An analysis of the Lego City deep space rocket | Hacker News Reader

An analysis of the Lego City deep space rocket

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201 pointswh31373 comments

My intuition, as a rocket scientist[1].

Once the boosters have come off, it feels like the drag from the nose is going to flip the rocket around. Those control surfaces on the nose are at the exact wrong end of the rocket.

They don’t even need to be dynamic — a set of static stabilizers at the rear of the liquid stage is should be added to provide stabilizing drag after the boosters have been detached.

This might not be a problem if the rocket has reached an altitude where it is subsonic or the drag from air is small enough to be countered by the torque provided by thrust vectoring.

Those engines have a gimbal, right?

[1] I’ve worked at KSC[2] since almost the very beginning[3] of the programme.

[2] Kerbal Space Center.

[3] 0.18.

...written with apologies to those who are actually qualified in this area.

I think the thing at the top of the rocket is meant to be a launch escape tower. Gimballed engines should give more than enough control authority. Look how the tumbling SpaceX booster regained control after relighting one of its engines [0].

All in all my first impression of the Lego rocket was that it is far too small (or short) to get to orbit and replacing the second stage with even more payload... well, that's not even a rookie mistake.

[0] https://www.youtube.com/watch?v=rkR7iaim4aw

Funnily enough, when I was first reading your comment, I was thinking, yeah, being a rocket scientist is great, but people who play KSP probably have better intuition about how wacky things like this would fly (as per the XKCD[1])[2].

[1] https://xkcd.com/1356/

[2] Written as someone who does neither

As someone who's done both, I agree with the article that basically the rocket isn't big enough. Or my feeling is that the cockpit part is actually too big in relation. But it still looks like a fun kit, as someone who also loves building legos. As someone who's built the Lego Saturn V kit, that one is a lot bigger. :)

I think its big enough already.

It took a four man team most of Christmas to assemble.

The additional thrust is provided by what creationists call "the guiding hand". Mid flight power into very high orbit provided by grandads new chairlift.

In a serious of unplanned aborts the SRBs proved the most resilient component.

Satalite payload went the way of Galileo shortly after lunch.

Looking at their nozzles, the boosters also appear to be SRBs. That's a real rookie mistake. Nobody has dared launch humans in a capsule atop SRBs. The general consensus is that, in an emergency, deploying a parachute under a hail of burning solid rocket fuel will not end well.

Is Nozzle design actually a method of determining Solid/Liquid?

I know that the design depends on expected stage of flight use and things like exhaust velocity, but i didn't think just seeing a nozzle would be enough to make a determination,

(1) Relative size. Solid fuel is much more energy dense. In this case, if the nozzle was for liquid then its tank is so small that it would burn out very quickly. So it is likely an SRB. Note the other nozzles for the relatively wider center tank.

(2) Shape. You can look to the shape of the nozzle to see which altitude it is optimized for. SRBs are generally optimized for lower altitude.

(3) Throat diameter. By following the shape of the nozzle towards the tank above you can estimate the throat. SRBs have wider throats.

(4) Gimble (not seen here) SRBs can be steerable but they don't 'gimble' the same as liquid engines. Their nozzles are moved rather than the whole engine (see shuttle). So any moving nozzle bits suggests an SRB.

So ya, there is much to read from just the nozzle. In fact deep in the CIA there are probably some experts with magnifying glasses studying pictures of a North Korean rocket nozzle doing exactly that.

1) I think relative size is a bit of a crapshoot. The size of the strap-ons is effectively the same size as the center core, The second stage of the core makes it look like a full sized LOX/LH2 tank because its painted orange, but it isn't.

2-3) Sure, but that is a general principal, not a sure sign its an SRB.

4) I don't see a sign of a gimbal. It may actually not even be a nozzle, and just the skirt.

Relative size of nozzles to attached tank. The sides are much much larger in proportion to their tanks.

SLBMs (ie SRBs) https://upload.wikimedia.org/wikipedia/commons/6/6a/SLBM_Com...

Atlas missile (liquid fuel) https://en.wikipedia.org/wiki/File:Atlas_2E_Ballistic_Missil...

By the time the all engines shut off, I would hope that the capsule is high enough where drag is effectively not a concern.

The nose is a Apollo/Starliner style launch escape system.

Clearly this line of reasoning is the wrong way round. Surely the correct approach is to calculate the gravity and size of the LEGO planet and it's moon from the characteristics of the rocket.

Okay, I did it. Lego Planet is 20 (edit: actually 5) times less massive than earth.

Let's presume Lego Planet has the same density as Earth, so that the math is easy and I can make fewer assumptions. If we assume a planet with the same density, then our radius is just a function of the mass. Then all we need to do is pick a mass that lets us reach escape velocity- I presume a Deep Space Rocket wants to fully escape the planet, after all.

V_escape = ((2 * G * M) / R)^(1/2). G is constant, M for earth is 5.97E24 kg, R is 6.38E6 m.

Doing some math, if Lego Planet has a mass of 3E23 kg (20 times smaller than earth), then the radius is 2.35E6 m (a little bit bigger than 1/3 Earth's radius), and the escape velocity is 4120 m/s. The author calculated this rocket would have 4246 m/s of delta-V, so that leaves this rocket 125 m/s of Delta-V to maneuver where it needs to go.

Edit later: Okay, people have suggested Lego Planet is made of Lego (sensible). So let's fix these numbers.

According to this[0] Lego bricks with optimal packing have a density 0.64 g/cm^3, which is 640 kg/m^3 - FAR less dense than Earth, which will have consequences. If we then guessed that Lego Planet's mass is 1.25E+24 kg, 1/5th of Earth's mass, the the escape velocity is 4213.2 m/s which is close to this Rocket's delta-V.

This also means that Lego Planet is 47% larger than Earth, by radius, and has more than double the surface area (which is based on radius squared). No wonder there's so many different Lego sets!

[0]https://www.brickodyssey.com/what-is-the-density-of-lego/

Lego Planet is made up of how many 2x4 bricks? (assuming they dont compress)

tldr: A little bit more than 400 million billion billion standard 2x4 bricks, roughly.

Given my calculated radius of 9.39E6 m (47% larger than earth), the volume of Lego Planet is 1.95313E21 m^3.

Brickepdia[0] (of course that's a thing) shows that a standard 1x1 block of standard height is 8 mm x 8 mm x 9.6 mm, or 6.14E-7 m^3. A 2x4 brick would be 8 times that, or 4.92E-06. Simple division then gives us that Lego Planet is made of 3.97E26 standard 2x4 bricks.

[0]https://brickipedia.fandom.com/wiki/Brick#Sizes

The Lego planet does not necessarily need to be smaller than earth. Remember, it's made of Lego bricks which are not 100% solid, they have pockets of air inside them when connected to other bricks.

You need to take into account the mass ratio of a Lego brick to be able to correctly correspond the mass to a radius.

Nice! I’m happy for LEGO planet to be Earth sized, which means the rocket has plenty of dV to visit deep space objects or land on a LEGO moon.

This assessment completely omits an important source of impulse -- the kid driven by imagination. This has to be scaled up too!

I've been locating some of our Lego models at home in places that defy traditional notions of gravity, can only guess what was the intended target of our little Commander at launch.

So the base assumption should be that the tanks are filled with Magic, and the main stage houses a warp-core. The destination is indeed Deep Space! (Picard to the Bridge)

It could be decoy program to hide development of an intercontinental ballistic missile?

I haven't seen any LEGO centrifuges in stores. Those could have been hoarded by someone to encrich LEGO uranium?

Lego City is a rogue state? This changes everything.

It is, or did you read anything about a proper governmental transition from SYSTEM to City? All hush hush.

They always seemed to have more police stations than supermarkets, always a bad sign

The only productive industry is Octan, the oil company, and in the movie, the president of that company is also the de facto ruler of the city. Is Lego City a classic petro-state?

A kit with an evil tinpot dictator figure, a graphite pile reactor, and a PUREX plant would be so cool.

LEGO(R), are you listening? Would buy one.

Its Lego! You dont have to wait for Lego corp to realise your dreams.

If you come up with a good design on their website they apparently sometimes publish them.

It's all a massive jobs program that doesn't need to accomplish anything to achieve it's true goals. Very realistic.

I believe the flaw is in assuming a Lego figure is not in 1:1 scale. So instead of upscaling the Lego figure we should just down-scale "earth" (in reality now Lego-earth) and re-run these numbers with Lego-earth gravity.

Lego figures are more bulky than humans. This would suggest that Lego-earth has actually a higher gravity than earth, right?

Lego minifigs are mostly filled with air, while humans are mostly water. Water is heavier than air, so wouldn't that mean lower gravity?

Good question, haven’t thought of that!

Solution is simple. LEGOTimespace functions based on consensus of observers and rule of imagination.

Thus, they need big rockets to merely get out of sight. Once they are unobservable, they only need enough "fuel" to convince the two astronauts aboard they are doing fine.

Choosing the Orion capsule as a reference seems unnecessarily large. The Gemini capsule had crew 2, and was 3,851 kg.

Not only that, it assumes the payload stage weights as much as the Orion capsule...

Yeah. This is a single person capsule so Mercury would be a better comparison. 1090 kg.

Without reading TFA: no. Probably could reach space. Maybe—maybe—orbit, but barely and I'd lean toward no on that, too.

Source: Kerbal Space Program intuition (hey, served me well when I took one look at that SpaceX starship design and went "oh that's not going past Earth orbit without an orbital re-fuel and nearly-dry top stage tanks at launch" and, sure enough, that was the plan)

The title was "Will the Lego City Deep Space Rocket Go into Deep Space?" when this was posted. We changed it to that of the article.

Lego city isn't on earth... So requiring earth amounts of delta-v doesn't make sense...

That's a good point! I'm assuming that it's on Earth as [this set](https://www.amazon.com/dp/B003A2JCOA?tag=duckduckgo-fpas-20&...) shows cities on Earth as destinations for the train.

$243... I finally understand why my parents never bought me these kits lol

The train sets have some really dedicated collectors. This is an old set I think so probably not representative of a regular LEGO set. But yeah they’re not cheap.

This is an old (no longer officially sold) set and Amazon is know to have completely over the top prices for them.

The original price is hard to find but seen to have been between 80€ and 130€ (probably the later).

Current prices for "new/sealed" (quality) sets seem to be around 135€.

Before corona it was around 100€:

https://www.bricklink.com/v2/catalog/catalogitem.page?S=6005...

Sadly LEGO doesn't produce many good train sets in recent years. They had quite a variety of not cheaper but also not very expensive sets when I was young.

Still I won't disagree that Lego had many overpriced not so good sets.

The routes include Moscow and Dublin. What city on Earth has train routes that can reach both of those cities? Another point in favor of Lego City not being on the Earth we know.

Instead of scaling the Legos up, assume that they're life size beings on a Lego sized pseudo-Earth. It's been years since I've done any physics so, I hope I'm not badly off, but I think the gravity will be around 1/900th that of Earth (if we assume radius of Lego world at 1/30 earth, mass of Lego world proportional to the cube). If we ignore the fact that there probably won't be an atmosphere on Lego world, I'm sure there are are various other applications of the square-cube law, etc. that will work in their favor as well.

I believe they'll make it.

It'll actually still be 1/30th of the Earth, since from Newton's law of universal gravitation:

F = G * (m_1 * m_2) / r^2

m_2 * a = G * (m_1 * m_2) / r^2

a = G * m_1 / r^2

If we make m_1 1/30th of its original size and to the same to r:

a = G * (m_1 / 30^3) / (r / 30)^2

a = G * (m_1) / r / 30

As escape velocity v_e is where GPE + KE = 0:

v_e = sqrt(2 * g * r) from the surface of the planet

If g is now 1/30th of it's original amount as shown above, the escape velocity would be sqrt(30) = 5.477 times less, or about 2042 m/s. So yup, they would make it if it's at 1/30th the size of the Earth, with thrust to spare for air resistance.

I'm sure there's a city with train services connecting Dublin, CA (served by the BART) to Moscow, ID. The question might just be when...

There is an underwater-tunnel between france and england with trains running. And there is the irish sea bridge-project with the goal to connect great britain and ireland by train and cars. Making such a route possible in some yet unbuild future reality.

Yeah, that's reasonable to consider. If we compare to the toy planets in Kerbal, for example, Kerbin is about 1/10 the size of Earth. Low orbit velocity is about 2.3km/s and escape velocity is 3.4km/s. A rocket with 4.25 of delta-v would probably be about right, after factoring in losses to gravity/drag while launching.

If it's not on Earth, why did the Lego figures bother building a Saturn V [0]?

[0] https://www.amazon.com/LEGO-Ideas-Apollo-Saturn-21309/dp/B01...

The saturn 5 model is not lego-city/minifig scale.

This brings up an interesting point in LEGO canon. If we assume that all LEGO franchises take place in the same universe, we'd have to assume that LEGO Angry Birds[0] took place somewhere in the canon timeline of the LEGO universe. Even more problematic would be trying to put BIONICLE[1] somewhere in the timeline, while somehow also being able to fit in LEGO Star Wars[2]. Perhaps the toughest one to piece together would be LEGO Mars Mission[3] and LEGO Life on Mars[4]. While Life on Mars depicts a peaceful relationship between humans and martians, Mars Mission centers on a conflict around powerful energy crystals, which just doesn't match up (especially considering the Martians look completely different in these series).

[0] https://brickipedia.fandom.com/wiki/The_Angry_Birds_Movie

[1] https://brickipedia.fandom.com/wiki/BIONICLE

[2] https://brickipedia.fandom.com/wiki/Star_Wars

[3] https://brickipedia.fandom.com/wiki/Mars_Mission

[4] https://brickipedia.fandom.com/wiki/Life_on_Mars

From the Mars Mission page it seems the non-peaceful aliens aren't Martian natives: > The Aliens have originated from an unknown place in the cosmos, and are also searching for the crystals. In actuality, they are not from Mars And the wiki suggests it's a soft sequel to the Life On Mars setting, making their place in LEGO canon easier to fit together.

In this high altitude video sequence of 'Lego Earth' I see no evidence of familiar coastal outlines. Case closed? https://www.youtube.com/watch?v=CvlXACoVVIo

If there's one thing we learn from E. R. Burroughs' careful multi-volume monograph on Mars, it's that the planet has room for numerous species, each involved in quite different political struggles.

My question exactly. I think the right approach is to do the delta v calculation, and then use it to estimate the size of Planet Lego, by assuming the rocket can achieve orbit.

The available amount of delta-v would actually be spot on for an orbital launcher on Kerbin (in Kerbal Space Program).

> A minifigure, 3 blocks high, is repesentative of an average American adult

Ummm... why? Lego is from Denmark.

An average Danish adult is 1.80 m, so I would be off by 0.05 m. That gives me an additional 9,000 litres of volume in the main stage, which can hold 3,240 kg of extra fuel. It also gives each SRB an extra 1.47 cubic meter of volume, or an extra 2,980 kg of fuel. This gives me an extra delta-v of 144.8 m/s, which is still a far cry from the 11,186 m/s you need for escape velocity.

[0] http://www.dst.dk/pukora/epub/upload/17958/headword/dk/29.pd...

Well, obviously, the brilliant scientists of LEGO City have invented better rocket fuel.

Some sort of direct matter-to-energy converter to power the engines, and then fill the tanks with something really dense, osmium or whatever.

This is indeed the case - as indicated in the award-winning documentary "LEGO City Adventures: 3,2,1"

https://www.youtube.com/watch?v=bKcR_RMxIBY&t=5m43s

("This is a specially modified rocket engine for my new rocket fuel that can get us to Mars in one month, as opposed to the normal ten months it would take non-geniuses.")

This is great. Would love to see this for the original space shuttle lego sets.

I hate when people criticize and don’t offer a solution. The obvious follow up would be to calculate exactly how big of a rocket must be built to reach the moon, or even deep space.

Being able to go to the moon also provides the ability to go to deep space (just point the rocket away from the moon). Escape velocity is 11,186 m/s, so (assuming a rocket with specific impulse similar to the one in the post):

11,186 m/s = 263.1 s * 9.81 m/s^2 * ln( x kg / 20,800 kg )

That's roughly 1.58591*10^6 kg of propellant, which is actually less that what's in the first stage of a Saturn V. Of course, this doesn't include air resistance or anything beyond just hitting escape velocity and leaving Earth's orbit.

Considering the fuel capacity of earlier Lego spaceships, I think the assumptions about the energy density of lego fuel are off by a couple of orders of magnitude.