Rockets are HUGE, complicated and expensive. As a matter of fact, the rocket that took humans to the moon, the Saturn 5, was 111 meters or 363 feet tall, and had more separation events than dating teenagers.
So why do rockets always split themselves into multiple parts. Isn’t that complicated and risky? Why throw so much away? I mean, there’s got to be a better way!!!
Well how about if rockets were only ONE stage? How awesome would that be? Well this idea isn’t new… it’s called single stage to orbit or SSTO and it’s often considered the holy grail of rocketry.
Well, today, I’m going to SMASH THAT HOLY GRAIL and explain why I think SSTO’s SUCK.
Woah woah woah, yes, I’m sure you DID build an awesome SSTO space plane in Kerbal Space Program that can put 2 large Orange fuel tanks to orbit, yeah me too… but… In order to drill this point in we’ll teach you all about the tyranny of the rocket equation and help you understand why every orbital rocket, well, ever is multistage.
Then we’ll take a stroll down SSTO history and look at some crazy designs that in some cases almost worked…
And not to be a huge downer, we will take a look at some SSTO designs that MIGHT actually work, including the Skylon spaceplane that uses the awesome SABRE hybrid engine.
Ok, Everyday Astronaut VS SSTO’s… but hear me out, let’s get started!
But what if I told you, I can’t hardly think of one good reason to build an SSTO in real life? Well there’s a few potential benefits, but do they outweigh all the negatives?
Ok… ok… so before I sound like some old grump here “Get off my lawn you darn kids with your SSTOs”, let’s talk about why SSTO’s are considered desirable by some and why people dream about a time when launch vehicles are only one stage…
Imagine a world where a launch vehicle takes off, goes to space, comes back, refuels and does that multiple times a day. No problem. Sounds like a real life millenium falcon.
When most people use the term SSTO it’s inferred that the vehicle would be reusable… it would go up in space and come back down all in one piece, throwing away only fuel to get it there and back.
And again, when talking about reusable SSTOs most people and concepts utilize a spaceplane design. A vehicle which takes off and lands like a plane.
Sounds great right? One vehicle to do it all! Nice and simple. Take off from a runway like any other jetliner at an airport, and then instead of leveling out at a boring old 10 kms or 35,000 feet or so, keep accelerating and increase your altitude until you’re in orbit!
Then when your mission’s done, just come back down from space and land on any old runway, again, like any other jetliner.
OK, SSTO’s don’t sound bad… do they? Uh oh, have I lost my mind??? Well, before we keep on dreaming about our potential sci-fi future, let’s take a quick look at the history of rockets to see why so far, SSTOs haven’t been attainable.
The first liquid fuel rockets ever made were just one stage. Basically a rocket engine, some propellant inside the fuselage and in most a cases, some kind of warhead on top. Rocket’s were mostly used as an advanced weapon delivery system at first, but lucky for us, they also so happen to be able to be used for spaceflight!
The V2 rocket built by Germany, were the first rockets to ever reach the edge of space. In June, 1944, a V2 rocket on a test flight reached beyond the karman line, the most commonly agreed upon boundary of space at 100 kms or 62 miles in altitude.
Sure, a single stage rocket could get up to space, but what about stay in space? Well in order to stay in space, an object needs to reach orbital velocity. To orbit the earth, a vehicle needs to be traveling at about 28,000 km/h or 17,500 miles per hour to stay in space.
Ok… so now we have a new goal for humanity. Get something into orbit.
Well on October 4th, 1957 the Soviet Union managed to place Sputnik 1 into orbit. The shiny metal ball with scary looking spiky antennas weighed only 83.6 KG or 184 pounds, hey that’s pretty much my weight!
The vehicle that carried Sputnik to low Earth orbit was a two stage launch vehicle most commonly known as the R7. This rocket was revolutionary and allowed the vehicle to ditch unnecessary weight on ascent by ejecting spent fuel tanks and rocket engines that were no longer necessary.
This is called staging. It was revolutionary and pivotal in putting anything of significance into space.
Staging is the number one cure for the tyranny of the rocket equation. The tyranny of the rocket equation is basically the diminishing returns on adding fuel.
If you were to double the fuel in your rocket, you don’t double the delta V or change in velocity… Due to now having to push around all the weight of the extra fuel and fuel tank, the rocket is able to burn longer, but it doesn’t receive anywhere near double the change in velocity.
And it only gets worse the further you go until the rocket actually gets too heavy for the engine and next thing you know you’re adding engines to lift the extra fuel and so on and so on.
So this is where staging comes in. Once the rocket empties a fuel tank, why not throw it away? Even better, if the rocket is throttling down an engine to stay accelerating at a safe speed, why not throw away the heavy, unnecessary engines.
This is what the R7 rocket did. It was basically a good sized rocket with 4 extra rockets attached to it. Once those side rockets were spent, they were discarded in this really cool formation known as the korolev cross, named after Sergei Korolev… who you can think of as the Werner Von Bruan of the Soviet space program.
BTW, you can still see the Korolev Cross on any of today’s Soyuz launches. I love watching that! So cool!
Separation events are often a breath holding moment in flight. There are two main types of staging events.
First there’s parallel staging where multiple stages are fired and active at once, like Soyuz, or the Space Shuttle, or Falcon Heavy.
Everyone holds their breath during staging because if a booster doesn’t separate, the mission will fail. It was definitely a big moment when the side boosters of the Falcon Heavy separated safely.
Or there’s an even more complex and nail biting type of staging called tandem staging. This is what a more traditional multistage rocket does.
It fires one stage first, then that engine cuts off, the 1st stage and 2nd stage separate, the 2nd stage fires and keeps going to orbit as a brand new fresh rocket.
Not only is the 2nd basically a new fully fueled little baby rocket, but their engines are also optimized for vacuum operation by having a much larger exhaust nozzle. We’ll talk more about this in an upcoming video, but, that’s another huge advantage for staging. Having different engines optimized for different environments.
The first rocket to ever do any kind of tandem staging was called the RTV-G-4 Bumper. It was literally a V-2 rocket with a small sounding rocket on top of it. It was launched between 1948 and 1950 and launched 8 times, creating a lot of valuable data on multi staging.
But this stage separation event was really hard to accomplish and almost considered impossible for a long time. That’s why many early rockets utilized parallel staging.
As a matter of fact, separation events, or more specifically a botched separation event almost lead to SpaceX being another forgotten aerospace company who went bankrupt after only 3 flights attempts.
Flight 3 of their Falcon 1 rocket had a separation event go wrong when the first stage had a little residual thrust after stage separation, causing it to bump into the upper stage, leading to a failed mission.
So if it was so coveted and necessary to make multistage launchers, why are SSTO’s so sought after?
Well, there’s something to say about making a less complicated launch vehicle. Maybe SSTOs are good for something…
Alright, so I can already hear you over there saying but but but… and the first but is probably “but Elon musk said the Falcon 9 booster could reach orbital velocities…”
Sure it can. Now good job. You just put a $30 million dollar piece of hardware up in space where it has no spare margin to carry anything more than a small backpack and no remaining fuel to re-enter or land. I think there’s better ways to put a backpack in space…
And again… Elon said that the BFS, the spaceship portion of the big falcon rocket is capable of reaching orbit by itself with a low payload but using the booster, it can deliver more than an order of magnitude more payload.
Well first off, it would require firing the 4 vacuum raptor engines at sea level which Elon says, “I wouldn’t recommend.” Ok, so combustion instability aside, let’s pretend it could put 15 tons into orbit AND somehow have enough margins to deorbit, and safely land…
So… now we flew a BFS to space and back so it could deliver something smaller than a Falcon 9 could do. The cost of range, personnel and of course refurbishment of the ship need to be considered.
These things considered, do you think it’s cheaper to just fuel up the booster too, since a lot of the costs are inherent to the launch cost and the fuel is relatively cheap.
Since the booster experiences much less reentry heat than the upper stage, it can be flown a lot more often without maintenance and refurbishment costs.
Just like how block 5 of the Falcon 9 should be able to fly about 10 times without any scheduled maintenance or refurbishment yet the upper stage is still elusive when it comes to reuse.
So why not utilize the entire vehicle? In the best case scenario you just launched 1/10th the payload capacity just to save a little on gas money. Once an entire vehicle is routinely rapidly and reliably reusable, woah that’s a lot of R’s… then who cares if it’s two stages or one.
In best case scenario 2 stages is better than one and in worst case scenario one stage doesn’t even work at all.
Ok, but let’s not be all poo poo. Let’s take a look at some SSTO designs past, present and future to see if there’s any that seem promising.
Let’s start off with some previously proposed and pursued SSTOs and look at why they failed.
Well, one of the earliest proposals was the One Stage Orbital Space Truck or OOST by Phil Bono of Douglas Space in the 1960’s. Later he proposed a reusable version called ROOST.
Another proposal from the 60’s was the NEXUS rocket which would’ve been HUGE. I’m talking HUGE. It would’ve been 122 meters or 400 feet tall, and a width of 50 meters or 164 feet!!!! HOLY MOLY.
Ok, so paper rockets are one thing, but how about a rocket that was actually being tested? Look no further than the DC-X or Delta Clipper Experimental made my McDonnell Douglas.
The DC-X was was an actual working 1/3rd scale prototype of a proposed DC-Y SSTO that was to be capable of putting about 1,300 kgs or 3,000 pounds into orbit.
The DC-X was just to demonstrate vertical take off and landing and it actually flew 8 times between August 1993 and July 1995 and pretty successfully… but it only reached a maximum altitude of 2,500m. Think of it like SpaceX’s grasshopper.
In 1996 NASA took the program and turned it into the DC-XA which made some improvements to the vehicle. It flew four times, including a 26 hour turn around and setting a new altitude record of 3,140 meters or 10,300 feet.
It’s last flight was on July 7th of 1996. After a landing strut failed to extend and a lox tank leaked, causing a fire and damaging the vehicle.
Despite a relatively low cost to repair and continue working, NASA cancelled the program and looked to pursue Lockheed Martin’s VentureStar… So let’s talk about that!
VentureStar is probably one of my all time favorite spacecraft designs. See… I don’t HATE SSTOs….
Lockheed Martin proposed a space shuttle replacement in the mid 90’s and received funding from the U.S. government to work on development.
The VentureStar ticked all the SSTO boxes. It would’ve been amazing and it actually got painfully close to flying… well at least a subscale, suborbital version. But here’s the rundown.
It had advanced carbon fiber construction, a giant linear aerospike engine and could take 20,000 kgs or 45,000 pounds to LEO. That’s close to a Falcon 9!
It would take off vertically like a rocket and land horizontally, just like the space shuttle.
Unfortunately, the subscale x-33 demonstrator was cancelled in 2001 despite being really close to flying. The X-33 demonstrator had 96% of its parts manufactured and was 85% assembled… and even the launch facility was complete!
The reason for its cancellation was a long series of technical difficulties, flight instability and excess weight. DARN YOU WEIGHT!
That got so close to flying, it hurts.
Next how about maybe the craziest proposed SSTO of all time. The Roton. This thing is hilarious. This… is a helicopter that could get to orbit.. Supposedly.
So basically it was a helicopter powered by jet tips… So some small thrusters at the end of the rotors. It would lift itself using the low powered jet tips spinning the helicopter rotors until the atmosphere got too low where it would then light up a rocket engine and ascend to orbit using the rocket engine.
The rotors weren’t just dead weight once in space either. Instead of providing lift, they would continue spinning to power the turbopump for the rocket engine. Then they would also be used to slow down through descent in the atmosphere and used to land softly.
Despite a full scale atmospheric test vehicle being built, and flying… well sort of… the program was cancelled in 2001 due to lack of funding and people saying the technology wasn’t valid and it was technically impossible on available technology.
Apparently it was horribly unstable when flying like a helicopter… actually the vehicle was found to be unflyable by anyone except the test pilots who even then had periods of being entirely out of control… yikes.
But I still LOVE that wacky thing.
Ok so what about some current proposals? It’s been more than a decade since those last proposals ended, there’s got to be some new technology we can apply and make these things happen, right?
Well, you can’t talk about SSTOs without talking about perhaps the most alluring SSTO, the Skylon space plane. The Skylon is being designed by the United Kingdom’s Reaction Engines Limited and utilizes an amazing combined-cycle hybrid rocket engine called the SABRE engine.
Now this is the one concept I can sort of get behind. The Skylon’s SABRE engines act like a fairly traditional jet engine. It uses the atmosphere to its advantage.
Instead of a traditional rocket that tries to get out of the atmosphere as quickly as possible by ascending virtually straight up for a minute or so, the Skylon will reach 5 times the speed of sound or around 6,000 km/h or 3,800 mph while pulling in oxygen from the atmosphere, just like a any other jet.
Then the SABRE engine then closes its intake and turns into a more traditional rocket engine where it’ll burn fuel and oxidizer to reach orbital speeds.
The SABRE engine has received even further funding from the United States’ DARPA, Boeing & Rolls Royce to build a high temperature test facility which hopefully will begin testing this year.
So the Skylon actually has some promise! But… even though it’s technically theoretically possible, even the makers of the Skylon seem to be backing down on its SSTO potential, at least for now.
Reaction Engines is currently pursuing non SSTO vehicles first, much before the Skylon will ever fly.
In 2017, Mark Thomas of Reaction Engines said they’re currently pursuing a spaceplane as a 2 stage vehicle where it would deploy an upper stage at a high altitude and a high velocity and then the space plane would turn around and land.
So… even with this crazy awesome air breathing engine, they’re finding it too impractical currently to make the Skylon an SSTO. Again, it’s just plain hard.
And one more thing while we’re talking about Skylon. Remember the SR-71 blackbird? Even though it could only reach mach 3… it still experienced so much heat that it needed to have large gaps in body panels since it expanded by 60 cms when flying at speed.
This led to it basically peeing its pants and dripping fuel when fully fueled on the runway. Now can you imagine something going even faster in the atmosphere? Well this is the 21st century after all, I don’t think the Skylon will pee its pants on the runway, but the crazy heat at high mach speeds in the atmosphere might be a huge hurdle.
I personally foresee there being many fairly substantial issues to making the Skylon actually work. It’ll be amazing if it does, but for now, it’s stuck in future hopes and wishes to me.
If you guys want me to do more about Skylon, let me know. We could probably do a whole article on it, it’s really a cool vehicle.
And lastly… we should probably mention Arca space’s Haas 2CA… This to me, is pretty silly.
It’s an SSTO rocket with a linear aerospike engine… cool. BUT, it’s only capable of 100kg or 220 pounds to orbit… ummm. Maybe they should just add a small upper stage on that thing and put 1,000 kgs into orbit since they’re throwing away the whole thing anyway.
My big question for Arca is… why. What’s the point? The ONLY thing they claim thats an advantage is they can launch inland because there are no spent stages… Hmmmm….
Not sure how I feel about that. I mean yay for aerospikes, but boo for a rocket with such limited capability.
Ok… so all past SSTOs have failed, all current SSTOs are either not going to be SSTOs or are kind of pointless in my opinion… and there really isn’t much on the table for a usable SSTO in the near future without some major breakthrough in material science or propulsion.
BUT WAIT. There’s ONE MORE THING. So far I’m here talking about how all SSTOs failed… that’s not true! As a matter of fact, one of the most famous spacecraft in all of history was an SSTO… the lunar excursion module!
Of course it could, but only on the moon! The LEM was capable of achieving lunar orbit using only one stage. But… that’s the moon. The moon’s gravity is much weaker than Earth’s gravity AND there’s no atmosphere to fight against.
And then we have SpaceX’s BFS which will be capable of not only SSTO from the surface of Mars, but even have enough performance to get all the way back to Earth from the surface of Mars in a single stage.
This is mostly due to Mars having only 38% the amount of gravity of Earth AND having only 1% the atmosphere of Earth. Making achieving orbit, much much easier.
So maybe SSTOs don’t suck. Maybe Earth sucks. It has just enough gravity to make it barely possible to achieve orbit with rocket engines and it has that pesky atmosphere which slows vehicles down on ascent.
So…. how are you feeling? I still stand by the fact that as cool as SSTOs are, and as much as I do actually love them, they just don’t really work in practice using currently available technology. Of course I’m not saying that’s how it’ll always be, but for now, give me them stages!!!
After all. I think we can all agree the most important aspect of an SSTO is the reusability thing. So what if a multi stage rocket IS fully reusable? Like the BFR? Isn’t that what matters most?
The first stage does what it needs to do then comes back and is refueled and reused. Same with the upper stage. So who cares if the vehicle does it in one peice or breaks off into two more dedicated pieces.
I can’t wait for a day when orbital flight is routine and reliable and fully reusable. And for the foreseeable future, I think it’ll continue to be done in stages…
So what do you think? Do you think SSTOs are still valid and practical or are you on team multistage? #teammultistage
Let me know your thoughts on SSTOs in the comments below. And PLEASE, spare me your Kerbal SSTO designs… It works in Kerbal Space Program is not a real argument… unfortunately.
And before you tell me all about how SSTOs can take off and land on runways… remember, that’s NOT exclusive to SSTOs. That’s an air launcher and or lifting body advantage. That is not exclusive to SSTOs, so I’m not going to say that’s much of a check box…
Let me know if you have any other questions or things you want me to cover in future articles!
As always, I owe a huge thanks to my Patreon supporters for helping make this and other Everyday Astronaut content possible. I owe a special thanks to those Patrons in our exclusive discord channel and our exclusive subreddit for helping me script and research. If you want to help contribute, please visit http://patreon.com/everydayastronaut
Don’t forget to check out my web store for shirts, hats, mugs, prints of rocket launches and original artwork and lots of other fun stuff at http://everydayastronaut.com/shop
Thanks everybody that does it for me. I’m Tim Dodd, the Everyday Astronaut. Bringing space down to Earth for everyday people.