Falcon Heavy prepares for debut flight as Musk urges caution on expectations

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Developing rockets is difficult – even when those rockets use existing rocket boosters.  Such is the case for SpaceX and the development of the Falcon Heavy.  Once operational, Falcon Heavy will be the most powerful rocket in the world.  While the path to its inaugural mission has been challenging, Elon Musk is urging caution surrounding expectations of the rocket’s first flight, which is expected later this year from LC-39A at the Kennedy Space Center, Florida.

Challenges to Falcon Heavy’s debut – Musk asks for realistic expectations:

The long wait is nearly over as SpaceX readies for the final series of upgrades/modifications to LC-39A at the Kennedy Space Center for the upcoming debut of the company’s heralded Falcon Heavy rocket.

Designed as a souped up version of the Falcon 9 – which has enjoyed a great deal of fame and attention this year with the first (and now second) reflights of previously-flown core stages as well as lofting the first reused Dragon capsule on the CRS-11 resupply mission to the ISS – Falcon Heavy’s initial design seemed simple.

But as Elon Musk stated at the keynote to the ISSR&D (International Space Station Research and Development) conference on Wednesday, “it ended up being way harder to do Falcon Heavy than we thought.

“At first it sounds really easy to just stick two first stages on as strap-on side boosters.  But then everything changes.”

Mr. Musk admitted, “We were pretty naive about that.”

To the specifics of the design challenges surrounding Falcon Heavy, strapping three Falcon 9 core stages together is tougher than it sounds.

Once a rocket has three cores working in tandem, the aerodynamics, structural loads, the Max Q envelope (period of maximum mechanical stress on the vehicle during ascent), and the subsonic to transonic transition region (where the vehicle accelerates through the speed of sound) are all greatly changed by the fact that Falcon Heavy is powered by 27 Merlin 1D orbital class engines.

Moreover, the tripling of the engines from nine to 27 creates a tripling of the vibrations and acoustics at play throughout the vehicle.

All of these elements, according to Mr. Musk, required a complete redesign of the central-core’s airframe.

“The amount of load you’re putting through that center core is crazy because you have two super powerful boosters also shoving that center core,” stated Mr. Musk.

“So [we broke] the qualification levels on so much of the hardware [that] we had to redesign the whole center-core airframe on the Falcon 9 because it’s going to take so much load.

“And then you’ve got the separation systems… and, yeah, it just ended up being way, way more difficult than we originally thought.”

In short, there are a lot of risks associated with the Falcon Heavy – as there are with all rockets.  Especially those making their first integrated flight.

In addition to the in-flight environment challenges posed to the Falcon Heavy, there is the ignition and initial climb out environment to consider as well.

For the first time in U.S. rocketry, 27 engines will be used together during first stage flight – the most engines ever for a U.S. rocket’s first stage.

The 27 first stage engines of the Falcon Heavy fall three engines short of the record for most engines used on a single rocket’s first stage – with the Soviet N1 rocket beating Falcon Heavy with 30 NK-15 engines for its first stage.

Those 30 NK-15 engines combined to create the most-powerful rocket first stage ever developed, with an impressive 10.2 million lbf at liftoff.

Falcon Heavy’s 27 engines will produce a maximum of 5.13 million lbf at liftoff, increasing to 5.549 million lbf as the vehicle ascends into vacuum.

But before Falcon Heavy can ascend, all of its engines need to light.

And lighting those 27 engines simultaneously poses a significant risk, with Mr. Musk admitting “There’s a lot that could go wrong there.”

While SpaceX has ample experience lighting all nine core stage engines of the Falcon 9 simultaneously, with every Falcon 9 going through a full duration hot fire at McGregor followed by a static fire on the launch pad before all nine engines are lit a third time for launch, no company in the U.S. rocket industry has experience in lighting 27 engines at the same time.

Part of the issue is that while each of the three Falcon Heavy cores – two flight-proven cores for the side-mounted boosters and a new core for the central core – has undergone hot fire testing at McGregor, they were all fired separately as the Texas test site is not built to accommodate three cores at the same time.

This means that the company will not gain a full understanding of how all 27 engines lighting at the same time will work in reality until the more-crucial-than-usual static fire event at LC-39A at Kennedy.

To this end, Mr. Musk has hinted on Twitter that SpaceX might conduct multiple static fires of the first Falcon Heavy at Pad-A ahead of its first launch campaign in an effort to gather as much data as possible about the simultaneous 27 engine ignition process and the stresses and vibrations it sends through the vehicle.

Importantly, the term “simultaneous ignition” is a basic description and an easy way to quickly understand the practicality of the process rather than the reality of what will actually happen.

Presently, NASASpaceflight.com understands that the Falcon Heavy’s engine start sequence will be staggered – like main engine start was staggered on Shuttle.

For the Space Shuttle, each of the three main engines started 120 milliseconds after the previous; however, to anyone viewing the engine start sequence in real-time, the engines appeared to light simultaneously.

It is now understood that a similar process will be employed with Falcon Heavy, with two engines being lit simultaneously followed by the next two… and so on until all 27 are up and running.

The time between each set of engine start commands will be quite short – giving the appearance of a simultaneous ignition.

Regardless, the staggered start will prevent a potential thrust torque (a thrust-induced rotation) scenario that could destroy or severely damage the octawebs at the base of each Falcon core that hold the Merlin 1D engines in place.

With all of these elements in consideration, Mr. Musk is urging caution regarding public expectation for Falcon Heavy’s first flight, saying that there is a “real good chance that the first vehicle [won’t] make it to orbit.  So I want to make sure to set expectations accordingly.”

Even more telling was how Musk continued on this point, stating that he hoped Falcon Heavy makes it far enough away from LC-39A before failing so the pad will escape significant damage.

“I hope it makes it far enough away from the pad that it’s not going to cause damage.  I would consider that a win, honestly,” said Mr. Musk.

Regardless, Mr. Musk encourages people to travel to Florida to see the first Falcon Heavy launch.  “It’s guaranteed to be exciting.  I think Falcon Heavy is going to be a great vehicle.  There’s just a lot that’s impossible to test on the ground.  

“But we’ll do our best.”

In addition to the technical and engineering challenges posed to Falcon Heavy from a design standpoint, there is still quite a bit of work needed to finalize LC-39A for its role in hosting Falcon Heavy.

Presently, 39A requires an additional 60 days of work to completely prepare the pad’s GSE (Ground Support Equipment), launch mount, and TEL (Transporter/Erector/Launched) for Falcon Heavy.

A majority of that work will not begin until SpaceX completes repairs and reactivation work at SLC-40 on the Cape-side of Florida’s spaceport.

Presently, SpaceX has not made any public change to the stated “August” completion date for SLC-40.

Therefore, assuming a late-August reactivation of SLC-40, that would place Falcon Heavy’s first flight in the November-December timeframe.

While not a requirement for Falcon Heavy flights, it now appears that the RSS (Rotating Service Structure) will be completely gone from LC-39A at the time of Falcon Heavy’s debut.

Workers at Pad-A have taken serious advantage of the Range-imposed downtime this month to make significant progress on the demolition of the RSS, which is noted to be coming down at a rapid pace.

So fast is the removal process proceeding that the RSS could potentially be mostly gone by the time Falcon 9 flights resume from the Range down period on 10 August with the CRS-12 launch to the ISS.

(Images: SpaceX, L2 imagery, and L2 artist Nathan Koga – The full gallery of Nathan’s (Falcon Heavy to Dragon to Starliner, MCT, SLS, Commercial Crew and more) L2 images can be *found here*))

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