- Hi it's me Tim Dodd, the Everyday Astronaut.
There's a new trend going around
in the commercial space industry
when it comes to launch abort systems.
All 3 commercial companies who are
putting abort systems on their crewed
vehicles have ditched that classic launch abort tower
we've seen dominate abort systems in the past.
Previous vehicles like The Mercury capsule, The Apollo
capsule and even the Soyuz capsule all used
an escape tower that sat on top of the crew module.
Capable of pulling the vehicle away
from a failing rocket in a hurry.
And to make this topic even more interesting
we're seeing another trend in abort systems.
SpaceX's crew Dragon capsule
and Boeing Starliner capusles, both are using liquid
filled abort motors instead of solid rocket motors.
So today we're gonna at talk about
the design considerations that have made
SpaceX, Boeing and Blue Origin ditch
abort towers on their crewd vehicles
and we're also going to re-evaluate why
the heck are Boeing and SpaceX
going with liquid rocket motors
instead of solid rocket motors.
And with both SpaceX and Boeing having
experienced serious setbacks and complications
with their liquid fueled abort systems
including the loss of a test vehicle.
It raises the question, is it even a good idea?
Now there's a lot of fun engineering decisions
behind each and every system, so lets get started.
(upbeat music)
I've talked about abort systems quite a bit in the past.
I have a video explaining why SpaceX's
Crew Dragon capsule has fins on the trunk.
Which helped keep it pointy and up and
flamey and down in the even of an abort.
And I have another video talking about
why the gemini capsule went with an ejection seat
instead of a launch abort tower.
So for a short run through on
mechanical abort systems, here's a brief overview.
If you're sitting on top of a skyscraper
full of explosive materials it's generally
considered a good idea to have a way
to evacuate quickly if things go wrong.
The way abort systems originally worked
was by having a trail of wires running down
the entire length of the tanks of the rocket.
If any two of them are severed it triggered
an abort system to fire up the motors and detach
the crew module within a fraction of a second.
The abort motors have to be powerful enough
to pull the vehicle away from a failing
rocket as quickly as possible.
These systems can pull up to 15g's
of accelaration for a few seconds.
Now I'm not saying that sounds like fun
but that actually sounds awful.
It sounds like been hit by a semi-truck
for a few seconds continuely, yeah no thanks.
And since the very beginning of human space flight
well atleast in the United States, engineer's
opted for a launch escape tower stuck on top of the capsule.
You'll notice some fancy scafollding
with a few really powerful rocket motors attached to it.
On top of the Mercury Capsule, the Apollo capsule.
The Soyuz capsule, China's Shenzhou capsule
as well as the upcoming Orion capsule
and India's Gaganyaan capsule.
All of these vehicles also used solid rocket motors
in the abort system because solid rocket motors
are very powerful for their size.
They can be lit in an instant.
They're simple and are relatively safe
to have sitting around idle.
This configuration with a tower is called
a tractor of a puller system.
Since the motors are in front of the vehicle
and they pull the vehicle free
as opposed to most rocket motors which are
at the bottom end of the vehicle and they push it.
The three newest capsules that will be
carrying humans to space have opted
to ditch the tractor abort systems
and go with a different approach called a pusher system.
And even more interestingly SpaceX, Boeing and Blue Origin
all have wildly different reasons for doing so.
So let's start off with Blue Origin
and their New Shepard capsule.
New Shepard is one of two players
in the sub-orbital tourism game.
Competing with Virgin Galactic
and their Spaceship To Space Plane
but Blue Origin's New Shepard is the only
one of these two that offers an abort system.
The abort system that Blue Origin
decided to go with is solid rocket motor
placed smack in the middle of their capsule
underneath what looks like a beautiful table
but is actually a large chunk of blast resistant
and high pressured tubing holding the solid rocket motor.
As mentioned before this makes it a pusher system
since the exhaust comes out of the bottom
of the vehicle when used during an abort.
Now an interesting note with this system
is how wobbly it looks when in use.
This is likely because the center of thrust
is very close to the center of mass
making it inherently less stable.
I talked about this briefly in the video
about why the Crew Dragon capsule has fins on the trunk.
Saying it looks like New Shepard could use
some aero-services to help keep it more stable
but then again as long as it get's
the heck out of there, it's probably fine.
Blue Origins use of a solid rocket booster
makes sense for this abort system.
Since it's only to be used in emergency.
It just kinda hangs out there hoping
to never need to be used.
Something a solid rocket motor does perfectly.
It's high thrust to weight ratio and safe storage
makes it a no-brainer for this.
So now we can ask ourselves, why didn't
they go with a more traditional escape tower?
First off, The New Shepard is trying to
operate as inexpensively and frequently
as possible by being fully re-usable.
It's booster very impressively lands
itself propusively and the capusle relies on safe
and reliable parachutes to make a soft touchdown.
The name of the game is little to no refurbishment.
A lauch abort tower is traditionally jettisoned
once it's useful window of operation
has passed to free up dead weight from the vehicle
but with a sub-orbital system that
only goes straight up and straight back down.
You'd think they'd probably just keep the tower on.
And this is espcially true since there's no
docking port that would need to be uncovered because their
New Shepard vehicle doesn't dock with anything
but say you did leave it on, it's pretty
safe to say you wouldn't wanna deploy your parachutes
in the vicinity of that launch escape tower because there'd
be a pretty big risk of them getting tangled up in it.
So in the case of a launch abort tower
mission success relies on that tower
being ditched or else the parachutes won't work correctly.
So that raises the question, is it better to
design a system that requires a separation
event every single flight in order to be successful?
Or do you wanna design a system that only
needs to be used in an emergency?
And besides that Blue Origin wouldn't want
to ditch the abort tower and flight
since it could land pretty close to their
facilties and it would be likely destroyed
with each and every flight, which would add to the cost
but there's perhaps even a bigger reason
why Blue Origin went with pusher system.
And by bigger, I mean bigger.
Take a look at these puller systems.
Do you notice anything missing?
Puller systems almost always have
a protective faring or a shell that covers
the windows and the rest of the vehicle
to protect the vehicle if the abort system is used.
It'd generally be a good idea to make sure your windows
are covered up so you don't completely oblitirate them.
The Mercury capsules had a window and an escape
tower but on the Mercury capsule the motors
were much higher up on the tower than other systems.
And we did things a little differently
in the early days of human space flight
that might just be frowned upon today.
Since puller abort motors and windows don't
typically go hand in hand, covering up
world's largest windows that go to space
would be a pretty big shame.
Since New Shepards entire purpose is to provide
the ultimate tourist experience with stunning views.
They sure wanna make sure seeing out those
big, giant, beautiful windows is a top priority.
So for Blue Origin the use of a pusher type solid
rocket motor abort system makes the most sense.
Next up, let's talk about Boeing's Starliner
and SpaceX's Crew Dragon Capsules which both
were designed for NASA to send crews
up to the International Space Station.
Now these are purpose built taxis to the ISS
first and foremost although both vehicles
may seek commercial use someday for private customers.
I have a video that dives really deep
into both of these vehicles and the rockets
they ride on and then I compared them
to the space shuttle and Soyuz, the other
vehicles that visit the ISS.
So if you need a rundown on what the history is.
The specs and all of the considerations
of these vehicles, definetely check this video out.
Both SpaceX's Crew Dragon capsule
and Boeing's Starliner opted to use a pusher system too.
They've chosen to ditch the tower
but both of these cheicles opted to use
liquid fueled abort motors instead
of the more traditional solid rocket motors.
Now you might be thinking, how can a liquid fueled
rocket engine be quick and safe enough
to be relied upon for an emergency abort.
After all, rocket engines can take several seconds to get
the pumps spinning and for ignition to occur so that
doesn't really sounds like a good option.
Well, these liquid fueled rocket engines
are using a similar system to what's used
in reaction controlled systems which offer
very quick and reliable ignition.
They do this by using utilizing a pumpless
rocket engine known as a pressure fed engine.
Pressure fed engines just have their propellants
stored in a very high pressure tank.
They open a valve and they let it rip.
And to make it even more reliable
the pressure fed engines used on these abort motors
run on hypergolic propellants.
Hypergolic propellants burn on contact with each other
so there's no need for a secondary ignition
source like a spark or a flame.
This makes the pressure fed hypergolic engine
very simple, quick acting and reliable.
A great choice for reaction control and abort motors.
It should also be noted that although hypergolic
fuels are very stable and reliable, they're
incredibly toxic, Cancerogenic and just really nasty stuff.
So why did both companies choose liquid fueled abort motors?
There's a few reasons but first and foremost
it's likely due to increased safety requirements from NASA.
They wants what's known as fuel envelope abort system
or the ability to abort safely from any point in the flight.
Now solid rocket motors can provide this option too.
For instance, the Orion capsule solid rocket
tractor system offers an abort mode
for the first two minutes of flight
after which the tower is jettisoned and the
second stage of the SLS system or the service
modules AJ 10 engine can provide abort options.
But with liquid fueled abort motors they can
fire them for less time or lower thrust
than a solid rocket motor which gives them
the ability to place the vehicle precisely
into a desired trajectory to either
re-enter safely or abort to orbit.
Which can make it easier to design
a full envelope abort window.
But quite frankly each company could have gone
with a typical tractor solid rocket motor abort system.
As a matter of fact, SpaceX's original concepts
for the Crew Dragon capsule or Dragon Rider
as they called it back then was going to utilize
a traditional tower and then jettison it on ascent
as we can see in this animation from 2010.
So the first reason each company opted for a pusher
system is the same reason we talked about before.
By removing a separation event, you're eliminating
a potential mission failure scenario.
By intergrating the abort system into the vehicle
it simplifies sequence of events needed for mission success.
But intergrating the abort system doesn't
necasarily mean they have to use liquid fuels, does it?
I mean, New Shepard get's away with it.
Well there's two reasons why utilizing liquid fuel
abort motors in these cases are actually
more useful or even necessary in an integrated abort system.
The first reason is control.
Liquid fuel rocket motors can steer
and point the vehicle by doing something
called thrust differential which is where they can
increase or decrease thrust on any of the motors
and that can help point it in the correct way.
This definetely helps keep the pointy
end up and the flamey end down.
Now there are ways to control a solid rocket motor
but thrust differential isn't one of them.
So in order to steer a solid abort tower
one cool way is by having a seperate solid
motor fire and having multiple valves open
and close on each side of the tower.
Take a look at this Orion attitude control motor test.
Inside there is a solid rocket motor that
lights and cannot be turned off once activated
but in order to provide steering, they can
vary how much thrust comes out of any of it's eight valves.
To provide neutral input they just open
up all eight valves equally which
then will produce no change in direction.
They then stick this unit really high up
on the abort tower to provide maximum
leverage over the vehicle which helps
provide ample control during the abort.
I think this is really cool.
In a pusher system there's no tower to provide leverage
so you can't put control services up there.
So using a solid rocket motor that can't
provide thrust differential can be a bit of a no-no.
But there's actually another reason why
you can't use solid rocket motors on these two vehicles.
And that's because these vehicles visit the
International Space Station and because they
visit the International Space Station
the abort motors also visit the station.
This means 32 times a day the vehicle has extreme
changes in temperature or thermal cycles.
Now hypergolics are happy as a clam
and stable hanging out in these conditions.
As a matter of fact, there's hypergolics
on the station as we speak.
The thermal cycling of a rocket motor that much
could potentially lead to some unintended booms.
And that's definetely not a good thing
when you're attached to the single most expensive
piece of machinery with precious human lives on board.
So why are they even bothering
intergrating the launch abort system at all?
This all sounds like an awful lot of trouble.
Besides the simplicity of an integrated
system not needing to separate, the two companies
actually have very different reasons for their integration.
Boeing modified some Aerojet Rocketdyne RS-88 Bantam
motors to run on the same fuel as the reaction
contol system that is part of the service modules
Starliner instead of the alcohol
and liquid oxygen they normally use.
The service module also houses the solar panels
and radiators of the vehicle.
By using the same fuel for the abort and the reaction
control systems it means an anominal mission
the abort system obviously isn't utilized which means
the extra propellent can be used for other things.
Besides having more fuel for orbital maneuvering
you also end up with spare propellent
to potentially reboost the International Space Station.
Now this is something only Russia's Progress supply
vehicle and the Signis cargo vehicle can currently do.
But the retired space shuttle and the ATV
vehicle could also do it previously.
Having the ability to reboost the station
is definetely a nice selling point.
Although we don't have any confirmation on whether
or not Starliner will ever perform this maneuver
but atleast having it is isn't a bad option at all.
But for SpaceX, perhaps their biggest reason
why they didn't wanna ditch their motors
is 'cause they designed the Dragon capsule
to be reused as much as possible.
This means by integrating the abort motors with the
capsule itself, they would recover them with each flight.
But ironically the Dragon capsule is less
reusable than the Starliner since the Crew Dragon
capsule splashes down as opposed
to the Starliner which will land on land.
This means that the Crew Dragon capusle
will never be never be reused for crew
and will only be used as a cargo vessel.
And what's even more ironic is the Starliner
which lands on land and can be reused up to 10 times
ditches it's abort motors with each and every flight.
As the service module which houses
the abort motors is jettisoned before re-entry.
So the vehicle which lands on land ditches
it's abort motors and the one that lands
in the ocean keeps it's abort motors.
That's interesting but the main reason why
SpaceX's Crew Dragon has the super Draco
liquid fuel abort motors in the first place
isn't just for abort options.
They originally intended to land
the Dragon capsule propulsively.
Yes that's right, they originally intended
to land the Dragon Capsule much like SpaceX
lands it's Falcon9 and Falcon Heavy Rockets.
Riding pillars of flames, so cool.
Now the original reason for landing propulsively
was for rapid and easy reuse of the vehicle.
Not only that but they also wanted
the Dragon capsule to land on Mars.
SpaceX's original goal with the Dragon
was to land it on Mars for a mission dubbed Red Dragon
but the whole propulsive landing idea got scrapped.
SpaceX gave up on pursuing propulsive landing
with the Dragon capsule mostly because NASA
really wasn't interested in the concept.
As it would require far too much additional work to certify.
So now that we've talked about why each
company ditched the tower and why both
Boeing and SpaceX has opted to use
liquid fuelled propellent's for it's launch abort systems.
It raises the question, is it even a good idea?
Both Boeing and SpaceX have had problems
when testing their liquid abort systems.
Boeing experienced an anomily with a Starliner
test vehicle in 2018 when preparing to do
a pad abort test, where the hypergolic
fuels leaked out of the vehicle.
SpaceX had something a little more
energetic happen in 2019 when preparing
to do a static fire test prior to an in-flight
abort test with their recovered DM-1 Crew Dragon capsule.
In my opinion these issues will get worked out.
These fuels using these systems are the same
or very similar to almost all orbital spacecraft.
After all, satellites often use hypergolic fuels
and a liquid motor to do their orbital maneuvering.
The space shuttle had hypergolic fuels
for it's orbital maneuvering system
and hypergolics are already used on
Dragon-1 for well over a dozen missions.
And there rarely has been any issues with these systems.
In general these systems can be safe, simple and reliable.
So why are we seeing such major
problems arise when testing them?
Well the public relation answer is, that's why we test.
But I kind of have a feeling it has to do
with how you test this stuff.
You often times go beyond what you'd normally be doing.
And doing things in these testing campaigns
that wouldn't normally be done on the launch pad.
As far as is it a good idea, I mean I think so.
The fuels are already on the vehicle in the first place
so it makes sense to use them for a launch abort system.
Liquid fueled abort systems are useful.
Potentially reusable and actually make
for a simpler spacecraft and mission profile.
Not to mention, when they're not used
they can be utilized for other purposes
such as reboosting the ISS or even propulsively landing.
Although we probably won't ever
see either of those things happen.
I'm sure each company is learning from
the failures of these systems and I have
little doubt this will all be sorted out
before we ever put humans on board.
And hopefully that's sooner rather than later.
So it is interesting that we went from one
common form of launch abort to suddenly
three 21st century companies using widly different systems.
But in the long run each company has a unique
reason behind their engineering decisions.
Which I find fun.
So does this help answer your questions
of why companies have scrapped abort towers?
Or why they're now using liquid fueled rocket motors?
Let me know if you have any other questions
about this topic in the comments below.
In keeping with this trend of talking
about escape systems, I plan on making a video
about why SpaceX doesn't plan to use
an abort system on their upcoming Starship
and help answer the question of
whether or not that's a good idea.
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Thanks everybody, that does it for me, I'm Timm Dodd.
The Everyday Astronaught bringing space
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(upbeat music)