Chapter 24: On the nutritional value of pointed hats

Author's Note: This chapter was half-written a few months ago, then other things got in the way. 100 Points to the first to guess when based on the title.

After a few more exams and another wrecked turbopump, it was finally time to pack our bags. The Leaving Feast was more or less the same calorie binge as the previous feasts, with the minor difference of the entire castle being decorated in an exorbitant amount of yellow and black.

On the Express home I spent a few minutes chatting with the guy driving the locomotive. Unfortunately he didn't have any tips on how to make a high pressure steam system not explode, because our train hasn't had any kind of problems or maintenance for decades. Apparently it's running on century old magic with practically nonexistent documentation, so the lack of issues is a very good thing.

After a few hours of chugging through the British countryside, we arrived in London. Saying goodbye to all my new friends in Ravenclaw and Gryffindor took a while, it still feels weird to have this many. All too often I ask myself, what did I do to earn those friendships? I suppose I kinda did use my celebrity status to reform half the school for the better and I introduced a few teens to their personal heroes and to how rocket science can be more fun than silly practical jokes. Some decent accomplishments, but l still experience a bit of what my psychology 101 book describes as 'Impostor Syndrome'.

The irony is not lost on me.

I took my luggage with a tiny bit of wandless levitation to help (after spending literal years on that skill I'm going to use it) and made my way through the crowd on the platform. A lot of people recognized me, some wanted to shake my hands. I'll probably never get used to being in public without a disguise.

I noticed Sirius talking to a blonde woman on the end of the platform and adjusted my trajectory through the mass of people.

"...not since you walked past me visiting your dear sister", I could hear Sirius say.

"Talking of family, under which circumstances are you here?", she replied coldly.

Sirius beamed. "I am here to pick up my son from school."

"I was not aware you had one. And I thought I knew the extent of our family tree."

"Well, I've only had him for a few years. And I haven't exactly had much opportunity to tell you the good news."

"I should congratulate you on having a family, but how is he old enough for Hogwarts? Did they start accepting toddlers now?"

Sirius pointed at me. "You can ask him yourself. There he is!"

"Potter?", Draco asked. He'd also arrived to the conversation. He looked at me, then at Sirius, then at me again. "How?"

"I'm adopted. Harry James Potter Black, pleased to meet you."

She did not take my hand. "Narcissa Malfoy."

As I'd suspected, the blonde woman who looks like a Malfoy, quacks like a Malfoy, turns out to be a Malfoy. Kinda strange that I'd never seen a picture of her, now that I think about it. I do know what her husband looks like.

Draco seemed confused. "Wait, you're a Black? You're Family?"

I put on my most obviously forced smile. "I kinda forgot to mention that, dear cousin."

And with that we left.

-HP-HP-HP-

I'd barely managed to settle back into my life at Grimmauld when I got a letter from the Flamels, inviting me and Hermione to visit them.

We Floo'd to their home in the Devon countryside, a typical old-school wizarding home.

What was less typical about it was the side building with the laboratory. It looked like you'd expect an alchemy lab to look like, with ancient tomes, scrolls of parchment, flasks, liquids bubbling and quite a lot of enchanted tools and objects. But then there were the things you wouldn't expect in a medieval setting: Modern printed muggle books, geiger counters, a Periodic Table and a nuclide map on the wall, blackboards filled with magical, chemical and nuclear reactions and Bunsen burners that wouldn't have looked out of place in a chemistry lab except for the fact that they ran without being hooked up to a gas supply.

"Nice. You've been pretty busy?"

"You could say that", answered Perenelle with a smile, "we've been doing a lot of work over the last few months. Especially Nicholas, he barely spends any time outside any more."

"We're sorry about that", Hermione apologised.

"Oh, no need to do that. I couldn't be happier! You see, when you've lived for centuries, things tend to get quite boring. But now we've got something new to do, and my Nicholas has been more enthusiastic about life than he's been for decades."

We continued our little tour of the Flamel's laboratory. A shimmering blue crystal sat at the heart of a complicated experiment, catalysing a reaction of a magical substance I've never heard of to another. Perenelle explained that it was a new take on an ancient idea, but now with different isotopes. The results were still useless, but in a different way now.

I noticed a bag containing a quite substantial amount of raw gold. To finance the Order's quite excessive spending. It was quite a bit more than that even, because raw gold counts as a muggle currency and the magical governments charge insanely high taxes on converting large sums between muggle and magical currency. It's one of the main reasons why all the get-rich-quick schemes involving differences between the magical and the muggle economy that you've probably felt very clever thinking about don't actually work.

Also the reason why my inheritance has significantly decreased in value when I converted most of it to tech stocks. Back then I rushed to convert everything to muggle, now we're getting fleeced converting the other way. Not exactly the greatest foresight on my part. Maybe that's why Dumbledore overdid it with the leaving feast decorations?

"I think you may have to constrain yourself a bit more", Perenelle told me. "I know Nicholas promised to help you with the war, but we can't keep up. What are you doing with all that gold?"

Okay, maybe buying Nimbus 2000s for everyone in the Order and then 2001s a month later was stretching it.

Hermione answered before I could think of an excuse: "Harry's creativity, mostly. But I thought the Stone makes gold without limits, now that you've figured out the isotopes?"

"It does. But it's not all the same isomer. There is a small percentage of metastable excited gold-197m. It releases a 400 keV gamma ray after a few seconds. We didn't notice it on the small scale experiments because the shield charm caught it all, but when we convert larger amounts it causes some issues."

"And there's no way to block them?"

"We cannot block anything between the Stone and the material. The Stone equalises the energy in the moment of conversion, but the delayed energy causes thermal problems. We cannot risk the Stone, so we're limiting ourselves to fifty grams at a time. And we still need it for Elixir, too."

Oh great. The moment I let the most overpowered item out of my sight it gets nerfed. Way to be subtle, universe.

AN: It's called balancing. Ever heard of it?

It's bad writing. If you don't want OP stuff being used, don't leave it lying around in the first place. Nerfing it afterwards is just not fair.

Look, I didn't leave it lying around, it was in the world as I got it. Blame Rowling.

Lazy bastard.

Fine. I'll let you have this.

"Oh hello Professor! Mr. Flamel!" Hermione greeted the new arrivals.

Dumbledore and Nicholas Flamel came through the door carrying a large hemispherical package.

After quick greetings, they revealed its purpose.

"So, since you two helped us so much, we thought we should give you something too", Nicholas said.

Dumbledore continued: "And I helped them pick something nice out for each of you."

I tried to pick up the mystery package. Very heavy, as if it was a massive rock. Maybe a rune stone?

Could it be?

Dumbledore saw the question in my eyes and nodded. Oh wow. That's amazing.

Perenelle stopped me from ripping it open right there and now. "Don't open it yet, your birthday is in a month."

No way I'll be able to wait that long.

Hermione and the Flamels started a conversation on 16th-century literature, I didn't really follow it. I could only think about the package and strolled across the laboratory.

Then I noticed it.

A shelf with hundreds of tiny glass bottles, each with the shimmering of an enchanted barrier, a small piece of metal inside, and a label of two letters and three digits, organised in a grid.

First I thought oh neat, a collection of all the mostly stable isotopes. Then pure horror.

"Mr. Flamel! Sorry to interrupt, but this is super important. That spell you created to separate lead isotopes, does it work on just lead or everything else too?"

"Oh, it's quite universal. Why do you ask?"

"Is it possible to hide the existence of a spell under a Fidelius charm?"

After a few moments of silence, Dumbledore replied: "Maybe. I don't think anyone has ever tried. Although that's exactly what one should expect if it had worked, isn't it?"

"We need to try it. Right now. The isotope spell."

The Flamels were shocked. "But that spell is essential for our alchemy research! If we'd hide it, how would anyone be able to reproduce our results?"

"And you know what else anyone could reproduce if we don't? Pure U235. And once someone has that, they only have to quickly assemble a critical mass, probably trivial with switching charms or instantly reversible transfigurations or something."

And just because I complained about taxes a bit doesn't mean I want a world where anyone can have recreational nuclear warheads, thank you very much. In fact, now that it's plausible I'm trashing all my plans and ideas relating to nukes. I don't even trust myself with that kind of power, no matter how satisfying nuking Voldie would've been.

"Harry's right," Dumbledore said, "the isotope separation charm is too dangerous to be released. But unfortunately I find myself already sustaining a Fidelius charm."

"As am I, on the current location of the Stone", Nicholas answered. "And Perenelle has cast one on our old home in France."

Hermione shook her head. "Neither I or Harry are anywhere close to being able to cast one."

"And Sirius did one at Grimmauld", Dumbledore continued. "Arthur will want to do one on his home, I am sure. Elphias is unavailable too, as is Hestia."

"Remus could do it. He's been learning it, but hasn't used it yet. He wanted to do the emergency base, the one on the other side of the world."

"Without a Fidelius, the emergency base would lose most of its utility."

"Doesn't matter. Won't save it if the entire world goes up in nuclear fire. Forget Voldemort, this is way more important."

"Unfortunately, I believe you are correct."

Dumbledore apparated away to get Remus. The Flamels went to get stuff for preparation.

"What do we do now?" Hermione asked.

"Now? We move to part 2 of my top secret master plan."

"Wait, what?"

I walked outside and picked up a pebble.

"You're better at transfig than me. Do you think you could turn this into a larger rock, the same bowl shape as my birthday present?"

Hermione looked at it. "Of course I could. Reshape with no material change."

"Then do it. Remus just arrived, we're technically legal. Also try to replicate the wrapping paper. I'm getting Kreacher for a cargo teleport."

Realisation dawned on her. "Are you trying to steal your own present while they're doing the Fidelius?"

"If I guessed correctly, that package contains a Pensieve. The most useful tool in the world that a time traveller with a head full of memories from the future could possibly have. And it's mine. Do you think I'd be able to wait another month to get my hands on it?"

-HP-HP-HP-

With the Pensieve safely at home, and Dumbledore busy with - what was it again he was doing? Oh crud.

Anyways, I started going through some interesting stuff from two decades from now. I managed to recover the full lyrics to that song about the exoplanet discovery in a few years, and tweaked my investment plans a bit.

"Is that comfortable?" Hermione asked, looking up from her book. "With your head in the bowl all the time?"

"Not really, my neck hurts. But it's necessary. I have to secure all my future knowledge before I permanently forget any more of it."

"What are you remembering?"

"Lots of stuff. It's going to take a while to sort through it all. Family stuff. Science stuff from muggle school. And I really wish I had paid more attention to what was going on with the stock market in the years before I was born."

She chuckled and returned to her book.

"Interesting?"

"It's about the history of New Zealand. Muggle history. Did you know Ernest Rutherford was from there?"

Rutherford in New Zealand… Wasn't there something?

I jumped from my chair. "Of course! Change of plans, we've got to try that."

"What are you talking about?"

"Sorry, I just remembered something. I've been so stupid. We could've avoided all our trouble if I'd just remembered the darn Rutherford cycle!"

I think that might have confused her even more.

"Is this about the Philosopher's Stone? Alpha radiation scattered on gold foil?"

"No, no. Not Rutherford the scientist. Rutherford the engine! Sorry, of course you don't know what I'm talking about, it won't be invented for another decade or two. Those brilliant Kiwis. You know how a rocket engine works, right? Liquid fueled?"

"Of course.", she replied with her near-perfect encyclopedic knowledge. "A fuel and an oxidizer are burned in a combustion chamber, the hot gases exit through a nozzle at high velocity and create thrust. Right?"

"Exactly. But there's a bit of a problem: The pressure in the combustion chamber is super high. Pressurising the fuel tanks to that degree isn't really practical for anything large. How do you get the fuel from the low pressure tank to the high pressure combustion chamber?"

"A turbopump. Some of the fuel is burned in a pre-burner and that spins a turbine which drives the pump. Sometimes multiple pre-burners are used, or no pre-burner at all and instead the heat from the nozzle is used, that's called an expander cycle. And some engines use a tap-off cycle where the turbopump is driven with hot gases from the main combustion chamber. Wait, isn't that how you built it with Fred and George?"

"Almost. We built our thing like the NERVA engine. It's simpler than a bipropellant one, it uses only one liquid instead of two, and instead of a chemical reaction in a combustion chamber there's a nuclear reactor to heat the propellant. We used a magic flame instead, same thing as far as thermodynamics are concerned. That part was easy. But the tap-off turbopump kept blowing up. And doing it that way was really, really stupid of me."

"Is that where Rutherford comes in somehow?"

"Yep. The Rutherford engine, developed in the 2010s in New Zealand. The world' first electric cycle engine. It's so brilliant because it doesn't use a turbopump at all. It uses battery powered electric motors to drive the oxygen and fuel pumps instead."

"Oh. Why isn't everyone doing that?"

"Because there's not a lot of energy in batteries compared to fuel. It's not super efficient even with next century battery chemistry, but the engines are super simple and cheap to make. And really there's no reason why we should use electricity, it's just the principle I should've thought of. We're not bound by the same rules as muggle rocket scientists, there's no reason why we need to use a complex thermodynamics nightmare turbopump when there's lots of other ways to run a pump."

-HP-HP-HP-

And now, Not-Really-Harry-Potter proudly presents, a simple tutorial in 3 5 9 easy steps:

How to build your own magic rocket!
Step 1: Start with the diagram of a real muggle rocket engine.

Step 2: Replace some key components with magic items you stumbled across in your previous adventures. Magic fire FTW, screw you Tsiolkovsky!

Step 3: Spend two months trying to get the turbopump to work despite having no experience with rocketry beyond reading a few books and killing way too many Kerbals.

Step 4: Realise you're a wizard. Replace turbopump with magic pump.

Step 5: Realise that you don't know how to make a magic pump. That wasn't covered in class.

Step 6: Floo to your friends whose father happens to have some experience enchanting muggle items.

Step 7: Explain the function of a rubber duck. (Debugging, obviously.)

Step 8: Realise that maybe rubber duck debugging isn't a thing yet.

Step 9: Avoid awkwardness...

"Uh, Fred? Can you tell me why she's staring at me like that?", I whispered, pointing at the girl who'd been looking at me the entire time like I was some alien from another world.

Which to be fair I kinda am but you get the point.

"Ginny? Why are you staring at our guest like that?"

She didn't answer though, and just fled to her room.

Weird.

Anyway, let's get back to work: Creating a magic pump that can move lots of water with really high pressure. Ideally make it controllable too.

By which I mean watching Arthur do the work while the twins show me his collection of batteries and plugs.

"This right here, that's an europlug, it works with most sockets used on the continent. It has just the two pins for power, no third one for safety, so it's only for plastic devices. That one here is a German one, it's round and has the grounding connector on the bottom and top. The Italian one has it in the middle instead. The British one is completely different and obviously superior, it has its own built-in fuse which saves a ton of copper running a different loop from the fusebox to every room…"

I'm not actually sure if they were really listening after the first twenty minutes.

-HP-HP-HP-

A few more hours spent testing different magic pump enchantments, geeking out over batteries (Arthur just recently added a Lithium one to his collection), a short quidditch game and a truly delicious meal later, we finally had a fountain of water shooting into the sky. Six self-refilling water bottles were being pushed to their limits by the new variable speed pump at its highest setting, and the pressure it was developing has massively improved from our first attempts too.

After that we hooked up the magic fire again, tried out a few different designs for the nozzle throat that I'd come up with earlier (had to transfigure them at Grimmauld to avoid underage magic laws) and we finally had a steady plume of hot, fast steam, about a meter long and a few centimeters wide, coming out of our apparatus.

To be quite honest, it feels a bit like cheating. We used magic bottles for propellant mass, magic fire for energy, magic to run the pump, cooling charms to keep it from melting, and most of the structure had been transfigured with magic too. Once you're using magic for everything, it takes all the challenge out of hobby rocketry.

But that aside, here's the important bit: According to the test stand we jury-rigged together, we achieved about 1.3 liters per second of conjured water supply and a specific impulse of 80 seconds, resulting in just barely more than one kilonewton of thrust.

For comparison, NASA's RS-25 Space Shuttle engine produces 1800 kilonewtons at over 360 seconds by burning hydrogen with oxygen. But it runs out of fuel after a few minutes, which ours won't.

So yeah, our hobby project would actually be a serious upgrade if installed on a Shuttle.

Let's see what it does to a broom!