Could Air Launching Rockets Be A Better Way To Launch?
- Rockets waste a ton of fuel just getting off the ground and a big portion of their fuel is consumed before they even get out of the earth's atmosphere. Now, despite this, we're still more or less just launching rockets the old fashioned way, the same way we always have. Isn't there a better way? Air launching is perhaps one of the most tantalizing concepts for launch vehicles. Just use an aircraft while you're in the atmosphere so we can use the atmosphere to our advantage to lift and accelerate a rocket and then just start the rocket in the upper atmosphere while it's already at a high velocity. Not only can we use jet engines in the atmosphere that are 10 to 20 times more efficient than rocket engines, but our rocket will actually perform better in the thinner atmosphere too. And this concept certainly isn't new from the Supersonic X-1, flown in the 40's to the hypersonic X-15 flown in the 60's to the first orbital air launched rocket Pegasus in the 90's, and the liquid fueled the LauncherONE rocket from Virgin Orbit that first flew successfully in 2021.
Air launching just feels like a no brainer. So why don't we see companies pursuing this? Or for that matter, why are pretty much all of the programs that have utilized air launch for orbital rockets no longer in existence? I'm Tim Dodd, the Everyday Astronaut. Today we're going to go over the pros and the cons of air launching to try and figure out why it isn't more common for orbital launch vehicles. Okay, let's get started. - 3, 2, 1. - Welcome to another episode of Why Don't They Just Where we try and figure out why seemingly fantastic ideas aren't used more often in the aerospace industry.
In the first one we answered, why don't they just launch rockets from on top of mountains so they're closer to space or from the equator so we can use the earth's rotation to help boost our starting velocity. So before you watch this video, may I recommend you watch that as well as my video about orbit versus sub orbit as they lay down some important concepts that will help make this video easier to digest. Okay, so let's start off with the pros of air launching. Using jet engines on a launch system makes so much sense on paper.
Like we said, they are easily 10 to 20 times more efficient than a rocket engine, meaning they can do 10 to 20 times more work with the same amount of propellant, and they don't need to carry heavy liquid oxygen on board. They simply utilize the oxygen that's in the air. So we can use efficient air breathing engines while we're in the atmosphere of course.
But don't forget there's also lift from wings that can help lift our rocket to help it get above a good portion of the atmosphere. If we can get it up to roughly 10 kilometers or 33,000 feet or so, the atmosphere is only about one quarter as dense as it is at sea level. This means not only does the rocket have less air to fight against, but just like we talked about in the previous video about why we don't launch rockets from mountains, if we're starting at a higher altitude, we can use a more vacuum optimized engine. Okay, now I'm gonna make some generalizations here, but let's say that you had a rocket engine that's meant to take off at sea level.
Obviously it has to have a smaller expansion ratio or a smaller nozzle than one that fires in a vacuum. Again, we talked about that with the Merlin 1D in that last video. You hopefully know this by now, but those expansion ratios of say a Merlin might be something like a 15 to one. Now if we took that same engine and optimized it to be able to start at 10 kilometers where the atmosphere is one quarter as dense, we could expand our nozzles expansion ratio to something more like 25 to one, which would give us a few extra seconds of specific impulse. Although that sounds like a pretty healthy increase and it is, how does that actually affect the performance of a rocket? Well, similar to launching from a mountain like we talked about in that last video, there is actually a pretty decent gain from just increasing the altitude at what you launch at. If you took a rocket that was taking off at sea level and you optimize it to take off from a stationary vehicle, like let's say a balloon or something that just starts at 10 kilometers, you could expect to see maybe about a 15% increase in payload capacity.
But you can get even more performance from air launching if the carrier vehicle has some additional velocity to inject into the rocket. So instead of launching from, you know, a weather balloon or something that's stationary at 10 kilometers, if you have a carrier aircraft that's traveling at let's say 800 kilometers an hour, you could get nearly a 30% increase in payload capacity over launching it from the ground. So doubling the advantage over just launching it stationary at that altitude.
But again, this does assume that we increased our nozzle size because of the thinner atmosphere. We could have a bigger expansion ratio, but otherwise we made no additional hardware changes to the rocket in order to air launch it. So this is a bit of a best case scenario estimate, but air launching has other advantages like the ability to take off from any runway that's large enough to host the carrier aircraft, which means the launch provider can come to the customer and launch on their turf, literally. Air launching can also fly above and around adverse weather conditions, which means they have the potential to have less costly scrubs due to weather, which is definitely a positive thing. And perhaps maybe the biggest advantage of air launching is its ability to hit very unique and sometimes hard to reach orbital inclinations since the carrier vehicle can align the rocket to any given orbit and it can fly to generally hard to reach places on earth in order to hit these orbits.
If a traditional ground launch provider has the capability to reach, you know, really any orbit, they likely need to have multiple launch complexes in order to reach the full array of target orbits, which of course is quite costly. Okay, so that's a brief rundown on the pros of air launching, but it can't all be good news obviously. So what are the cons of air launching? Why isn't it more common? To be honest, the cons of air launching can pretty well be summarized with just one word practicality. And the biggest limiting factor in what makes air launching possible is the size of aircraft that it takes to actually launch a rocket big enough to put a meaningful payload into orbit. I think people tend to forget just how big and heavy rockets generally are in the first place. For instance, picture you're sitting inside of a Boeing 737 very common jetliner generally used for mid-range travel, typically flying around 170 people.
Now picture a Falcon 9 rocket like this, it's the workhorse of the 21st century in space flight capable of taking over 17 metric tonnes to orbit while still being able to land the booster. So it can be reused. And of course when it has a Crew Dragon capsule on top, it can ferry four humans to and from low earth orbit. If you're anything like me, you probably envision the 737 as a larger vehicle than the Falcon 9. So let's go ahead and put a 737-800 next to a Falcon 9.
Yeah, even a medium lift rocket like the Falcon 9 is actually pretty gargantuan compared to the 737. They actually have nearly the same body diameter with the Falcon 9 at 3.7 meters wide. And the 737's fuselage is actually pretty much exactly as wide, albeit the fuselage is four meters tall, but at 39.5 meters long, the 737-800 isn't even as long as the Falcon 9's booster, which is 45 meters long. Or I guess when it's standing it's 45 meters tall and as wild as the size is, things get ridiculous when you start to consider they're mass a 737-800 when fully fueled and loaded up with its maximum takeoff weight tips the scales at 78,000 kilograms, which is nothing compared to a fully fueled Falcon 9, which weighs an astonishing 550,000 kilograms. Now you might be asking yourself, why did we just compare a 737 and a Falcon 9 two very different vehicles? Well, I just wanted a baseline to help compare the size and the mass of these two objects to help put into perspective just how big of an aircraft you'd actually need in order to lift a rocket that's fully fueled.
Let's do this another way. Let's take a look at one of the largest jets ever produced a Boeing 747-400. This monster is 70 meters long. So yeah, about exactly as long as a Falcon 9, but its fuselage width is massive at 6.5 meters wide and its maximum takeoff weight is about 410,000 kilograms.
So let's imagine we stripped one out and had this huge jetliner lift a big rocket off the ground, got it above about 75% above the atmosphere, and then let go of the rocket while traveling at about 800 kilometers an hour, which surely would help the rocket do much more work with less propellant since the aircraft did a lot of the work. Well, we don't actually have to imagine this at all because this is exactly what Virgin Orbit did with their LauncherONE rocket and cosmic girl 747-400, and this was actually a decent sized rocket too at 1.8 meters wide and 21.3 meters tall or long and it was 25.8 metric tons when fully fueled. So how much payload could that rocket actually put into orbit? It seems like it kind of got a free ride through half of what would be its first stage burn. So it's almost like having free boosters.
So its payload capacity should be much greater than a comparable rocket of its size, right? Well, it was capable of about 500 kilograms to lower earth orbit, which still puts it kind of in the the lower middle part of the small lift launch category. But now in terms of performance and size, how to compare to other rockets in this class, I think the closest rocket to fly in terms of capability and size is SpaceX's Falcon 1. Falcon 1 was almost identical in size at 21 meters tall and 1.7 meters wide and it weighed 28 tons. So let's have a guess at what the capability of the Falcon 1 was. 470 kilograms, so just 30 kilograms shy of what Virgin Orbit claimed was the maximum payload capacity of LauncherONE. And that was just the normal Falcon 1.
There is plans to have it just be a little bit bigger, a little bit more powerful for the Falcon 1E and that would've far outperformed this. So in this case, there doesn't really appear to be much of a noticeable payload increase with air launching. But why, I mean these are such similar rockets, but one gets a massive boost from a 747 both in terms of its starting altitude and its starting velocity.
Why was this performance so marginal? Shouldn't it really have shined and really be performing like a much larger rocket with that free boost? Now, to be perfectly honest, this kind of puzzles me too, I think it mostly came down to a lot of additional mass that was added to the rocket to make it air launchable in the first place. And the dry mass of the rocket is really one of the biggest factors in its overall performance. Any additional mass that you add to a rocket structure is mass that you can't take to orbit as your payload. Now in the upper stage, it's a 1:1 payload penalty. So for each kilogram of mass that you add to the structure, you lose a kilogram of payload capacity.
And on first stages it's more or less usually around a 4:1 ratio. So in other words, add four kilograms to the first stage, lose a kilogram of payload, so it's not quite as big of a payload penalty as it is on the upper stage. But still you just don't want to have any unnecessary mass in the rocket structure period. And there's a lot of structural considerations when you need to hang a fully fueled rocket off the wing of an aircraft horizontally. And not only does this rocket need to be able to sit there and handle all these forces, it also needs to be able to handle the normal forces during ascent through the length of the rocket just like a normal rocket does. So now it needs to be able to handle these forces and these forces and that's actually a lot structurally.
Another consideration is the liquid oxygen on the rocket was loaded on the ground almost four hours before launching, and it was completely disconnected from the ground support equipment at T minus 40 minutes. So the locks tanks needed really good insulation to keep what was on board in a liquid state, which again adds even more mass. Now this is more specific to say LauncherONE because the Pegasus rocket was a solid fueled rocket and didn't have to worry about boil off and other liquid propellant issues like sloshing and air starting. However, liquid fuel rockets have the potential for a lot more performance, which is exactly why Virgin Orbit opted for liquid fuel. But another reason why the advantage isn't as big as you might think it would be is because a rocket usually has a lot more momentum by say 10 or 11 kilometers of altitude than the velocity that a 747 can give to a rocket by that altitude.
Most rockets are typically traveling two to three times faster than LauncherONE was when it was dropped by the 747. Not to mention a normal rocket is already traveling along its exact correct trajectory or its ideal flight path, which at 10 kilometers in altitude is usually at roughly 60 to 75 degrees vertical in order to continue getting above most of the atmosphere quickly. Although it is pitching over to begin its gravity turn.
On the other hand, an airliner flies more or less horizontally most of the time. Now because of that, before letting go of the LauncherONE rocket Cosmic Girl, the 747 pitched up substantially to help align the velocity of the rocket closer to that ideal flight path. And despite that pitch up that you saw when cosmic girl let go of LauncherONE, the rocket still needed to do a pretty substantial pitch up maneuver in order to get on that optimal ascent path. So in other words, not even all of the 800 ish kilometers, an hour of velocity that the jet essentially gave to the rocket for free are actually even aligned with the optimal trajectory. The rocket still has to do a pretty major course correction, which is costly. So that air launch advantage isn't quite as strong as it you know, kind of could be in theory.
Now, I should specify here real quick that Air launchings performance advantage is fairly minuscule on an orbital rocket because the aircraft does such a proportionally small amount of work in the grand scheme of orbital velocity because it's, you know, it's only that 800 ish kilometers an hour or so out of the 28,000 kilometers an hour needed to actually get into orbit. However, on a suborbital launch system like Virgin Galactic Spaceship two Air launching makes a lot more sense since the aircraft does proportionally a lot more work, both in terms of altitude and starting velocity of that suborbital flight profile. Now, again, for more on that exact topic, my video on orbit versus sub orbit should help drill this all in. But for me, the whole air launching thing is a bit of a flawed concept. Instead of thinking of the 7 47 as a first stage, it's probably almost better to think of it as a flying launchpad. And when you think of it that way, it doesn't really seem as appealing.
And then when you think about it more, it seems like they were almost doing the whole thing backwards. Instead of just packing up your satellite and flying it over to the rocket and over to the launch complex, they were doing it backwards. They were flying the launch complex and the rocket essentially over to the payload. And somewhat ironically, they even had to ship the entire rocket inside of another aircraft probably 'cause it was safer to do so instead of hanging the rocket on the wing the entire journey, which means that now you're flying an entire 747 and a C-17 to the customer, which I'm sure is not cheap, instead of just shipping that relatively tiny satellite to the launchpad. To me it'd almost be like if you ordered a pizza and instead of just delivering the pizza, they brought the entire store and the oven and everything to you right to your front door just doesn't make as much sense. However, one of the biggest selling points of Virgin Orbit was the ability to launch from anywhere by flying the system to their customer.
But in general, who cares where on earth a payload is actually launched from it's going to space, it's going into orbit. So as long as the launch site permits good enough performance to reach the destination with that rocket, the launch site itself is completely trivial. But I think the biggest problem with air launching is the risk. And in my mind there's two major risk items that make it just simply not worth it. The first is the risk of letting go of a rocket and then lighting its engine.
Now obviously this is a necessary safety precaution for those on board the jet, but the common thing to do on the ground is to light the engines, hold onto the rocket, ensure everything is okay, and then let go. As you likely know, sometimes launches are scrubbed after engine ignition because the flight computers just weren't happy with the conditions and it ends up shutting the engines down - Ignition, lift off, disregard, we have an abort. - This doesn't do much good when your rocket is already falling back towards the ground. But to me, the biggest risk was the risk to human lives with each launch by having what's essentially a highly explosive, pressurized device strapped onto the wing of the aircraft that these humans are flying.
Now during most traditional launches, all humans are several kilometers away from the launch and they're even evacuated once the rocket is pressurized. Luckily, nothing bad ever happened to any humans during any of Virgin Orbits launches, but I still personally don't really love the idea of people being so close to a fully fueled liquid rocket in such a dynamic environment. And again, this was a bit different with Pegasus since it was solid propellant and it was actually quite inert before Ignition, in my opinion.
LauncherONE is one of the final nails in the coffin for air launching. I think they actually did absolutely everything right to maximize the success of the program. They used a Boeing 747, which has a ton of qualified pilots, trained mechanics, spare parts, and it even actually has a mount under the wing to secure the rocket that was originally used to transport engines. It's almost like that jet was made for this. This all means that operating the aircraft was financially much more reasonable than say, a custom one-off plane like StratoLaunch's massive jet called ROC. Now, I'm sure ROC can lift more, but at what cost? Obviously that platform hasn't really worked out as an orbital air launch vehicle either.
ROC is the world's largest jet, but despite this, it still can't even lift a rocket nearly as big as a Falcon 9. So you have an even more expensive carrier aircraft to fly and operate that could lift a large rocket, but even that big rocket still won't even have as much performance as even a really early basic Falcon 9's performance. And we could keep going with this thought experiment. Now, yes, you could potentially use a supersonic aircraft like an SR-71 or a Concord or maybe get into some wild hypersonic air breathing carrier aircraft, but the results are generally going to be about the same. You'll be spending a lot of money and time on research and development to build and operate an even more expensive vehicle that is at best still just doing a small fraction of the work that a traditional first stage would do.
You still have the risks and the scaling issues that you do with any other carrier aircraft. But now with even more considerations like aerodynamic heat loading at supersonic and hypersonic speeds and of course likely much, much, much more costs. I thought about going on and on with this and scaling it up more and more, but considering it was nearly impossible to make a business case using an off the shelf and relatively affordable 747, sadly, I think it's hard to take Orbital air launching any further. - So to summarize, I'm gonna say basically the same thing I said at the end of the last video.
Rockets are already pretty close to the edge of what's physically possible. So I understand why we crave to try and find ways to help squeeze even more performance out of them, optimizing them as much as possible and using existing technology and future technology to our advantage. But rocket science is really just the art of compromise, knowing what things are actually worth it, what things aren't worth it, and what things are actually just more expensive and harder and not beneficial in the long run.
Finding these compromises is the true art of developing a successful program, but when it comes down to it, the old K.I.S.S method, like my dad would call it or keep it simple, stupid, is what actually wins out. At some point. Building just a little bit bigger rocket is often much, much easier and more feasible than building a more costly and potentially risky launch system. But what would you do if you were building a launch system? Would you be building a traditional rocket or would you try to maximize performance any way possible? Let me know your thoughts or any other questions in the comments below. And stay tuned for the next part where we look at just adding jet engines to a traditional rocket instead of air launching it. We'll go over the pros and cons again and dive into the feasibility of adding high performance air breathing engines to rockets.
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2024-02-12 13:16
