Hi it’s me Tim Dodd the Everyday Astronaut!
We’ve all seen SpaceX land the first stage of their Falcon 9 rocket and now Elon Musk
says he wants to try and land the second stage…
In order to understand why his engineers were probably crapping their pants when he said this,
we’re going to go explain why recovering something from orbit is significantly harder
than recovering the first stage.
We’re also going to show you some options for second stage reuse including one option
to just keep it up in space where it might be a better use than back on earth.
Now bare with me on this one, it’s going to be a little bit more technical and a little
longer, but with a little help from Kerbal Space Program, I think we’ll be just fine!
SpaceX has gotten so good at recovering the first stage of their Falcon 9 that we forget
how almost impossibly hard it is.
Right up until they actually landed their first first stage on December 21st, 2015 for
mission Orbcomm 2, it was still thought by most people in the industry to be simply impossible.
We’ve previously talked extensively about how SpaceX lands and also gotten into some
fundamentals of determining if they’ll have enough margins to land their Falcon9 on land
or on the drone ship in the ocean.
So if you don’t understand those things, please check out those videos before
watching this one.
This one's going to get a little more technical but we’ll try and make it
as easy as possible.
So let’s start off with a few numbers here before we get into some examples.
The first stage of a typical Falcon 9 will get up to speeds of around 5,000 mph (which
is about 2.3 km/s).
As you hopefully are aware, even at this speed, the first stage still needs to do a fairly
substantial entry burn as the stage enters the atmosphere so the heat of reentry does
not tear the stage apart.
This requires the first stage to use up a portion of its already scarce fuel in order
to survive hitting the atmosphere.
The second stage on the other hand is traveling much much faster.
An object in low earth orbit is traveling at around 17,500 mph or 7.8 km/s.
Then we have missions where the second stage needs to loft its payload up into geostationary
transfer orbit.
This requires it reaches speeds of 22,000 mph or 9.8 km/s!
So as you can see, depending on the destination of the payload, the second stage may end up
to 4 times faster than the first stage.
This is a big problem when trying to recover it.
The primary reason being heat on reentry increases by speed cubed!
Or in other words, this means if the reentry of the vehicle goes from 2 km/s to 8 km/s
heat would increase by 64 times.
So now I hear you saying, “But we bring stff down from space all the time, why is
this any different?”
To answer that question, let’s take a look at some common reentry systems.
We’ll start with what’s probably the most common.
A capsule.
The reason a capsule works so well is because it’s the simplest, safest and most stable
of all designs.
The blunt leading edge and tapered walls create an exceptional environment to dissipate heat
and stay pointing heat shield first.
Next most common is what the space shuttle was, a lifting body.
A lifting body allows for the vehicle to stay in the upper atmosphere longer, allowing the
vehicle to slow down for a longer period of time, which keeps peak heating and g forces
to a minimum.
This also allows for a lot of control when coming in, so the space shuttle and others
like it, have a great cross range capability leading to flexibility and reliability in
hitting its intended landing area.
The space shuttle isn’t the only lifting body used.
There’s the upcoming dream chaser by Sierra Nevada and Boeings secretive X37B both of
which sort of look like mini space shuttles.
They’re similar by concept, but are even more of a lifting body since they don’t
have delta wings like the Space Shuttle did.
And lastly, although not currently used for orbital flights on Earth, is using retropropulsion
like SpaceX uses for the first stage.
The primary reason we can’t use retro propulsion is because it would take as much fuel
to slow the vehicle down prior to reentry as it took to speed the vehicle up to orbital
speeds.
This means even after the second stage has completed it’s long burn to speed up, it
would have to somehow have enough fuel to turn around and do that entire burn again…
which just simply isn’t happening.
Some of the other major problems facing second stage reentry is stability.
The vehicle will want to enter heavy end first, or in the case of an almost empty second stage,
engine first.
Say we put a heat shield up on the top of the stage, it would take some serious design
considerations for the vehicle to maintain its proper heat-shield-first orientation during
reentry.
Believe it or not, SpaceX actually released this video in 2011 which showed the second
stage with a heat shield on the front of it.
Another issue is when it comes to landing, the second stage cannot simply use its engine
at sea level.
With its massive nozzle, it would be too unstable at sea level, meaning we can’t actually
use the Falcon 9’s vacuum Merlin engine for its final landing.
But perhaps the biggest enemy is weight.
Sure, we can solve any of these issues if we tack on a spare set of engines for landing,
a set of wings for orientation, additional thrusters for maneuvering, a heat shield to
survive reentry and some landing gear…
The problem here is for each pound we add to the second stage, we will have to remove
a pound from the payload.
By the time we add all of this new hardware, there’s a chance we won’t have any margin
for payload at all!
Think about it, the 2nd stage pushes its payload all the way to its intended destination.
All of the mass of the 2nd stage is literally joined to the payload right up until its mission
is complete and it lets go of that payload.
This is different from the first stage.
The first stage doesn’t take nearly as much of a payload penalty for each pound added.
Elon Musk quoted it’s only about a 5:1 ratio for first stage.
This means they could add about 5 pounds to the first stage before you have to remove
one pound from the payload.
This is why the first stage of the Falcon 9 can have large landing gear, nitrogen thrusters
and grid fins and still put a substantial payload into orbit.
So let’s take this over to the computer game Kerbal Space Program where I’ve set
up some examples of how SpaceX just might pursue second stage reuse and just how
hard it actually is.
Alright, so we're here inside the vehicle assembly building at Kerbal Space Center and
I'm building a rocket that’s similar to a Falcon 9.
We're going to call it the Falcore5.
For those of you unfamiliar with Kerbal Space Program, it’s basically a game / simulator that will
suck the life out of you.
Do you have a significant other?
Do you want to keep that significant other?
If the answer is yes, then you should not play Kerbal Space Program.
It is that addicting.
Because we want this to be somewhat realistic, I cranked the gravity up two times in this game.
So this will make our margins a little bit thinner and that much more realistic.
Our payload for this mission is the start of a new space station and it weighs 4.8 Metric Tonnes.
With this bog standard Falcore5, it will just barely be able to push this thing up into
low Kerbin orbit at an altitude of about 250 KM.
So that will leave us with only enough remaining fuel to de-orbit ourself and that's it.
Now for reference sake only, do notice that the vehicle is showing as having 7,828 m/s
of DeltaV.
This is only a reference, this doesn't really line up to anything kind of in real life,
it doesn't really line up to most things in Kerbal, but we're going to use that as we
change the mission profile later.
We're just going to remember what we started off with.
Ok let's put this baby out on the pad.
Full disclosure.
I'm using a mod called MechJeb which will actually fly the rocket for me.
It's not very fun but it at least will keep all of our missions exactly consistent and
it will compare apples to apples so that we kind of eliminate the variable of me flying it.
Alright here we are, we're loaded up on the launch pad ready to go!
All systems go, 3, 2, 1, HIP HIP!
Alright I'm gonna speed this up because we're going to be doing this many times and I don't
want to have to watch every single second of every single mission.
Alright here's our gravity turn, and there's Max Q, and now we have stage separation that
first stage has enough fuel to try and land.
Fairing deploy.
And now you can see we're getting into orbit.
Alright now we're going to coast up to its highest point or apogee and do one more burn
to circularize so that will put us parked in our 250 km orbit.
And now let's deploy our payload.
Ok it's good to go!
Now notice how little fuel we have left.
This was at the absolute upper end of what our standard Falcore5 is capable of pushing
into orbit.
So we're going to do our deorbit burn and we're going to get down to about 50 km's into
the atmosphere so it blows up on reentry.
And there we go, kaboomy.
So now we have two options.
Since we had literally no fuel remaining to attempt any kind of recovery at all, we can
either do one of two things.
We can shrink the payload down or we can build a bigger rocket.
I think we all know the answer to this one...
Introducing the FALCORE HEAVY!!!
Ok so we're putting that exact same payload, that space station, up into the same place
in space in the same orbit.
But now we're actually starting off with 9,296 m/s of Delta V as opposed to just 7,828 m/s.
So let's see how much fuel we have remaining at the end of this mission to see if we have
any chance of recovering the second stage at all.
Ok here we go 3, 2, 1 HIP HIP!
Alright gravity turn, Max Q, those side boosters deploy and they have enough fuel to land back
at land now the center core when it separates that will have to try to land out at sea on
the drone ship.
Ok now we're going to get into orbit.
Let's see how we're doing.
Ok so the payload deploys and we have a decent amount of fuel.
Let's see how much fuel we have left over and try to recover the second stage after
our deorbit burn.
Ok so we have just over 1,200 m/s of Delta V to slow ourselves down as we enter the atmosphere.
Now as the stage heats up I'm going to throttle up to help slow ourselves down so the compression
from the freestream air behind the bow shock doesn't heat us up too much.
It's a lot of technical stuff but basically that's the stuff that will kill us.
After a good entry burn we still have around 400 m/s left over, now will this be enough
to slow us down as we get into denser atmosphere?
Ahhh throttle back up!
Ahhh no.
Bye bye!
There she goes!
So uh, we just simply cannot slow ourselves down enough.
So just as we had talked about earlier.
We need to slow ourselves down enough to survive reentry which we just simply don't have the
margins to do.
So let's use that atmosphere to our advantage.
Let's stick a big ole heat shield up on the nose of that stage.
And then we'll try and point it nose first as it goes through the atmosphere to slow
us down.
Our biggest problem here is we know that engine is going to want to go first because its the
most massive part of that stage.
So we're going to try to keep it oriented using those thrusters and stuff like that,
but we'll see how it works.
Let's check this out.
And WA LA!
We now have a large heat shield at the nose of the second stage.
See that beautiful pizza crust looking thing?
Oh yeah there it is, between the second stage and the payload.
It is just delicious!
We're going to go ahead and skip to payload to deploy and watch our reentry.
Now notice after our deorbit burn we're left with only 675 m/s.
Having to push around that big heavy heat shield really took its toll.
So now we had 1,200 m/s on that last reentry... so hopefully we won't need to use our engines
at all to slow us down so let's see how this goes.
Ok so our RCS thrusters are keeping us oriented nose first.
Heating up... it's getting spicy!
Ohh!
Oh shoot yeah.
There we go and that's what happens.
The RCS thrusters were not strong enough to keep it oriented.
It would take a lot of RCS thrusters to keep it oriented like that.
Ok it's time to get serious.
That last attempt didn't even have an engine that could land on sea level anyway or landing gear.
So let's go all in on this one.
Ok I'm adding fins for stability, Some separate engines to land with and some large landing
gear that will allow us to land upside down.
Now again, this is only for reference, but by the time we add all this recovery hardware
onto the second stage, we're looking at only 8,093 m/s of DeltaV.
That's a ways down from the 9,296 m/s that the original Falcore Heavy had.
Ok so we're going to put this into orbit and deploy the payload.
Let's see how things go on reentry.
Well look at that those fins are keeping it oriented heat-shield-first.
Eh yikes.
Oh ooo!
Wow ok.
They're barely keeping it oriented.
Woah ok.
Eee!
Ok we only have 100 m/s left over to land with.
We're traveling over 200 m/s, I can see where this one is going.
Into the drink.
BOOO!
So remember before when we had to decide either to make a smaller payload or a bigger rocket?
Let's make a smaller payload on this one.
Introducing a large tele communications satellite that weighs in at exactly half the mass of
the space station piece at just 2.4 Metric Tonnes.
We're only showing 8,219 m/s which is up from that 8,093 m/s we had with that larger payload.
But the interesting note here is we won't have to use as much going up, so it does mean
we'll have a decent amount more to work with on the way down.
Ok so boom we're in orbit.
Payload deploy and now we're deorbiting.
Ok much better.
We now have 605 m/s left over to try recover this baby.
Eeeee that's a spicy meat ball!
Ok and let's and flip it over here.
Oop woah!
Getting real spicy down here...
Annnd alright let's do our landing burn here.
Annnnd full throttle!
Annnnd touchdown!
Yeah!
Oh that took me way too long.
I don't wanna tell you how many times.
Ok now don't forget Kerbal is not a perfect analogue, but it at least helps illustrate
some of the challenges involved in trying to recover a second stage.
Ok so moral of the story is, yes, there is potential to land the second stage.
It ISN’T impossible.
It will require some MAJOR design changes and even if they get are to get it to be recoverable,
there might be such little remaining margins that they might not be able to launch a significant
payload.
They might end up using the Falcon Heavy to launch cube sats or something all for the
sake of reusing the second stage, which would not be economical.
Only time will tell and I’m really excited to see what they come up with for their solution.
Now, before the comments get blown up with “BUT THE SPACE SHUTTLE” I did want to
point out that the orbiter portion of the Space Shuttle was essentially a recoverable
second stage.
It fully succeeded in bringing home those expensive and wonderful RS-25 main engines.
The problem is it took such a big payload penalty.
Despite having almost the same amount of thrust at sea level, the space shuttle could only
put a 28,000 kg payload into orbit, which is nothing compared to the Saturn V which
could put 120,000 kg’s into low earth orbit.
This is because about 100,000 kg’s of weight was the orbiter itself with its wings, and
engines, cargo bay and landing gear, this greatly reduced how heavy of a payload it
could actually deliver.
Now I did mention there’s another proposal for keeping a second stage in orbit and reusing
it while it’s up there.
United Launch Alliance better known as ULA which is a joint venture between Boeing and
Lockheed Martin has a really cool idea for the second stage of their upcoming Vulcan
rocket.
They propose the idea that why try and bring a second stage back down at all.
Why not keep them in orbit eventually they can populate a large number of 2nd stages
with some extra fuel in them.
Then they would then be able to dock them and top off a single stage so you could eventually
have a full second stage sitting in orbit waiting to be used.
This means you could push a substantial payload way beyond Earth orbit on just the fumes of
otherwise doomed stages!
It’s a pretty brilliant idea and I’m excited to see them work on this concept.
Unfortunately we won’t see their Advanced Cryogenic Evolved Stage or ACES fly until
2023 at the earliest.
Regardless, it’s really exciting to see companies taking this stuff seriously.
My hope is that within the next decade we're going to see FULLY reusable rockets that help
bring the cost of space down significantly, that'll be a really exciting time!
If you have any other questions about second stage reuse or any other question about rockets
in general, leave them in the comments below.
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Well that does it for me!
I'm Tim Dodd, the Everyday Astronaut.
Bringing space down to Earth for everyday people.