Ep 480: Rockets pt. 2- Multi-stage Boosters

Missions, Physics, Space Flight | 0 comments

The vast majority of rockets are multi-staged affairs. Why is this? What makes this kind of rocket so successful? Today we look at the ins and outs of multi-stage rockets.
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Show Notes

Space Launch System
Space Shuttle system
Multi Stage rocket system
Tandem launch – Saturn V style (stacked on top)
Parallel style – Falcon 9 and Falcon Heavy and Space Shuttle (rockets on either side)
BFR (Big Falcon Rocket)


Transcription services provided by: GMR Transcription

Pamela Gay: This episode of Astronomy Cast is sponsored by Casper. Get $50.00 toward select mattresses by visiting Casper.com/Astro and using promo code Astro at checkout. Terms and conditions may apply.
Now, it may seem a little bit weird that this astronomy podcast has been sponsored for so long by a mattress company, but the thing is Fraser and I both adore our Casper mattresses. Both of us started out with our own 100 day trial of a mattress that both of us – Canada and the USA – could get a free return on – or free shipping on the return more to the point – and we kind of fell in love and have ended up buying more mattresses over the years and slowly filling our separate houses.
And I’ve actually upgraded from my Casper original to having a Casper Wave, which is a mattress that has various densities in various places to shape all the curves and crevices of the body to make sure that everything is supported the way it needs to be supported for the natural shape of your body.
Now, one of the other things that we kind of adore is – just like you can get our show anytime you want on the internet without having to interact with any human beings – you can get a Casper mattress anytime you want on the internet and it arrives in a cardboard box that may be made of neutron star material because it is amazingly dense and somehow unpacks into this glorious, full-sized, thick, glorious mattress.
And it’s gotten to the point that when I’m traveling, there are basically three things I miss and this isn’t always the order. I miss my husband, I miss my dog, and I miss my Casper mattress. So, get yourself a mattress that makes you miss it when you travel. Go to Casper.com/Astro and use the promo code Astro to get $50.00 toward select mattress purchases. Thanks.
Fraser Cain: Astronomy Cast Episode 480: Multi-Stage Rockets.
Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today. With me – as always – Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey, Pamela, how are you doing?
Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Welcome back from your trip.
Pamela Gay: Thank you and welcome back from yours.
Fraser Cain: Yes, Iceland. That was awesome – super fun. Before we get onto this week’s show, I want to thank the folks at Sumo for sending me a awesome beanbag chair. It’s not an ad, but they sent me one so I got a free thing, but it is like this gigantic easy chair, but made of beanbag – like a beanbag cylinder – and what it’s gonna be great for is actually setting it out in the yard in the summertime with my binoculars and watching the night sky because you can sort of lean back at different angles and rest yourself in the beanbag and look at the sky. So, this will be its purpose.
So, thanks to Sumo. I actually tried one out back at PAX many years ago and they’re pretty great and I do like these chairs. So, when they said, “Hey, we can send you one,” I couldn’t resist. And also thanks to people who have been sending me video games.
Pamela Gay: That’s awesome.
Fraser Cain: I got a copy of Stellaris’s new Apocalypse expansion, which was awesome. So, yeah, it’s really hard to walk that line between getting free stuff and maintaining your journalistic integrity. So, fair disclosure involved he says sitting in front of the telescope that OPT sent us. Alright, let’s move onto this week’s show. The vast majority of rockets are multi-staged affairs. Why is this? What makes this kind of rocket so successful?
Today we look at the ins and outs of multi-stage rockets. We were talking about this before the show. We kind of did this backwards that the multi-stage is the classic rocket that everyone is familiar with. The single-stage rockets are the more advanced, more technologically complicated, more brute force. So, why did we not do this in the other way? I don’t know. It’s fine.
Pamela Gay: Well, I can tell you why because I was going in numerical order.
Fraser Cain: One, many.
Pamela Gay: Yes, yes.
Fraser Cain: That’s done – answered.
Pamela Gay: That’s it. That’s all the logic there was.
Fraser Cain: Alright. So, what is the logic behind – speaking of logic – behind a multi-stage rocket? How does this thing work?
Pamela Gay: The basic idea is, “Why do you keep carrying your trash with you when you can just dump it off?” Now, as human beings, ideally we’re dumping it off in the recycling bin or the garbage, but if you’re a spacecraft, you dump it in the ocean. And so what happens is – as these rockets take off – they used up the fuel and a segment of the rocket – either the side boosters or the first stage – and then they get rid of that empty fuel containing now empty unit so they don’t have to carry the weight of that fuel tank with them. And that’s really all there is to it is, “Throw out your garbage as you go.”
Fraser Cain: But I think that people don’t realize the scale of how much of a rocket is fuel and how difficult it is to get to space; that it is worthwhile to get rid of that tiny little bit of aluminum and rocket engines so that you can carry more fuel. So, can you talk just a bit about the tyranny of the rocket equation and how – why this is even necessary?
Pamela Gay: The basic idea is you can accelerate faster by applying a force to a smaller mass and – as you take off – you have that constant, which is the mass of your spacecraft and all of the things and stuff that are currently attached to it. So, this is the whole space launch system. And then you also have the mass of the fuel and – as the mass of the fuel decreases – the mass of the spacecraft doesn’t unless you drop bits and the more mass there is, the harder it is to accelerate it. And so you just drop stuff. It’s just force equals mass times acceleration. You want to accelerate more; you hold your force constant and reduce your mass.
Fraser Cain: Right, but that point like 99 percent of a rocket is the fuel. It’s a big can of fuel. When you see a rocket on the launch pad, it is just this gigantic tube–
Pamela Gay: Or three cans if you’re the space shuttle.
Fraser Cain: Or three cans or three cans stacked on top of each other, but in general, it is cans of fuel with tiny little payloads on top of it. I think that’s the part that blows me away – pun intended – is how much of a difference staging rockets versus single stage is in the amount of launch capacity – of payload that you can get to orbit.
Pamela Gay: And one of the things that we talked about last week is there really isn’t a successful way to launch a human carrying or ISS restocking amount of mass into space without using multi-stages at this point. Now, when we say multi-stages, we’re referring to actually two totally different kinds of systems. We have the surreal or tandem launch – which is where you have the old Saturn Vs – one stage stacked on top of another.
The Saturn Vs were actually three, separate stages. Then, the other kind of system we have is the parallel stage, which is what the space shuttle was, what we saw with the Falcon Heavy. Falcon Heavy was actually a complicated mixed mode launch, but what we see with the space shuttle was its full launch system was those two solid rocket boosters that fired in parallel with the main engines on the space shuttle, which were drawing all of their fuel from that external tank.
Fraser Cain: Yeah, and I think the space shuttle is a great example of a fairly complicated array. You’ve got that space shuttle – that main fuel tank, which is liquid hydrogen and oxygen – with the orbiter bolted on the side with its RS-25 engines providing some of the power. And then you’ve got those two gigantic solid rocket boosters. And the thing with solid rocket boosters is once you turn those things on, you can’t turn them off again.
Pamela Gay: Right, and this is one of the extreme concerns for safety for a lot of these systems and this is where we’re starting to see interesting things going on with some of the new designs that some of the companies are coming up with. While space launch systems is continuing to have that solid rocket boosters mounted onto the liquid core, we’re starting to see the Falcon IX, this is stacked liquid fuel and – as we look at the Falcon Heavy – we’re now looking at essentially a central core that has two Falcon IXs mounted on the sides and it’s just gonna get crazier from there.
Fraser Cain: Right, exactly. Now, what is the downside? I mean if the upside is that you can essentially use this staging system to – as the cans of fuel are emptied out you throw the cans away – decreasing the amount of mass that you have to carry to space, continue, continue. What is the downside of going this way?
Pamela Gay: The biggest downside is you now have many more points of failure. With a single stage, you’re relying on one set of engines. You’re relying on one set of electronics. You aren’t worried about the explosives that separate the stages failing. You’re not worried about just all of the different pieces that can stop working. And this is something that I think every space loving little kid learns when they start launching model rockets.
An Estes rocket will teach you this in detail. You can run into problems where your first stage doesn’t separate correctly and now you’re carrying around basically a bomb that’s trying to fire while blocked. Now, luckily, that’s not a thing that seems to happen in the real world. It only happens to people like me as small children. But you have all of these different points of failure.
Fraser Cain: Hey, everyone, Fraser here. So, as a space journalist, I come across a lot of amazing stories, photographs, videos, and more and I just don’t have time to cover it all in video form, on Universe Today, talking about it in Astronomy Cast. But I’ve decided to put all this information overload to use and write a weekly email newsletter. I’m personally choosing the stories. I’m personally writing the newsletter.
No robots will be involved in the crafting of this newsletter. Just good old fashioned, artisanal Fraser neurons, but this is also a conversation, a way that you can comment back to me on the stories that you really like. It comes out once a week on Fridays. It contains a dozen or so of the most interesting news and stories I saw this week. Not just stuff from Universe Today, but from across the space reporting sphere. And of course, you can unsubscribe anytime. So, if this sounds like something that you’d like to receive, go to UniverseToday.com/newsletter and sign up. Now, onto Astronomy Cast.
Pamela Gay: With stage rockets, another one of the real frustrations that we brought up last week is they do have to drop these earlier components. So, we see with the Falcon IXs and the Falcon Heavy now that those first stages are currently returning to nice, friendly landing sites and happily landing upright in this amazing, synchronized routine. But we still don’t trust that this is going to happen consistently and – if they missed and landed on a suburb – that would be bad.
And so currently for most of our normal lower Earth orbit launches, many of our different geosynchronous orbits, we want to launch from the East Coast so that we can take advantage of the rate of rotation of the planet, launch out towards Europe, and then make sure that our early stages either land on a barge, land on the coast with nothing between them and the ocean to crash onto. Yeah, garbage collection it’s a problem. Don’t throw garbage at other people and – yeah.
Fraser Cain: We mentioned this last week that the Chinese rockets – because they don’t have a lot of space and they don’t have an eastern–
Pamela Gay: Well, they have an east coast. It just has Japan off of it.
Fraser Cain: Right, right, right. But the point being they don’t have a lot of space and they’re launching their rockets over populated areas and these boosters are coming down in forests and things like that. But you talked about that sort of a lot of moving parts. Anyone who has played the Kerbal Space Program, I defy you the first time you play that game to get the staging right. What you’re gonna do is you’re gonna stack up all your rockets in your first multi-stage affair and you’re gonna mess up the staging because it’s like a kind of programming.
So, your rocket’s gonna take off and then the whole thing’s just gonna come apart or the middle stages are gonna go off or the stages are gonna fail and you have no way to make it go. So, I think people have done model rocketry, but now Kerbal Space Program lets you experience this very quickly that staging is a very, very complicated affair. We talked about the Saturn. We talked about the space shuttle. What are some different configurations that we can see these staging arrangements happen in?
Pamela Gay: One of the big ones that many of us are eagerly waiting for the next launch of is the Delta IV Heavy. This is one of the most used primary rocket situations the Delta. It just goes kind of trustworthy system and the Falcon IV Heavy, it’s been kind of sort of in use since 2004 – by which I mean there was a partial failure early on.
But since then, it’s been used to do things like an uncrewed test flight of the Orion multi-purpose crew vehicle. We are waiting for it to launch the Parker Solar Probe and this is another one of those systems that has that multi-stage central core and then it has the side boosters attached. And so, we have this combination of parallel and tandem where you have the central core with multiple stages and then you have these side boosters as well.
Fraser Cain: Let’s go back to one of the most famous ones, of course, which is the Saturn V. How did that one operate?
Pamela Gay: So, the Saturn V was a series of series stages. There were three different stages that each got to a different part in the trajectory. And it was from the Saturns that we actually learned one of the big problems that you have with these multi-stage rockets is they leave junk everywhere. Now, that first stage it’s coming back down. But stages above that, periodically get abandoned in orbit where we stop calling them useful rocket stages and start calling them debris – potentially harmful debris – debris that might explode.
And it’s from these early rocket launches that we learned the necessity to – this was a new work I learned prepping for the show – basically increase the passivity of these rocket stages where the idea is you have to jettison anything that might explode so you now have an empty cylinder floating through space instead of an explosive, empty cylinder floating through space.
Fraser Cain: Like you’ve got to jettison whatever fuel is left inside, right?
Pamela Gay: You have to jettison whatever fuel – whatever explosive components it might have – because a lot of these things will use minor explosives to separate the stages, to separate off the boosters on the side. So, there are lots of little things that can go boom in the night and you have to get rid of them. Throw out your explosives, people. Throw out your explosives.
Fraser Cain: But when you really think about the Saturn V and the whole Apollo mission, they were more than a three-stage rocket. I mean when you think about, right, there was the three stages just to get the main part of the spacecraft into space. But then you had to go out to the lunar orbit and then you had to land on the moon and then you had to come back and then you had to boost from the moon back to Earth, etc., etc., etc. Someone did a calculation.
There was – I feel like it was Amy and I believe the technical number was a bajillion rocket nozzles used on that mission when you added up all of the attitude stuff and all of the booster rockets and all of that. It just went on and on and on.
Pamela Gay: And this is where you start separating the what are stages from what is you have liberated your internal spaceship to go do its own thing. We recently saw with SpaceX a gorgeous demonstration of this when they launched their Tesla. I’m not gonna go into the politics and the morality of launching a Tesla. I am going to say this was an excellent production that explains all of the different parts because we did see those first stages for the side boosters come down and land.
We did see the central core not land and then we saw that second stage that continued to push it higher and higher and higher and then we saw the fairing separation. And it’s in that fairing separation that the cargo is set free and that cargo is separate from the space launch system. With the Apollo missions that cargo was the lunar landing module, the command module, all of these different parts that allowed us to make it through this lunar space, get around the moon, land, return, and basically make it back home.
So, that is a separate computational electronic engine firing entity. A good way to think of it for the children of the ‘80s like us is I don’t know if you remember the Knight Rider TV show where Kit would drive up into the back of that semi-truck?
Fraser Cain: Mm-hmm. Sure.
Pamela Gay: Well, Kit was not part of the semi-truck. So, you can think of the semi-truck as the rocket that’s carrying around the cargo that also has its own capacity to – in this case – drive or if you happen to be the Apollo modules, to fly.
Fraser Cain: So, let’s talk about some of the interesting multi-staged affairs that are coming up shortly. I think the one that we should definitely talk about is the BFR, the Big Fantastic–
Pamela Gay: Fabulous.
Fraser Cain: Fabulous Rocket coming from SpaceX. That’s a multi-staged affair. How is that gonna work compared to say something like the space launch system coming from NASA?
Pamela Gay: So, with BFR, what we start to worry about is details of just how do they get all of the parts to fire simultaneously? With the space launch systems, it’s that tried and true central core, two side boosters, central core, two side boosters. We did that with the Delta. We did that with the space shuttle. We’ve seen with the Falcon Heavy different fuel components.
It’s no longer the solid rocket and liquid core, but this is something we know how to do. I’m trying to pull up the latest development pictures of the BFR because it’s supposed to have a lot more than two side booster, if I remember things correctly.
Fraser Cain: Well, the BFR’s got the – is essentially a two-stage rocket, right?
Pamela Gay: Okay.
Fraser Cain: So, the bottom stage of the booster, the BFB, the Big Fabulous Booster, is gonna be very similar to the existing SpaceX first stage, right? But it’s gonna have a ludicrous number of these brand new Raptor engines that SpaceX is working on. I believe it’s in the 30s. I mean there were 39. I forget the exact number right now.
Pamela Gay: It’s 31.
Fraser Cain: Thirty-one, okay.
Pamela Gay: Yes. So, I misremembered it. You’re right. It wasn’t side boosters. It was central engines.
Fraser Cain: Right, and then you have the top portion – which is the BFS, the Big Fantastic Spaceship – and that is going to have as well its own engines and be able to fly. And so you’re gonna stack the BFS on top of the BFB. That whole stack is going to fly to space and then both are going to be fully reusable.
So, the BFB is gonna just return to roughly the launch site – in the same way that the Falcon IX boosters do – and then the top part, the BFS, is going to deliver the cargo to space, do what it needs to do, and then do the same thing and return to Earth and land near the launch facility or they’ll be able to be used in an expendable manner where the whole thing is destroyed, but a lot of cargo gets blasted into space.
Pamela Gay: And it’s going to be interesting to see all the computational work that’s required to get all 31 of those Raptor engines firing simultaneously enough, steering, maintaining balance. And this is where a lot of these systems start to cause heart palpitations is if you don’t have simultaneous firing of all of these things, it’s sort of like jumping with your right foot before your left. You’re probably gonna fall over sideways.
Fraser Cain: the Soviets learned this horrible mistake with their N-1 rocket when they were trying to do their own trip to the moon. It was, well the Saturn V was this stack of a fairly small number of engines. The N-1 was a gigantic number of engines and this was before fancy computers and they couldn’t get the timing right and these things just kept detonating. The Falcon Heavy had 27 engines, right?
Pamela Gay: Yes.
Fraser Cain: Three times nine … no, wait. Yes, 27. So, that is more engines than have ever been operated simultaneously.
Pamela Gay: Yes. So, we’re making great progress by being able to use new and more kinds of engines in parallel to control all of these different things in truly crazy and awesome ways. And I can’t wait to see what comes out of all of these different plans. We had the original idea coming out of SpaceX for their ITS launch vehicle – which was another one of these ludicrous vehicles, their interplanetary transport system that was going to be able to launch to low Earth orbit 550 tons and we haven’t been able to imagine this kind of mass in my entire life.
Now, we’re like, “Yeah, why not? Let’s just launch instead of building a super lightweight James Webb. Let’s just build a new version of the VLT and just make some minor adjustments and we’re good.” This is a new way of thinking.
Fraser Cain: So, if stage rockets are the way and ideally reusable, what are the limits to this? Why aren’t there ten-stage rockets?
Pamela Gay: It really starts to come down to then you’re adding weight with all of the control systems on each stage. So, you have to find this magical balance – and it’s not magical. You have to find this computational convergence of your equations balance – which feels like magic.
Each stage has electronics, has engines, has all the bits and pieces that allow them to separate and all that mass adds up and then you are losing that mass, but the first stage still has to launch the six stages above it and all of their engines. And there’s tradeoffs on how many stages do you want to use so that you can drop the appropriate amount of garbage and how fast does the weight from each of those stages add up limiting your ability to launch more effectively.
Fraser Cain: And it is that, as you said. You know it’s the weight of the nozzles, the weight of the electronics that really decide whether you’re going to because I guess in a perfect world, if all of that stuff weighed almost nothing – although the tank part weighed a lot – then you would make more stages. You would just slough them off as quickly as you could and then you would – in the end – have the perfect rocket equation.
Pamela Gay: And this is one of those things that you see in model rocketry where people do launch eight-stage, 12-stage insane rockets and this is because the only thing you need to go between the stages is a special fuse that will use your first stage to launch your second stage, use your second stage to light your third stage and you have this super slow acceleration at the beginning where that first D is like, “I don’t know how I’m gonna do this” but then it gets faster and faster and faster as it goes because you can accelerate over a much longer period of time. It’s pretty awesome.
Now, the one thing I do want to hit on is the first multi-stage rockets actually pre-date our countries. This is one of those things we forget that rockets aren’t a modern thing. Sure, Goddard was the first one to figure out how to do liquid rockets, but folks in China and Korea were in the 1400s and onwards working to develop fireworks, working to develop rocket-powered arrows, working to create baby missiles that they fired at one another. And so this is not a new concept.
It is just a modernly perfected to go to outer space instead of to destroy thing concept. But as early as the 1400s, they were able to get rocket arrows that went 200 meters and that’s kind of cool.
Fraser Cain: I’ve never done any model rocketry.
Pamela Gay: What is wrong?
Fraser Cain: I know, I know, I know, I know. I think I’ve been around for like one model rocket launch.
Pamela Gay: You have got to do this.
Fraser Cain: I take it you’ve done a bunch of it?
Pamela Gay: I have launched so many rockets and exploded so many rockets and climbed trees to retrieve rockets and–
Fraser Cain: Well, let’s cue that up for an episode then and we can talk about your experiences and how to get into the hobby of model rocketry and how not to blow your hand off.
Pamela Gay: Well, I think – well, I actually tried that one as well, but the most important lesson I learned is on the rockets that are designed to launch things and critters, only launch critters with exoskeletons because critters that don’t have exoskeletons can’t sustain high G’s. And when you can’t find a cricket to launch, do not replace the cricket with an earthworm. You will regret all of your choices and the earthworm will regret them more.
Fraser Cain: I’m gonna suggest don’t ever launch anything alive.
Pamela Gay: Oh, crickets are fine. They may not like it, but they’re fine.
Fraser Cain: Alright. Alright, well we’ll figure out what to do into the future. Thanks, everyone, for joining us and we’ll see you next week, Pamela.
Pamela Gay: Sounds great. Bye, bye.
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[End of Audio]
Duration: 31 minutes

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