So hot it can boil iron, so noisy it can be heard 60 miles away,
and so dangerous it’s hosed down with a million gallons of water every three minutes.
Rocket testing, suffice to say, is quite an intense process.
But what actually goes down at a typical modern testing facility?
Join us today as we get all fired up for a look at how space rockets are tested.
This month marks the 60th anniversary of Russian
cosmonaut Yuri Alekseyevich Gagarin’s pioneering first flight into space.
So you’d be forgiven for thinking rocketry was pretty well understood by now.
Well, there’s actually still a very long way to go. Vast new rockets,
from SpaceX’s awesome Starship to NASA’s moon-bound SLS giant
are grander than anything attempted during the old-school space race.
There’s also different mission priorities these days. And even different fuel sources.
As such, the latest generation rocket engines, be they Elon Musk’s almighty Raptors or Uncle
Sam’s RS-25s from the Shuttle programme, need to be rigorously put through their paces on
terra firma before they’re trusted with precious payloads. And of course delicate human beings.
Rocket tests fall into two broad categories – sea level ambient, and altitude.
This distinction is critical, because air pressure is greater at sea level than way
up off the ground. At altitude, rockets produce more thrust in the thinner air,
and have to cope with less heat transfer. And these factors have major engineering implications.
For sea level tests, the ambient atmospheric conditions surrounding the test area work fine.
But testing rockets at altitude is a lot more challenging, as technicians need to simulate
conditions way up in the blue sky, but on the ground where they can actually analyse them.
For this, rockets are placed in a sealed chamber, where pressure is sucked out with
mechanical pumps to around 0.16psa, roughly equivalent to the prevailing conditions
100,000 feet above sea level. This sealed environment obviously creates its own problems,
so a process known as INERTING is introduced, whereby gaseous nitrogen
or helium is fed into the chamber to prevent explosive build up of rocket exhaust matter.
Whichever type of test, sea level or altitude, that ferocious exhaust
needs to be directed somewhere, typically into a so-called flame bucket of trench.
This funnels the heat and energy in a direction where it can’t do any harm.
The particular fuel source of the rocket is a big determinant in how the rocket is oriented
during the test. Liquid rocket engines are typically fired in a vertical position,
as gravity is all part of the fuel intake process. Solid rocket engines,
alternatively, can be fired horizontally, which requires a smaller and cheaper flame trench.
However this comes with its own issues, not lease noise, which we’ll come to shortly.
So, where actually are these testing grounds?
Most modern rocket testing facilities are situated in the southern United States, in order to be near
launch sites, which themselves are frequently close to the earth’s equator. Why are launch
sites near the equator? The land around the equator moves at around 1670 km per hour, while
halfway towards the poles land only moves 1180 km per hour. This means launching from the equator
helps spacecraft blast off already 500 km/hour faster, without any additional input of energy.
As such, SpaceX has a large rocket testing facility in MacGregor, Texas,
which is handy for the company’s Boca Chica launch and assembly site.
But today we’ll mostly be focussing on Nasa’s John C. Stennis Space Center in Hancock County,
Mississippi. This is where new SLS rockets are being tried out in preparation for the
first Artemis launches later this year, part of an ambitious programme
to put the first woman and next man on the moon in the next three years.
Stennis has a fine pedigree in this regard, as it was where rocketry for the first and
second stages of the Saturn V lunar landing were tested back in the 1960s. The magnificent landmark
A and B test stands are even registered as official National Historic Landmarks.
From 1975 to 2009, Stennis also tested the main
engines that powered 135 missions of the iconic NASA space shuttle.
Indeed, those same RS-25 engines that powered the shuttle, with some choice
upgrades naturally, will hopefully be sending the SLS to the moon soon.
The facility is so integral to American space lore, that it’s said by proud Stennis workers
‘if you want to go to the moon, you first have to go through Hancock County, Mississippi’.
Stennis is indeed perfectly situated for rocket testing. It’s isolated from major population
centres, on a 13,800-acre site surrounded by a 125,000-acre buffer zone. There’s great access by
road and seven-and-a-half-miles of specially dug canals, plenty of local water, a supportive local
government, and a climate that suits testing. At least when hurricane season isn’t in full swing.
So what happens in the actual test?
During the recent SLS warm-ups at Stennis, a variety of rocket systems were checked over.
Modal testing is a way of assessing the overall structural integrity of components,
essentially by striking them with a finely calibrated mallet and measuring
their resonant frequency. If the frequency is off in some way it could suggest faulty welds,
cracks, or other problems that might jeopardise the ultimate mission.
There’s a so-called ‘power up’ procedure that must be followed, to make sure everything comes on
correctly and in the right sequence. The avionics – that is, on board mechanics and electronics
– must all be tested, and thrown phoney curveballs to reveal any unforeseen weaknesses.
A crucial aspect of rocket avionics is the gimbaling system,
which orients the rocket boosters in order to maintain overall trajectory.
This system has its own self-contained hydraulics, and must be tested thoroughly.
A series of simulated countdowns will take place, ramping up to a so-called
‘wet dress rehearsal’ where propellants are flowed into the engine but not ignited.
To facilitate the SLS tests at Stennis, some 700,000 gallons of propellant – that’s
200,000 gallons of liquid oxygen, and 500,000 gallons of liquid hydrogen – are
piped in from six floating barges. That’s about 114 tanker trucks worth.
At the moment of ignition, when the rocket does finally fire, technicians need to keep an eye on
some fairly hair-raising physical changes to the rocket. With temperatures ranging from minus 400
degrees Fahrenheit – thanks to that liquid fuel source – then rising to a few thousand
degrees Fahrenheit while alight, the metal tank can grow, and shrink, by several inches.
That infernal exhaust will then jet into the flame chute, requiring two
separate water suppression systems, both fed by a dedicated 66 million gallon reservoir.
240,000 gallons of water every minute gets pumped over the chute to cool
the engine exhaust during testing. In addition to that, some 92,000 gallons
of water a minute are sprayed through a separate nozzle system to suppress noise.
This noise suppression isn’t just to help the local property prices. It’s to prevent
all that acoustic energy disrupting the finely calibrated test equipment, or affecting the
material performance of the rocket itself. Those gigantic plumes of white smoke you see emanating
from Stennis testing rigs are in fact steam from that colossal torrent of much-needed cool water.
Stennis has its own High Pressure Industrial Water Facility to manage
this aspect of its operation, by the way and a High Pressure Gas Facility of
HPGF that helps pump gaseous nitrogen into the core state during testing.
This prevents outside air entering the testing zone during those all-important altitude tests.
As well as the mechanical side, special software is also deployed during testing,
to control the avionics and simulate the different atmospheric conditions the rocket
may find itself subject to. Software is believed to be the reason French rocket
Ariane 5 suffered a disastrous mid-air mishap in June 1996, so there’s a lot at stake here.
All these processes working in concert should help run a test that produces, in the case of the SLS,
1.6 million lbs of thrust – or 2 million lbs at altitude – consuming, as it fires,
some 2.6 million litres of propellants.
Thrust is measured using the TMS, or Thrust Measurement System,
which uses a series of load cells to calculate how much
upward movement the thrust generates, with fuel weight taken into account.
Once rocket firing is complete, the real work begins.
Some 1,400 sensors arrayed across the test site produce terabytes of data which technicians can
pore over in order to make subtle refinements ahead of the big launch.
The impact of rocket testing, in particular at Stennis,
goes far behind just firing payloads into space by the way.
The onsite Advanced Technology and Technology Transfer Branch was founded by NASA to develop
and share the knowledge acquired at Stennis in a way that enriches daily life for everybody.
Not least the Mississippi economy. In 2018 alone
it’s reckoned the work at Stennis benefited the local economy to the tune of $583million.
In case you thought it was just a load of pyromaniacs
out there in the Deep South, having a blast.
What do you think? Is the ground testing of rockets every bit as exciting as the lunar
missions themselves? Let us know in the comments,
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