once operational, the BFR will replace both the Falcon 9 and the Falcon Heavy as the mainstay of SpaceX’s commercial launch activities. It will be constructed in a new facility SpaceX is building at the Port of Los Angeles, at an 18-acre site at Berth 240, a location the company has leased for 10 years, with options on two 10 year extensions. Until that facility comes on-line, the spaceship portion of the craft is expected to be fabricated at the company’s existing facilities at Brownsville, Texas, and may even be tested ahead of the rocket core.

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