We’ve seen rockets blast off from here on Earth. But that’s only half the story. Rockets have additional stages to push them into trajectories, like transfer orbits and various orbital maneuvers. Let’s talk about what happens after the rocket is long gone, beyond our sight.
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Pamela Gay: This episode is sponsored by Casper. For $100.00 off your Wave purchase, visit Casper.com/Astro100 and using promo code Astro at checkout. Terms and conditions may apply.
Fraser Cain: Astronomy Cast Episode 481: Upper Stage and Beyond.
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: I am doing great. I have no news. Is there any news, anything interesting to talk about? Should we just get right into this week’s show? There must be something.
Pamela Gay: So, I got to see some of the wonderful folks that were with us on the eclipse trip while I was in Amsterdam and that was fabulous. Thank you, everyone. And I will be in Tokyo in a couple of weeks and Kyoto and a city that begins with the letter F that I can’t pretend to know how to pronounce because I don’t. I will be announcing all of these things and, hopefully – if you live in Japan and you’re one of our listeners – I’ll be able to get to meet up with you too.
Fraser Cain: Right on. And we finalized our trip to Australia. So, I’m flying out from Vancouver to Australia July 3rd and I’ll be there for two weeks. So, I know we’ll be getting to the Brisbane area and then I think we’re gonna go north from there after I’ve fulfilled my speaking gig. But I know some folks in Sidney may want to get their hooks into me. So, there you go. That’s when we’re gonna be in that neighborhood.
Pamela Gay: And I found out I’m going to Sidney and probably Melbourne in October. So, you get both of us in two separate months.
Fraser Cain: Right on. We’ve seen rockets blast off from here on Earth, but that’s only half the story. Rockets have additional stages to push them into trajectories like transfer orbits and various orbital maneuvers. Let’s talk about what happens after the rocket is long gone beyond our sight. I think this is great. We get a chance to see all of these rockets take off. We’ve seen the space shuttle launch. We’ve seen video of the Saturn V launch. We experienced the Osiris Rex launch in Florida.
It’s one thing to see the rocket go, but that’s really just a couple of the engines on that entire rocket. Everything else that goes on – all the various orbital maneuvers, all of the different additional engines that fire – they happen when you can’t see the rocket anymore. So, let’s talk about some of those additional stages that you learn about, but really beyond just them being names, numbers, times, burn durations, things like that, you don’t experience it in the same way that we experience that first stage taking off from Earth.
Pamela Gay: It’s true. So, the most important part is getting off the planet because if you don’t get off the planet, everything else is kind of for naught because you’re now smashed into the planet probably. Once you get off the planet though, you have to make sure you don’t come back in unexpected ways and the way orbits works is they want to be an ellipse and – if you’re taking off from Cape Canaveral, you’re taking off from New Guinea, you’re taking off from wherever you’re taking off from – once you get to the highest point in your orbit, if you don’t fire your thrusters, you’re coming back.
Fraser Cain: Right. No matter where you launch from, no matter how you did this, your choices are escape velocity from Earth or you’re retuning. I mean I love this idea that you can’t in one orbital maneuver leave the Earth, that Superman can’t punch you into orbit as much as that has been described, right? Superman can punch you onto a trajectory that’s gonna bring you back down to Earth and it might take a while.
Pamela Gay: He can punch you out of orbit.
Fraser Cain: Right.
Pamela Gay: So, you can just leave Earth behind and keep going, in which case it wasn’t so much an orbit that was an ellipse as an orbit that was a parabola or a hyperbola, as opposed to a hyperbole, which also doesn’t work for keeping you in orbit.
Fraser Cain: Right.
Pamela Gay: So, in general though, unless you escape Earth entirely, you have to fire thrusters again. So, we see rocket, rocket, rocket, rocket, rocket shuts down and then, if you keep listening – which most people don’t do – to that SpaceX broadcast, that NASA TV broadcast, what happens next is you hear about an orbit basically circularization of orbit. This is where when yo8u get to that peak of your initial take off.
Fraser Cain: Your apogee.
Pamela Gay: Your apogee. I was trying to explain it with words that everyone would know.
Fraser Cain: Right.
Pamela Gay: Once you get as far away from the Earth as you’re going to get before you start coming back, you have to fire your rockets and you have to change your position so that you’re firing them in the direction around the planet that you wish to be going to increase your velocity in that sideways direction so that you don’t come back.
Fraser Cain: And so you are circularizing your orbit.
Pamela Gay: Verb it – just verb everything.
Fraser Cain: You are orbitizing your orbit. You are–
Pamela Gay: Circularizing. It was already an orbit, it was just a ballistic orbit.
Fraser Cain: Ballistic, right. So, you’re just removing that hard landing.
Pamela Gay: Yes.
Fraser Cain: You fire this – and there’s this really – and I know you haven’t played Kerbal Space Program much – but there is this really satisfying point when you launch a rocket in the Kerbal Space Program. Your rocket gets out of orbit. You can see this ballistic trajectory. You then have to wait while your rocket keeps moving along this ballistic trajectory. You get to this high point – this apogee – and then you reignite your rocket, continue to fire it, and you see that ballistic trajectory turn into an orbital trajectory and you’re like, “Whew. I don’t have to kill another Kerbel today.”
Pamela Gay: But they look so happy.
Fraser Cain: They don’t care. They’re fine. They’re troopers, but you can see that trajectory change from this ballistic trajectory to this orbital trajectory and you know they’re safe. Now, they may never come back down, but they are safe and they’re not gonna crash into the planet and it is always this incredibly satisfying process every time you do it. Again, I’ve mentioned this in the past, if you want to understand this play Kerbal Space Program. You’ll get it deep in yo8ur bones. So, you fire that rocket and what is that about? Now, in the real world, what does this?
Pamela Gay: When you’re in an orbit, your velocity is constantly changing and this combination of how fast you’re going and what direction you’re going in – at any point in combination with the mass of the thing you’re going around – and you can ignore your own mass unless it’s huge. So, the mass of the thing you’re going around, your velocity – which includes speed and direction – those two things will pretty much allow you to figure out, “Am I going to die?”
If you’re on an elliptical orbit, you’re going to be sweeping out a significantly smaller angle over time. You’re gonna be moving slower when you’re further away from your center object and then you go fastest when you’re in close or about to die in the case of ballistic orbit. And if you’re instead on a circular orbit, you’re gonna be going at a constant rate the entire time and that constant rate at that same distance is much faster than that slower rate where you’re about to get pulled back in.
So, what you want to do is increase your velocity from the, “I’m going to die” plunging back down to the planet slower velocity to the “Oh, I’m just going to keep a constant distance from the Earth, from the Sun” from whatever your going around velocity. Now, the neat thing about this is if you then decide, “Hey, I’d like a bigger orbit.”
The way you get there is you then fire yo8ur engines again, increase your speed, and that will set you on a new – not new circular – but a new elliptical orbit where your closest point is that starting point where you fired your engines. And then you’re going slow when you get up to the top and you have to fire again to circularize your orbit. You can constantly increase yo8ur orbit by going from ellipse to circular, ellipse to circular. Normally, you don’t do this too many times, but it’s a good, cautious way to do things.
Fraser Cain: And what is the gadget that you use to do this?
Pamela Gay: We might call them thrusters.
Fraser Cain: Right, or rockets.
Pamela Gay: Exactly.
Fraser Cain: Upper stages.
Pamela Gay: Upper stages. We also have onboard engines. We have for a while there were these neat little add-on engines that they had when they let things out of the space shuttle cargo bay. The Canada arm would reach in and grab out the thing and it would fire off and that was kind of cool.
Fraser Cain: So, what’s an example then of an upper stage that we use today?
Pamela Gay: Well, so there’s the upper stage and then there’s the specific add-ons that you have rocket for rocket for rocket. So, one of the ones that everyone is probably most familiar with is that weird, black thing that they mounted Elon Musk’s Tesla to.
That was an upper stage. It looked like a whole bunch of aluminum pipes painted matte black that they mounted a car to, they mounted some engines to, and they were able to change their orbit where they went around the planet a few times. We all watched it. Then they fired the engines attached to their platform of Tesla and that was their next stage to get them on their way out to the asteroid belt because they needed to miss Mars, apparently.
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Fraser Cain: One kind of interesting story from space history is the Galileo spacecraft took a fairly long journey from Earth to get to Jupiter because it had to use gravitational assist from several of the planets because it didn’t have a powerful enough upper stage rocket and the reason it didn’t have a powerful enough upper stage rocket was because of the Challenger accident.
They decided that they were only going to permit a certain essentially explosiveness of an upper stage in the space shuttle, which released the Galileo spacecraft. So, it’s interesting to think about how they’re not little putt-putt engines. They can be really powerful engines required to kick things onto the kind of orbit that takes them to have the kinds of velocity changes that are needed to reach some of these outer planets in the solar system.
Pamela Gay: And one of the things that I have to admit is of great pain to myself is one of the main systems that was used for a long time is the PAM booster, which is where I thought you were going and I was like, “I am not going to say that crap. He didn’t go there. I had to say it.”
Fraser Cain: No, I wasn’t, but I should now.
Pamela Gay: So, it was the Payload Assist Module and it was just an upper stage that McDonald Douglas put together and it was used on a bunch of the Atlas and Delta class rockets. There was one that was used on the Ulysses spacecraft. And so there’s a bazillion different of these. That one has been my personal curse. There are certain things you don’t want to be name after; non-stick spray and orbital booster rockets are both on that list, but – beyond that – where we see these coming up over and over the main ones that a lot of us have heard about are the Centaur Upper Stages.
They’re used with the Atlas and the Titan series rockets. There’s all sort of different – there’s the Agena, which is again used with the Atlas, the Thors, the Titans. But then you can have completely custom systems – which is pretty much what SpaceX did.
Fraser Cain: So, what is going to be your decision into what you’re gonna use as an upper stage? Because they don’t have to work in the atmosphere of the Earth, so what are your design constraints to building an upper stage?
Pamela Gay: The kinds of things that you worry about are what is the Delta V I need to accomplish? How many times do I need to fire? How long do I need to be able to keep adjusting what my spacecraft can do?
Fraser Cain: Sorry, Delta V?
Pamela Gay: The change in velocities that you have to be able to accomplish. So, it’s one thing to say, “I need to get out of the Earth’s orbit and vaguely heading towards Mars and I’ll use gravity assists as needed” in which case you probably want to head towards Venus for the gravity assist and then get whipped around towards Mars.
But if – on the other hand – you’re like, “I’m just pulling a new horizons and I’m just gonna get going as fast as I can and aim toward Pluto” you have different constraints in how you’re going to do it and then there’s also the case of things like Rosetta – where it wasn’t so much the upper stages is I think the wrong term – but Rosetta had to wake itself back up after years and years, catch itself up to that little comet – and you don’t orbit a comet. They don’t have enough mass for that to happen. So, it had to have onboard thrusters capable of not orbit maintaining but station keeping next to the comet.
Fraser Cain: What are the kinds of propellants that they’ll use in some of these upper stages that might be different from what they’re gonna use for the lower stages?
Pamela Gay: I think the chemical that we’re all most familiar with from when things go terribly wrong is hydrazine. This is a terribly, terribly poisonous chemical that also has a great ability to burn and produce a lot of expansion, which means a whole lot of thrust. And if you want to carry something that will get your spacecraft where it needs to go, hydrazine’s a great thing to have. But when it comes back down to the planet, we have to collect all of the bits and pieces and make sure that we aren’t destroying our planet because we like our planet, usually.
Fraser Cain: Interesting sort of reference to this. If you remember The Martian and he was trying to make water, he was making that from hydrazine – which is also incredibly toxic and would have killed him if he’s breathed any of the vapor or if he’d gotten it on his skin it would have been bad for him – but it has lots of nitrogen and hydrogen in it and hydrogen is the key.
Pamela Gay: It’s N2H4.
Fraser Cain: Yeah, and the hydrogen is the key because that’s the thing that helps you make water.
Pamela Gay: So, this is an inorganic compound – thus the lack of carbon atoms in it – and you can oxidize it with all sorts of different things and it’s this oxidation process that leads to the glorious, upliftyness of rockets that we get and death if a human is nearby – unless, of course, you are The Martian – in which case you’re good.
Fraser Cain: Well, that’s why the liquid oxygen hydrogen of the space shuttle was so lovely when those big, main engines go off. You just see just water vapor. It’s like clear. It’s like you could just take a drink from the exhaust plume – except for the solid rocket boosters. You don’t want to drink from that.
Pamela Gay: And there are other techniques and we’re gonna talk about some of these next week. For instance, with the Dawn Mission, instead of carrying around all this terrible fuel, they knew that they needed to be able to go and go and go for years and years and years and they wanted to have maneuvering capabilities and they didn’t care if they got anywhere quickly. And this gets to other questions that you have to answer. Do you care how long it takes you to get to your place? Then, if you don’t, go slow, go stead, and do it in a way that doesn’t weigh a lot like with an ion engine.
Fraser Cain: Right. So, we’ll talk about ion engines and maybe some other exotic forms of propulsion next week. But we saw one of the problems with rockets with the recent Falcon Heavy launch. The twin boosters landed side-by-side. It was a beautiful thing, but the central core didn’t land because it ran out of ignition juice.
Pamela Gay: Well, it didn’t run out. It failed to fire. Well, I guess that was ignition juice because they didn’t ignite. They had all the fuel; they just didn’t light the fuel.
Fraser Cain: And this is one of the challenges in the back of this thing you know you can just continuously keep your rocket burning to put yourself on a new orbit, but the most efficient way to do it is you want to turn your engine off. You want to coast to that new orbital position. You want to relight your engine and then you want to continue onward. And so that’s a challenge as well, that relighting, that restarting. It’s always a very tense moment.
We’ve seen this many times with various missions. Cassini had to do it. Galileo had to do it, as you said. Rosetta had to do it. Almost every time you send one of these deep space missions are sent out there, there is this moment where it has to relight. It has to perform an orbital maneuver and the question is, “Has the last couple of years of it doing nothing caused it any problems?”
Pamela Gay: And it’s kind of terrifying. And when we’re in low Earth orbit – and by we, I mean people who aren’t you and I – when our spacecrafts are in low Earth orbit, when the astronauts are in low Earth orbit, it’s not the long-term does it turn on? It’s the how often can we turn these suckers on and off as we have to maneuver around this, that, or the other thing that was unexpected and is heading our way?
There’s also the matter of station keeping – In this case, just trying to keep yourself above the planet because there’s so much drag on these systems when they’re in lower Earth orbit. There’s a lot of different reasons that you have to stick a thruster, stick an engine, stick a driver – call it what you will – based on the technology that it is.
You have to move your spacecraft and the Apollo missions, you pointed an amazing article by our good friend Amy Shira Teitel out to me where there were dozens upon dozens of engines necessary for the Apollo missions to first get off the planet, then make their way through lunar space, then to make it around the moon, onto the moon, back around the moon, come back to Earth, and then just get themselves lined up so their heat shield was pointed in the correct direction so they didn’t die at the last moment.
Fraser Cain: Yeah, there were 83 engines according to Amy. She saw there was some placard at some rocket place that she was at and she then did all the research to find out that there were 83 separate engines as part of that mission. And that is where we look. Each one of those has weight, has mass. You have to cart it around. You use it, then you dump it. And so you’re always having to go back to that original discussion that we had last week about these staging rocket systems.
Where do you hang onto the rocket and fire more propellant out of this existing engine or where do you dump the fuel tank and the engine because that saves you more overall fuel, decreases your mass, makes your next stage simpler? If rockets were kind of as efficient as cars, for example, then you would never throw any part of it away. And that’s what SpaceX – with the BFR – is trying to move towards. But right now, we’re still not there.
Pamela Gay: And one of the greatest concerns they’re gonna have technologically in getting the BFR going is they’re going to achieve this by using not 84 engines that are used at all different times, but rather they’re gonna have a few dozen going off at once to first get off the planet, then to do the thing wherever they’re doing the thing – whether it be Mars, moon, or asteroid or whatever – and then they’re going to have to reignite those engines and get themselves home. And every time you add and engine, you’re adding more complexity and you’re adding more points of failure – which is always a bit terrifying.
Fraser Cain: For sure, but you’re also potentially adding redundancy as well, right? You get kind of both failure and redundancy. I’m sure there’s a word for there – faildundancy.
Pamela Gay: Yeah, I call it risk.
Fraser Cain: Risk.
Pamela Gay: But acceptable risk and necessary risk and that’s the thing is as technology gets better, we’re able to figure things out in real time. We’re able to have computers that are able to turn off this, adjust that, at hundreds of processes a second – even with slow systems. And it’s this ability to do so many computations on the fly that is going to allow the BFR to do things the Saturn V folks could never dream of. They were working when computers were still new and young and wouldn’t fit in the room I’m sitting in.
Fraser Cain: Yeah, yeah. I mean that is – when you look at the capability of the BFR and the Falcon Heavy and the way you can have a rocket land again – that’s a computer learning to land a rocket. It has to be very smart and that kind of thing never would have been possible back in the Apollo age. So, they had to go more simple: one rocket, one engine, maybe four engines maximum. Have them all go at the same time. But now to have 27 separate engines on say the Falcon Heavy and for them to launch but also turn off and turn on and do their job, it’s very complicated and requires powerful computers.
But I just imagine this future. I mean imagine if you saw the back in the Apollo era and the rocket took off and all of the stages separated and then all the chunks came back to Earth and they could stack them all back up and launch another one later on. It’s so different where we’re moving with the levels of reusability that it’s probably going to call into question all the way just even that concept of, “What is an upper stage? What is a maneuvering thruster? What do you need?” All of that is gonna be sort of up for redevelopment again.
Pamela Gay: We’re going from the point where with the Apollo missions, they had that joystick and those push buttons that they used that became our 1980s video game controllers essentially to now a human being doesn’t have acceptable response times to do a landing with one of those rockets. We all – if we’re frequent flyers – have had that moment of coming in for a landing when a gust of wind grabs your airplane and you’re like, “We’re gonna die” and then no, you’re fine. You are fine. You may land at a crazy angle, but it’s fine.” A human being can handle that.
What a human being can’t handle is you have a vertical object which is much more subject to torque and – if you have a gust of wind that is significantly different at the top of that rocket and vertical is the distance that gusts of winds tend to differ – if you have different gusts of winds at different heights on that rocket, you have to compensate that and you only have – It’s not rear wheel drive, butt end drive. I’m not sure what to call this. But you only have one set of thrusters. Those rockets are doing the job of balancing a baseball bat on the palm of your hand.
Fraser Cain: Yeah. So, as we close up, I think it’s important to note that the upper stages – the maneuvering thrusters – they are the unsung heroes of rocketry that they’re the ones that you count on. They spend a long time in the dark and the cold and they need to fire when they have to fire or the mission is a failure and we appreciate all of the upper stages and maneuvering thrusters out there for their hard work. Next week, we’re gonna talk about exotic forms of propulsion like ion engines and–
Pamela Gay: Rail guns.
Fraser Cain: Rail guns. Whoa, alright. Settle down. Alright, we’ll see you all next week.
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Duration: 30 minutes