Super-Earths in Need for Extremly Big Rockets (2018) | Hacker News Reader

Super-Earths in Need for Extremly Big Rockets (2018)

arxiv.org

36 pointsweird_user50 comments

There's a lot of loopholes you could plausibly escape through. For one: you could use hybrid atmosphere-breathing engines to get most of the way to low orbit. (Using ambient atmosphere as a reaction mass circumvents the rocket equation). From low orbit, you can switch to electric thrusters with Isp much higher than the engines considered here: it's no longer necessary to have a thrust/weight ratio greater than unity.

A hydrogen planet would be particularly easy, since the light molecules would maximize Isp for a (non-combustion) thermal engine. A nuclear scramjet on a Hycaean world would have some truly impressive performance.

Yea. Some kind of nuclear scramjet would be the way to get off a big planet with a dense atmosphere.

To explain for all of the non engineers:

On earth scramjets work because airflow enters the front of the engine and the shape of the inlet compresses the gas, fuel is added and burnt, then the exhaust gasses go out the back really fast.

With a nuclear scramjet the fission reaction heats up the middle of the scramjet so air comes in, is compressed then heated so it expands and goes out the back really fast.

That would move you very quickly and efficiently through the dense atmosphere, using almost no fuel, and you do not have to carry much reaction mass.

Then once in space you feed compressed hydrogen into the front of the engine heat it up and send it out the back really fast, just like the nuclear rocket motors that nasa is developing right now.

A hybrid engine like SABRE might also be viable? https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

Or a jet stage followed by rocket stages.

Perhaps "slow" ascent would be a option, i.e. not reaching escape velocity but just steadily ascending until you are far enough away that gravity is lower. I know here on earth it is far more inefficient, which is why you always go for "ballistic" trajectories where you gain enough velocity that inertia carries you on.

Maybe there is something you could "ratchet" against? Thrust a bit upward and have something prevent you from falling back down. Maybe the denser atmosphere would provide an option. You could deploy large sails as intermediate launch pads in the atmosphere for example.

I once had a fun week playing Kerbal Space Program building solar-powered quadcopter launch platforms...

Basically a quadcopter which is mostly a big platform with a rocket payload in the middle. The quadcopter slowly ascends to the highest feasible altitude, bypassing all of the worst of the air resistance, and greatly reducing the delta v needed to get into orbit as a result.

This was mainly helping with the atmospheric drag problem, though; you would presumably need a lot of atmosphere to get far enough from the planet to help with the increased need for horizontal speed with a super massive planet.

Oh hey I found an old screenshot: https://imgur.io/zq8iyV0?r

Not a rocket scientist or even a physicist.. but if i remember right, the bulk of your energy in a rocket is expended on your horizontal speed not the height gain. I came across this when I was looking up if it made sense to launch a rocket from an equatorial mountain like Kilimanjaro (6000m asl)

Keep in mind that KSP happens in "easy mode" where all of the atmospheric drag, gravity pulling, and orbital speed are smaller, but they are smaller by completely non-proportional amounts.

Last time I looked, there was a mod that made Kerbin like Earth. I suggest you try it.

Were you able to land the quadcopter part and recover the cost?

Sadly, once you lose focus on a craft in atmosphere it is lost. So shortly after you start piloting the launched probe, the quadcopter gets garbage collected by Kerbin's atmosphere.

Yes and no. So the slow/efficient acceleration is realized today with electric propulsion [0] which is very efficient in terms of fuel consumption because it accelerates the particles to such high speeds. But, it is very low thrust so it is something that is on for days/weeks/months at a time versus the small amount of time that chemical rockets are on. Because it's so low thrust though its not something that will actually generate enough acceleration to get you off the ground in the first place. You will still need a high thrust device (i.e. rocket or something more exotic like SpinLaunch) to get you off the ground and in to a low orbit. So while you wouldn't have to reach escape velocity (11.2 km/s on Earth) you still need to get to a very large fraction of that (6-7 km/s for low-earth orbiting satellites).

https://en.wikipedia.org/wiki/Spacecraft_electric_propulsion

As gravity increases, buoyancy also increases. A high-g civilization with access to hydrogen would be able to float to the top of their atmosphere, and then proceed to launch into space.

Not a physicist but I thought the main problem is not getting high, but getting fast in the “horizontal” dimension to balance out high gravity and stay in orbit.

This really wouldn't help as much as you'd think. For a high-gravity planet, the atmosphere is mostly a trivial problem compared to gravity, and the gravity at the top of the atmosphere would be barely lower than it is on the surface.

A much more realistic and useful option is to just go air-breathing nuclear, and use the atmosphere for reaction mass.

My understanding was that (at least for the earth) the strength of gravity is nearly identical at the top of the atmosphere as it is at the surface. So I think that doing this would only benefit you in the sense that launching from the top of the atmosphere means you don't have to push through the atmosphere and it's associated drag, but you are still going to need nearly the same Delta V to make orbit.

But I am very far from an expert (not even a Kerbal player) so happy to be corrected if I'm wrong.

The gravity from earth at the ISS is 9/10ths the gravity at the surface. The curve for decrease of force is not that curvaceous, to negate the gravity from earth requires significantly further distance than you think.

Read a sci-fi book recently with that being a central plot point. Reasonably advanced species unable to escape the gravity of its home world. There is also a dearth of fissile material in the solar system that prevent a "nuclear option". Book was written as a bit of homage to "The Mote in God's Eye" with wanting to leave to planet being seen as a "Crazy Eddie" idea.

https://www.goodreads.com/book/show/59554147-cold-eyes

It’s clearly more difficult, but authors are a few individuals briefly thinking about a problem not entire civilization attacking the problem across generations.

My first thought is balloons work based on relative densities so they can still reach very low density air on a high gravity world. That doesn’t help much with rockets, but firing a gun or using something like spin launch is much easier if you can start from very low atmospheric pressure.

No idea what actual engineers could come up with.

Mostly it's moar stages. Each stage of a kerolox engine can get you about 5 km/s, and LEO is about 8 km/s, so two stages works pretty well. Velocity for LSEO (low super-earth orbit) might be 11 km/s, so you'd need a third stage, and each stage would be 3-5 times the size of whatever it's launching.

It’s not that simple a falcon 9 upper stage on a 5g super earth would collapse from its own weight during takeoff.

It’s a double hit as your lower stages are also losing ~5g’s of acceleration due to gravity. So if you want to add 3g the entire rocket needs to be able to withstand an effective 8g, and you need a rocket engine + fuel to provide 8g’s worth of force. On top of this the time between each stage becomes extremely costly.

Imagine non-super earth's alien civilizations are probably writing up articles about how planets like ours are hard to escape.

You dealt the hand you're given.

> You dealt the hand you're given.

Bottom line: Chemical rockets are never going to take us any place interesting in the universe.

I would argue that the moon, Venus, Mars, Titan, Enceladus, and others are quite interesting :P

But I see what you're saying. Excluding our own solar system, yes, chemical rockets are super limited :)

Even within our own solar system, anywhere beyond the moon is likely to end up as a one way, dead end trip.

Why is it so important to get back? That implies people need to go there, I think? Why?

And would it really not be possible to get to Enceladus and back (w/o people)?

The gravity there is so much less than the moon.

Or is it just the distance to and from Enceladus, and possibly nothing for a slingshot?

I can look at pictures of Machu Picchu on Google Maps but the experience of being there is so much better (from what I hear). That's why we want to send people. Presumably those people want to survive and come back too.

> I can look at pictures of Machu Picchu on Google Maps but the experience of being there is so much better (from what I hear).

Right, but you can't even look at pictures of these other planets because we haven't gone at all.

And if your choice was between spending $4T to go to Macchu Picchu yourself, or look at pictures on Google Maps for free - you would almost certainly not spend $4T "for the experience".

The reality is - Nasa estimates it would cost $4B for another manned moon mission. India just sent a lunar probe to the moon for about 1/100th that price.

Even if we could send someone to Saturn - the price would be more prohibitive than the chemistry.

If it costs 100-1000 times more to send humans than robots - it's probably not worth it.

16-Psyche[1] is finally conquered.

The first remote factory ships arrive, and not long after the pile collectors go out, the machines start building new rockets.

New starships, constructed from the mineralogical ultra-super-mega-wealth that is this remote asteroid treasure, begin to rise among the golden landscape. These are not Earth-rated rockets, but rather planetary transfer super-structures that can move entire cities worth of materials, slowly and surely, out to somewhere great.

We might find a spot to park some of these cities where the weather is always going to be great. We might find a way to build them into mini Dyson-spheres, parked around a bit of fusion, or so.

I think rockets can take us to great places. Imagine a single-pass 3D printed Starship made out of titanium and other such alloys 16 Psyche might provide ..

Its a pretty big universe. A lot of it is interesting.

[1] - https://en.wikipedia.org/wiki/16_Psyche

Chemical rockets are good at escaping a planet's gravity well. We don't currently have a better alternative. Any of the 'low and slow' methods like ion drives are only good for travel between gravity wells.

They seem pretty good for getting to low orbit, and once you're there, lots of possibilities for propulsion open up.

> It should be noted that, while the subject of this paper is silly, the analysis actually does make sense. This paper, then, is a serious analysis of a ridiculous subject, which is of course the opposite of what is usual in astrophysics

From what I can tell, this was published in August of that year, though with the silliness toned down significantly [0]

[0] https://www.cambridge.org/core/services/aop-cambridge-core/c...

> water (H2O) cannot become radioactive itself

This was interesting. I researched it a little and found this on https://www.quora.com/Why-is-water-the-only-thing-on-Earth-t...

When hit by neutrons "hydrogen, move to another stable state and only become unstable when that particular atom gets hit twice" "Oxygen takes three absorptions to become radioactive" and underwater neutrons "activated mainly the sodium in the sea salt."

I'm curious how much harder it would be on these planets to launch 1) Sputnik like satellite 2) Hubble Space telescope

> We find that chemical rockets still allow for escape velocities on Super-Earths up to 10x Earth mass. Much heavier rocky worlds, if they exist, will require using up most of the planet as chemical fuel for the (one) launch, a rather risky undertaking.

From the abstract. I love papers with a sense of humor.

I thought because gravity gets smaller by squared distance, large planets would not have crushing gravity on the surface because you are far away from the center of mass. Is that true? If so, how large would a super earth have to be to have an equivalent earth gravity on its surface?

It's because volume increases by cubed distance. For a larger planet to have the same surface gravity as ours, it would have to be less dense.

good points. I think the less dense part is pretty 'easy' though, earth is a bit dense for its size. (probably a result of the collision that created the moon).

It is completely reasonable to have a planet that is more massive than earth with the same or lower gravity due to differences in density.

However, the density variation of rocky planets is pretty small so as they get larger you will see higher surface gravities.

One option that doesn't seem to be mentioned is the used of beamed energy such as laser (visible or infra-red spectrum) which provide energy directly to a rocket.

This might be used in a secondary process (e.g., ion or plasma generators) or directly (heating atmosphere and/or fuel) to generate thrust.

The advantage is that the power source is on the ground, and need not be lofted, which removes part of the rocket-equation limit. It's still required to source or carry reaction mass, and I'd suggest that at least a fair portion of that be obtained within the atmosphere.

I don't know what a launch trajectory would look like, though I suspect something which went relatively slowly vertical (to minimise low-elevation drag), then began a hybrid lifting-ballistic flight at the highest possible levels of the atmosphere, powered by a planet-ringing set of laser stations, and acquiring reaction mass from the atmosphere itself, might be within the realm of reason?

It also strikes me that a world with sufficient mass would tend to retain hydrogen gas itself (though that would still likely react with oxygen to form water vapour), but at higher elevations there might be a significant differential fraction of H2 to other atmospheric components. Root mean squared velocity of H2 at 27 C (300 K) is about 7,000 kph (~4,300 mph).[1]

That's already less than Earth's escape velocity, so the problem is the molecules which have higher velocity that "boil off" into space.[2] I don't have the chops to compute this.

But a laser-pumped mesospheric hydrogen ramjet rocket might be able to take advantage of highly-energised (heated or ionised) hydrogen to gain escape velocity on even a significantly larger Super-Earth.

________________________________

Notes:

1. <https://chem.libretexts.org/Bookshelves/General_Chemistry/Ch...>

2. Earth has lost roughly 25% of its primordial hydrogen (and water) by this mechanism. <https://sciencenordic.com/chemistry-climate-denmark/the-eart...>

Here's the wiki page for ways around it. Of course, it's all hypothetical and theoretical:

https://en.wikipedia.org/wiki/Non-rocket_spacelaunch

Spin launches have become less theoretical.

https://www.spinlaunch.com/

I would assume that such planets have a very large surface area, larger than Earth and Mars combined? In this case, super-Earthlings have plenty of exploring to do, just in a different way from us. It can still be challenging and time consuming, for example air travel may not be practical, or may require balloons floating in denser atmosphere rather than jet engines. By the time we explored Mars and they explored their entire surface, or prospects for going further may not be so different. Either we have developed technology to travel much further, or we have to accept that that's all the planetary surface we are going to see and we better take good care of it.

Maybe? But as they say, necessity is the mother of all invention.

A super earth 2x the diameter of Earth but with the density of Mars would have the same surface gravity as Earth (according to my back of the envelope calculations).

The density of an object at hydrostatic equilibrium is a function of its gravity, which is a function of it's mass, assuming rocky and similar composition in aggregate. The likelihood that a planet would be 2x the diameter of earth and less dense is extremely low.

i completely disagree. Mercury is extremely dense and small, Venus is almost exactly the same size as Earth and is in fact ... less dense than Earth. There are many many things that leads to a planets density, and they call fall into a very wide range. A planet 2x the size of Earth with mars density seems very reasonable to happen - even if unlikely. Kepler-22b is very close for example

just think of the countless civilizations of blind space whales living in subsurface oceans for whom merely getting to the surface of their planet is as difficult as it is for us to get to space

We think it's hard launching a can full of air with some people inside is hard, try launching a can full of water.

Extremly?!

Reply on news.ycombinator.com