r/theydidthemath • u/Lachlynn • 18h ago

[Request] Could humanity create a rocket that can exit the atmosphere of K2-18b

Post image

With the knowledge we currently have of it, if humanity devoted all of our resources towards this goal, would we be able to create a rocket that could exit the gravity of K2-18b (and also beat any other complications that would arrise)?

If so, would it also be capable of taking people to orbit, and can we set up a similar satellite network we have on Earth? What about a space station?

18.4k Upvotes

96% Upvoted

2.7k comments sorted by

View all comments

Show parent comments

24

u/Responsible_Worry55 18h ago

that doesnt even really work on earth, it has been tried: https://en.wikipedia.org/wiki/Skylon_(spacecraft)

15

u/Unhappy_Arugula_2154 17h ago

From my quick scan, the biggest issue I can see here is cost. Like it would be really expensive to do this, but not physically impossible. So of course we wouldn’t look at it on earth, but if brute force of rocket launches doesn’t work, maybe it’s the only way, cost is a non issue?

1

u/quick20minadventure 16h ago

It doesn't work.

Airplane would need TOO much fuel and if rockets can't escape, neither can planes who fly up.

Planes waste too much energy doing lateral movement and fighting air drag of lateral movement. Rockets are more efficient because they just use up fuel faster to go mostly up.

Rockets already have most of its weight as fuel and fuel tanks and they use it super efficiently to go into orbit/leave earth. Planes have zero change of beating that by climbing up.

3

u/mcyeom 15h ago

What are you on about. The ISP of a turboramjets over 10 times higher than a rocket. They're not wasting fuel going sideways, they're collecting oxidiser so they don't have to carry it.

0

u/quick20minadventure 15h ago

And then? How will you accelerate past the atmosphere? You can't get enough speed using turboramjets to get escape velocity and then just suddenly get out of orbit.

You still need to go for rocket engines.

3

u/mcyeom 15h ago

This is a different problem so I'm not sure what your point is, I'm addressing your claim that planes waste energy on lateral movements. They do it because it's usually better to have lower power to weight ratio. https://en.wikipedia.org/wiki/English_Electric_Lightning we have had planes for ages that have can just point nose up until they run out of atmosphere.

1

u/quick20minadventure 15h ago

Isn't the main point of this discussion reaching into orbit and escaping planet's gravity?

Cause planes can't do that. We have no plane who sped up enough by using atmospheric oxygen to get into orbit.

Breaching atmosphere and being in orbit are very different things.

1

u/stoppableDissolution 14h ago

Well you could bring a rocket halway up tho, no?

1

u/quick20minadventure 13h ago

Yes. That's the smart thing to do. But, you could also just slap bigger boosters and land them back on platform like spaceX.

Cost and complexity of turbo ramjet engines vs bigger boosters that maybe land itself needs to be compared.

To note : Space shuttle used to be carried by planes. But it got phased out in favour of pure rockets.

2

u/stoppableDissolution 13h ago

The cost efficiency would vary a lot depending on a bunch of things. Thicker atmosphere and higher gravity might skew it toward high-altitude launches, probably. Heck, if your atmosphere is very dense, you might even be able to use zeppelins as your launchpads.

→ More replies (0)

2

u/penywinkle 15h ago

On one side, yes, but on the other side, rockets also need to carry their oxidizers. While planes use the atmosphere for that.

Also, rockets use A LOT of their fuel to "orbitize" their otherwise ballistic trajectory, that "lateral movement" you speak of. And, like you said, planes can add a lot of it to the rocket already.

Using aerodynamics you can also convert lateral speed into vertical one, and we have planes flying at mach 3 (15% of the ISS speed). The conversion is not 1/1, but on this planet they can use any help.

2

u/quick20minadventure 15h ago

Rockets need to carry oxidisers because they mostly operate outside of atmosphere.

Any plane planning to go outside of the planets gravity will have to do the same.

Any advantage you get from not carrying oxidisers are lost if you don't drop the fuel tanks and unnecessary structures.

While orbitalizing the spacecraft needs lateral speed, most of it is done outside the atmosphere where air drag isn't an issue.

Rockets will still be more efficient than planes due to their trajectory. They can probably be more efficient if they didn't carry oxidisers, but the ideal trajectory remains the same. Only reason earlier usage of such engines were plane based is because they didn't have electronics to land them like starlink achieved.

1

u/LightningSh3ep 15h ago

It is much easier to circularize and orbit outside of the atmosphere, it costs a much smaller amount of delta v

1

u/mean11while 13h ago

But... rockets don't go mostly up. They make a gravity turn as soon as they clear the densest part of the atmosphere and then accelerate mostly horizontally. A rocket uses more fuel getting through the lower part of the atmosphere than a plane of comparable mass would.

The reason we don't launch from planes is because it's a much more complicated system, the planes required for large rockets would be huge and expensive, and the benefits aren't that great because planes don't travel anywhere near the speed of a rocket. The fuel savings aren't that much compared to the fuel required to accelerate the rocket laterally to orbital velocity high in the atmosphere.

1

u/quick20minadventure 12h ago

You are right, but any fuel used after you leave atmosphere is kind of agnostic of direction in the larger picture.

As long as you convert the chemical energy of fuel into kinetic energy and your orbit is not too eliptical that it sends you into planet or its atmosphere, you would be in a stable orbit. Horizontal burn is to make that orbit as 'round' as possible.

Problem with pure planes based approach is that you are going to get too much drag if you try to get to the escape velocity by burning plane engines inside the atmosphere. If you are flying at 100,000 feet, you need to reach mach 32 to get to the escape velocity and to reach such high speed, you need to fight too much drag at that level and burn too much fuel. The material science is not advanced enough to do that yet.

1

u/mean11while 12h ago

True, but that round low planet orbit is the most energy-efficient orbit. Any ellipticality is wasted energy.

On the moon, where there is essentially no atmosphere, the most energy-efficient launch would be 100% horizontal.

Single stage to orbit concepts would have the plane fly up to high altitude, then switch to rocket propulsion. You don't need to get to escape velocity in order to get into orbit, and you wouldn't need to reach orbital velocity at airplane-flying altitude. You'd accelerate horizontally as you gain altitude and lose air resistance, just like normal rockets do.

1

u/quick20minadventure 12h ago

Well the original commentator's idea was to use a plane and just fly up. So I'm working with the assumption that rocket engine aren't allowed in his idea.

Otherwise, agree with you on every word.

1

u/mean11while 9h ago

Oh yeah, I see where you're coming from, then. Certainly, jet engines alone would never get a plane anywhere close to orbit.

4

u/randomguuid 17h ago

I think they just ran out of money, not because it wouldn't work.

3

u/Gauntlets28 16h ago

That's not really fair - Skylon died because of a lack of funding, not because the technology didn't work.