SpaceX Dragon advancing the Launch Abort System to new heights
SpaceX are continuing to make progress with their NASA commercial crew contract, with the latest milestone involving the firing of their SuperDraco thruster engine, an integrated element of the Dragon which will be used as its Launch Abort System (LAS). However, these engines are hoping to gain additional roles, including the ability to land Dragon propulsively on land.
Launch Abort System:
Historically, Launch Abort Systems (LAS) – or Launch Escape System (LES) – have appeared as towers, attached on top of the crew capsule, ready to “pull” the capsule – and its crew – away from a failing vehicle, be it at the launch pad, or during early ascent.
In the event of a nominal launch, the tower would be jettisoned midway through the ascent to orbit – at a point in time where a major issue would result in the capsule simply separating away for an abort – usually resulting in a splashdown.
These LAS towers can be seen on the early crewed launch vehicles, having first been tested in 1960 – when the “Beach Abort” practiced the abort technique on the first production Mercury capsule at NASA’s test facility at Wallops Island.
While Gemini used ejection seats, the towers became part of the Mercury and Apollo programs, even earning a place in Hollywood movies, such as when Tom Hanks – playing Commander Jim Lovell in the movie Apollo 13 – reached forward to manually jettison the tower during second stage flight during the film’s launch scene.
The early flights of the Space Shuttle only employed the ejection seat capability, as much as it was hinted using such a system – only available for a limited time during first stage ascent – would have provided the escaping astronauts very little chance of survival. The Shuttle mainly relied on abort scenarios involving the return of the crew with the orbiter.
Thanks to the strict safety record of crewed launch vehicles, the use of the LAS has only been called for once during an actual emergency, with the Russians.
Another abort – Soyuz 18A – aborted in flight, resulting in the crew landing safely near the Chinese border – however, it is believed this event came after LAS jettison, with the abort carried out by the Soyuz engines.
The clear use of the LAS during an abort event is a famous one – mainly due to the footage finding a large audience on youtube – as the crew of Soyuz T-10-1 underwent a pad abort, just seconds before their failing vehicle exploded on the launch pad.
The video shows a line of the Soviet top brass witnessing the dramatic abort, acknowledged only by one General calmly adjusting his collar.
It was reported that the crew landed safely, just four miles away.
LAS For Orion:
For the defunct Vision for Space Exploration (VSE) – a direct fallout of the Columbia disaster – crew safety for the next launch system was paramount, as NASA reverted back to a capsule design, with a full Launch Abort System.
In 2007, a major trade of several LAS concepts were evaluated by NASA managers, namely the Multiple External (x4) Service Module (SM) Abort Motor concept, the Crew Module Strap On Motors (x4) concept, and the In-Line Tandem Tractor (Tower) concept – the latter of which was baselined into Ares I/Orion design.
The trades request the preferred design should ensure the risk of losing the crew in an abort scenario would be no greater than 1:10, noting such a system is no guarantee for crew survival during an emergency.
The winning concept – the Tandem Tractor (Tower) LAS design – comprised of a Nose Cone, Attitude Control Motor (Eight Nozzles), Canard Section (Stowed Configuration), Jettison Motor (Four Aft, Scarfed Nozzles), Interstage, Abort Motor (Four Exposed, Reverse Flow Nozzles), Adapter Cone, and Boost Protective Cover (BPC).
The primary role of such a system is to save the crew during the ‘three stages’ of an abort, the first involving the firing of the spacecraft – in this case Orion – away from a failing vehicle on the launch pad to a safe distance, before deploying the parachutes on the Orion, for a landing in the Atlantic ocean within a 3450 ft radius due east of the launch pad.
The second stage of an abort is noted as “mid altitude” – which has a different characteristic when compared to pad abort. This stage of abort works for up to 150,000 ft, involving the LAS remaining on the vehicle after firing the Orion safely away from the failing vehicle until the point of drogue chute deployment, which becomes necessary at that altitude.
The final stage of abort, which would still require the LAS, is at an altitude of between 150,000 ft and 300,000 ft – the latter being the point the LAS would be jettisoned on a nominal flight. After being pulled away from the launch vehicle, Orion would revert to a flight profile similar to that used during the end of a normal mission.
During the evaluations, some engineers called for changes to the system. However, due to the mighty struggles of the time – involving Ares I’s mass to orbit ability, or lack thereof – the main focus turned to using the LAS, in the event of a successful launch, to still perform a firing, in order to assist Orion’s ride uphill.
“LAS Abort Impulse used for Ascent Assist: Theoretically can increase mass to orbit by 1000 lb. However, additional tension loading on the Command Module requires additional structure that leads to overall decrease in mass to orbit,” noted an extensive NASA presentation (acquired by L2 – Link to Document).
Managers also evaluated the use of nozzle inserts in the LAS motors, which would reduce the thrust and thus structural loadings on the vehicle. This option would mitigate any concerns of Command Module (CM) mass penalties.
‘An Alternate Option Using Nozzle Inserts: Reduces Abort Motor Thrust, Increases burn time. Relieves Command Module Compressive load – no tension loads. Increases the Payload Mass-to-Orbit by ~650 lb,’ added the presentation.
This option weakened as evaluations progressed, with the final note on such a use for the LAS pointing to only a 400lb mass increase.
This system has enjoyed one test launch, lofting a boilerplate Orion into the skies of White Sands, New Mexico, durng the successful PA-1 (Pad Abort 1) test in 2010.
While abort motors on the Service Module lost out during the trade studies, a new concept came forward called the Max Abort Launch System – or MLAS (named after Maxime (Max) Faget).
Although it was never publicly admitted, this system was often mentioned by sources as a potential solution towards a growing movement associated with cancelling Ares I and human rating the Ares V, as the Constellation Program (CxP) began to falter.
It also had the backing of then-NASA administrator Mike Griffin, which would not have come as a surprise, given MLAS was an evolution of two of the original three LAS concepts studied by Constellation, one of which made the LAS trade study in 2007 via a rather amusing hand-drawn sketch, created in 2006.
The MLAS concept combined the boost protection cover of the service module mounted escape system with the command module mounted motors, in turn reducing the overall height of the vehicle – something desired by the Ares V HR advocates, who were worried about being able to stack and rollout the vehicle – with a LAS tower – under the height restrictions of the Vehicle Assembly Building (VAB) doors.
The MLAS utilized a ‘bullet’ boost protection cover over the capsule to house four Mk 70 Terrier solid motors separation motors – as opposed to locating them on a tower above the capsule.
Two orientation parachutes are attached to the top of the fairing to re-orient the vehicle, with the blunt heat shield to aid in fairing separation.
The design resulted in the aborting vehicle re-orienting immediately after abort motor cut off during a pad abort, but would fly with its nose “into the wind” on a mid-altitude abort. The orientation parachutes would then activate quickly before the fairing separation.
In the event of a high altitude abort, the fairing would come off immediately, in order to allow the Command Module Reaction Control System (RCS) to stabilize the vehicle for entry.
The design of MLAS changed several times during its development, gaining fins for stability during later cycles, becoming more in line with another hand drawn sketch.
This time the artist was former Constellation head Scott “Doc” Horowitz – as seen in the second of two MLAS presentations acquired by L2 (Link to Presentations) – over a year after Mr Griffin’s conceptual design.
The final version of the MLAS flight test vehicle weighed in at over 45,000 lbs and was over 33 feet tall – and this vehicle actually got to fly for real, after being shipped to Wallops for its one and only hop off the ground.
The pad abort test proper began seven seconds after burnout of some specially attached solid motors, as the vehicle rose into the Virginia morning sky at 6:25am local time on July 8, 2009.
Video of the launch showed a perfect test, as the vehicle rose on a stable flight path, before reorientation and further stabilization, followed by crew module simulator separation from the MLAS fairing, and parachute recovery of the crew module simulator.
Other tests were planned for MLAS, including a high altitude abort – which will involve the fairing being released immediately after abort is called, in order to allow the Command Module Reaction Control System (RCS) to stabilize the vehicle for entry. However, the program was put on the backburner, as the Constellation Program found itself cancelled.
At this time, the Space Launch System (SLS) will launch Orion with the previously chosen Line Tandem Tractor (Tower) design as its LAS.
Despite being late to the game, when compared to the Constellation development path, SpaceX still came up with an arguably superior system for use with their Dragon spacecraft, a system not only fully integrated into the body of the spacecraft, but one that also holds future uses, through to those that aren’t even related to launch abort.
The first major difference relates to the traditional use of solid propellant, mainly because of the speed it can ignite and reach full thrust – something highly desirable when moving human lives away from a failing rocket.
However, Dragon sports a series of eight liquid SuperDraco engines, built into the side walls of the Dragon spacecraft, capable of producing up to 120,000 pounds of axial thrust to drive the Dragon away from its failing launch vehicle.
SpaceX are deep into developing the engines – just nine months after their Commercial Crew Development (CCDev) contract noted the LAS is required – with the latest test fire taking place at the company’s Rocket Development Facility in McGregor, Texas.
Referencing back to the benefit of solid motor abort systems, SpaceX’s SuperDraco produced full thrust within approximately 100 milliseconds of the ignition command. It also fired for five seconds, which is the same amount of time the engines would burn during an emergency abort.
Advantages of the SuperDraco liquid thruster include how the engine can be put through a series of throttling ranges, in turn allowing for redundancy, with SpaceX claiming they could lose one of the eight abort engines and still recover the vehicle and crew successfully. The engines can also be restarted multiple times.
Another advantage is the fact it’s not a tower. As noted previously, the LAS tower normally requires jettison shortly after first stage flight. Any failure of this key sequence of ascent would end the mission, given the flight profile wouldn’t be designed for carrying the LAS along for the ride.
Because the system is integrated into the Dragon itself – as opposed to departing the spacecraft during jettison – the spacecraft can technically abort within much longer periods than the tower version. With Dragon returning with the engines on board, they can also be reused on future launches.
There is also a large amount of commonality between the 18 maneuvering engines built into Dragon and the SuperDraco LAS engines – bar the fact the SuperDraco engines would burn through propellant 200 times faster.
The biggest long-term advantage of this system is related to the potential use of the engines to land Dragon back on land propulsively, as seen via SpaceX’s Reusable Falcon 9 concept, which returns all of the launch vehicle and spacecraft hardware to the ground for reuse.
Parachutes would still be onboard the Dragon, for a contingency event resulting in problems with the SuperDracos, allowing the spacecraft to land on water, as it is currently designed to do.
However, Earth isn’t the only landing destination for Dragon, with SpaceX holding ambitions of landing on the Moon and more notably Mars. Nicknamed “Red Dragon” – SpaceX have made no secret about heading to Mars, even publishing a graphic of their spacecraft touching down on the Red Planet.
Such a mission is deep into the future, although Elon Musk, SpaceX CEO and Chief Technology Officer, included the full range of use for the SuperDraco’s when announcing his pleasure with the recent test firings.
“SuperDraco engines represent the best of cutting edge technology,” Mr Musk noted. “These engines will power a revolutionary launch escape system that will make Dragon the safest spacecraft in history and enable it to land propulsively on Earth or another planet with pinpoint accuracy.”
NASA’s own Mars plans are a mix of old mission outlines and revamped videos, but do show they also have propulsive landing ambitions – in tandem with large parachutes – with the latest conceptual Mars mission videos showing massive cargo landers and crew habitats, with the crew riding down to the surface on board the hab lander, touching down under large amounts of propulsive power.
Such missions are likely to be in the 2030s at the earliest, with the main focus for the entire US space program being the urgency to regain their own domestic crew launch capability, following the retirement of the Space Shuttle.
The successful development path of the SuperDraco engine has literally pushed SpaceX one step further down the road for NASA and the United States to achieve independence from purchasing seats on the Russian Soyuz – a vehicle which still uses the same tower LAS that caused one Soviet General to check if his collar was straight.
(Images via NASA, SpaceX, and L2 content)
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