SpaceX Isn't Planning to Reuse the Falcon 9 Rocket's second stage | Hacker News Reader

SpaceX Isn't Planning to Reuse the Falcon 9 Rocket's second stage

bloomberg.com

32 pointsrbanffy52 comments

I think this is just confirmation of something that's been true for a while. Stage 2 reusability went from "we're working on it" to "yeah, that's interesting, we'll take a look at it when we have time".

I think they were a little burned by the Falcon Heavy we well. It's an awesome rocket, but it'll take a while for them to even break even on its R&D, as it looks like it won't ever fly more than 2 times per year and then it'll be quickly supplanted by the BFR.

And so, while we won't see a reusable Falcon Stage 2, we will see some kind of weird test hybrid / mini BFS build around the falcon stage 2. I imagine it's just a Stage 2 with the shape and control surfaces of BFS to test their re-entry procedures.

https://spacenews.com/spacex-to-modify-falcon-9-upper-stage-...

I read somewhere that the Falcon Heavy will find a new niche in direct-to-Geosynchronous launches. Currently, most launches take you to a transfer orbit, then you have to use your satellite's fuel to position yourself in synchronous orbit, a maneuver that can take months. The Falcon Heavy skips the second step and you save your satellite's fuel. Can't seem to find the article at the moment though.

1. It only takes months if your satellite uses only ion engines. If it has regular hypergolic propellant engines it only takes hours.

2. ULA's (Lockheed Martin and Boeing) Atlas V and Delta IV can make direct-to-GEO insertion, and basically only the DoD wants that. SpaceX plans to use FH to get DoD contracts, but it's not a very large market. It used to be very lucrative for ULA, but SpaceX will lower prices so it won't be as lucrative for them.

Months going to GEO are months of revenue lost, and mass for propellant is less mass for payload. That's the tradeoffs that may not be compelling for some customers.

SpaceX's first direct-to-gso order was Oct 16: https://spacenews.com/swedish-firm-buys-falcon-heavy-launch/

It makes sense. If BFR works, it will make all the rockets obsolete, by simply being fully reusable. Hence the only sensible thing to do is to concentrate 100% of the efforts on it.

What all is different about the BFR heat shield compared to the shuttle? I assume there is much newer tech since the 70s, is it easier to refurbish?

Newer material. But big issue with shuttle was that each panel was unique and had to always be placed in a specific spot

The shuttle didn't do any re-entry engine burns. It fully relied on the friction.

It's not really friction that's slows stuff on reentry. The heat is from compressing the air. Which is why the heat spikes away from the vehicle.

Anyway, the rocket equation means if you wanted to slow down before reentry you would effectively need to accelerate to twice the final speed 0 to X then X to 0. That's simply not going to happen.

Why not? Isn't that what they currently do for the falcon 9s? Sure, the rocket equation makes it more difficult since it's an exponential increase in fuel, but that's not impossible, right?

> Isn't that what they currently do for the falcon 9s?

It's not (not even close). Falcon 9 first stage isn't reentering from orbital velocity.

It's essentially impossible, and if it weren't it would be terribly wasteful. Orbital velocity is going to be primarily shed by atmosphere.

It takes much, much less energy to slow down a nearly-empty shell than one that was full of fuel and oxidizer.

You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place.

Yes. SpaceX surely knows this, and reckons that the cost is worth full reusability with reduced refurbishment cost.

No, they are using a heat shield which is unnecessary if you slow down before reentry.

What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.

You can compare the weight of the heat shield required for atmospheric reentry braking to the weight of the fuel you need for propulsive reentry braking: if the fuel weighs more, use the heat shield instead. This could be the difference, for example, between 100t of fuel versus 15t of heatshield.

It eats into the payload, of course. That's why they have different landing trajectories (RTLS, barge in the ocean) and sometimes don't land at all, depending on the payload mass. The propellant itself doesn't cost much (Elon once noted that fully fueling a Falcon 9 is about 200k, which is nothing compared to the 62M that a launch costs).

Falcon uses air to slow down most of the way, they even use parachutes to get as slow as possible, before turning on the engine for the past possible second.

Falcon 9 doesn't have parachutes as far as I know. The fairings do, but not the first stage.

They did for some early tests. I was thinking Dragon not falcon 9, it’s funny how memories can just be off for stuff like this.

But, as you say Falcon does use air breaking.

SpaceX opted to use fuel to decelerate over parachutes partly because a similar delta-v could be achieved with less mass in fuel than in parachutes, with better re-usability of the whole and much better control.

They are not slowing down from orbital velocity, the first stage of falcon 9 also benifits from air breaking. So it’s not much fuel specifically becase they don’t need to slow it down very much.

An empty rocket can’t fire the engines. And even an empty rocket is heavy so you need a significant amounts of fuel to slow down from orbital velocity, but you need even more fuel to lift what amounts to cargo aka the fuel used to slow down into orbit. Or, you can just use a heat shield which is really light weight when you are only returning an empty rocket.

PS: Compare the amount lifted to LEO with the weight of an empty rocket. You find the weight of the rocket is significant which is why they use multi stage rockets even though this means lifting multiple sets of engines.

I don't think BFR is going to do any substantial re-entry burns. The reason is that it's completely impractical to slow down from orbital or interplanetary speeds with rockets, so you have to rely on your heat shield. BFR will only do very small de-orbit burns in order to bring it's trajectory into the atmosphere, just like the shuttle did.

In contrast, the Falcon 9 first stages are at suborbial speeds and don't have a heat shield, so they need to do a substantial re-entry burns to bring down their speed before they hit the atmosphere to avoid damage.

The falcon 9 first stage is not just suborbital, it’s vastly slower than LEO at just Mach 10. It’s also still in very thin atmosphere at 80 km, it’s also spending more time in very thin air as it’s sperates while going up, which is why they don’t need heat shields.

It’s true they fire the engines briefly at the top, but doing so to slow slightly from Mach 10 is very different vs slowing from Mach 10 to zero or Mach 22.7 to Mach 10. And this still significantly reduces the cargo they can take to LEO.

PS: Remeber kenetic energy is velocity ^ 2 so your bleeding off 1/5 the energy. Further, adiabatic heating is from the compression of gas, lower speeds means lower compression.

Everything you've said is agreeing with me except maybe the clarification that the Falcon 9 first stage doesn't quite cross the the Karman line, right? In particular, I think "sub-orbital velocity" is commonly taken to include trajectories that are substantially below orbital velocity

https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight

like the Mercury-Redstone mission Freedom 7 that achieved a peak speed of ~5,100 mph, less than Mach 7.

https://en.m.wikipedia.org/wiki/Mercury-Redstone_3

Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage.

https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosp...

(You don't explicitly say otherwise, but one could misread your comment as suggesting that the re-entry burns are optional or negligible.) My main point was that the re-entry burns are about reducing the velocity significantly (30%, or whatever), not just for steering the trajectory into the atmosphere like for de-orbit burns.

> Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage.

With the current design and mission profile yes they need to reduce speed via thrust. But, a large chunk of this is Falcon 9's aerodynamic profile. Even if heat's not an issue long cylinders are not stable but you could build a rocket with a different profile. Remember, it's just fine without a heat shield (other than the upper stage / engine area) when being used as a rocket and peak velocity is at separation.

So, I agree with this design that burn is necessary, my point is in terms of design space it's one of many options.

Further, to be clear it's not a meaningful option when you start talking about orbital velocity. The shuttle kind of gave heat shields a bad rap, but they can be really light and simple. The shuttle's problem was trying to build a reusable heat shield at the limit of what was possible vs a huge range of much simpler designs.

> Remember, it's just fine without a heat shield... when being used as a rocket and peak velocity is at separation.

This is a useful point for me to keep in mind, thanks!

Everyday Astronaut explained this pretty well: https://www.youtube.com/watch?v=SCCw_M8MAU0&t=110s

The Shuttle's heatshield had to deflect significantly more energy because of the different landing approach.

The shuttles largest issue was it's weight. It's the old square cube law. You need to remove more energy based on the mass of the object, but you remove it over what amounts to a 2d area.

Unfortunately, part of the design requirements was to return a large amount of mass from orbit which means it needed to be designed to dump a rather obscene amount of energy into the upper atmosphere. This meant it could not do a rapid decent and because it would overheat, and thus needed to stay in the upper atmosphere for a long time. Which meant it's thermal tiles needed to operate for much longer time periods requiring extreme amounts of thermal protection.

PS: The wings also added even more weight issues.

The Shuttle was one of the finest examples of MI complex waste and negligence there has ever been.

The Shuttle was designed for a use-case that evaporated, and it's not necessarily inaccurate to suggest that it was manufactured so as to spread "pork" to as many Congressional districts as possible. However, wasting money on space is not necessarily a waste, particularly if no one else is doing it. And for all their faults, the Shuttles flew high, and accomplished much.

well it is all fun and games until you put actual people in it.

There is a counter argument, if you continually shift your focus you will never allow your products to get to the point where they're really making money. Reusable stage two might be a year away, BFR five. That's four years of lost revenue.

They're already making money; reusable second stage would only increase profit a little.

Thankfully, Falcon 9 is already making money. Reusing stage 2 is an optimization that represents an allocation of capital - capital should generally go to the highest-expected-return uses.

Weighed against the investment to get a reusable second stage. It may still be logical.

Maintanance, fuel costs and other operating costs would probably still leave small enough rockets competitive (because of the rocket equation rockets get exponentially smaller as the payload gets lighter).

But I agree in your point the Falcon 9 will become obsolete once the BFR is proven to be reliable and manages a sufficient launch frequency.

For small, specialized launches there are already other companies filling the niche: https://en.wikipedia.org/wiki/Electron_(rocket)

Rockets get exponentially larger as delta-v requirements increase.

As payload increases or decreases, rockets merely need to scale linearly. Conceptually, this is quite simple to understand: simply launching two identical rockets will result in double the payload.

The mass part of Tsiolkovsky's equation is simply mass-initial / mass-final. Halve both the numerator and denominator, and you have the same fraction.

Not directly related to the topic, but I wonder if there's a way to at least partially recover energy, spent on lifting stuff to the orbit, from kinetic energy rockets have on reentry.

This question also applies to the heat generated from atmospheric friction during liftoff and landing. And also to payloads.

How about a giant space seesaw?

If you watch Elon's interview on the Joe Rogan podcast, he actually discusses VTOL electric aircraft using regenerative descent braking. essentially the only added energy is air resistance plus charging inefficiency, his back-of-the-envelope math was somewhere in the 92% end-to-end range.

When I want those kinds of perspectives, I usually check out the 'Isaac Arthur' channel [0].

[0] https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g/vid...

Guessing as my uninformed self, I suspect we could recover energy by building a tower that acts as the opposite of a rail cannon. If the craft is emitting a magnetic field as it descends, it could create an electric current in the tower. Of course, it would need to nearly touch the tower or emit a very strong magnetic field, so the conditions probably contrive against this scenario.

<edit adds succeeding> Or, descend through a tower and let the escaping air drive a turbine.

I think the problem is that the energy has high intensity and low duration. It would be similar to asking if we can get some residential power with a military grenade.

I’ve seen a suggestion that magnetic breaking of the form you suggest is a better way to generate power from mining the moon than He3 — difference in gravitational potential and extremely low concentrations of the latter in the lunar surface.

Yes, but.

Momentum exchange-electrodynamic hybrid teathers would partially solve that problem. Like all the good launch systems, all variations are either totally impossible or absurdly expensive unless you already have cheap access to space:

https://en.m.wikipedia.org/wiki/Momentum_exchange_tether

People were looking at tether tech a decade or so back. You throw the payload into a higher orbit which gives you a negative delta v.

But the timing has to be pretty precise on that release...

What's the status of their efforts to recover and recycle the payload fairing? I remember reading that they expected to save $1M or more per launch by doing that.

No success yet. They are going to move the fairing catching boat "Mr Steven" from the west coast to the east coast to then try again. And the fairing is ~$6M so they can save ~$5M per launch if it works.

I have no information other than the timing but it seems like that spate of firing he did enabled both the starlink redesign and the focus switch to bfr.

Were there firings at more than the Starlink office?

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