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electric_ionland t1_j5v0opz wrote

The main issue is that you would need to match the ISS orbit.

A probe coming in from deep space will have a velocity of more than 11.5km/s. ISS orbits Earth at only around 7.5km/s. This means you need enough fuel to slow down by more than 4km/s (nearly 9000 mph!). This propellant would be way heavier than a heatshield. The few deep space missions that have brought things back have not bothered to slow down to orbital speed. They just slam into the atmosphere and let it do all the braking for free.

Heatshield are actually really convenient (if technically difficult to build) ways to slow down. Think of how big a rocket needs to lift something to orbit. If you did not have atmosphere to slow you down on you way back you would need nearly as big of a rocket to land.

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Drzhivago138 t1_j5v7fy6 wrote

>They just slam into the atmosphere and let it do all the braking for free.

It helps that unmanned probes don't have to worry about pesky things like crew survivability during reentry.

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crazunggoy47 t1_j5wu80j wrote

In a pinch, they can also lithobrake!

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DoubleDot7 t1_j5wuusq wrote

Does that mean letting the ground do the breaking for free?

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danielrheath t1_j5wxivw wrote

Yes, as in "Craft underwent rapid unplanned disassembly after an unintentional lithobraking maneuver"

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Controlled01 t1_j5xofup wrote

Is that how they described that Martian lander that plowed into the dirt all those years ago

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gandraw t1_j5xqxmn wrote

That's why it's important to remember the difference between aerobraking and areobraking when you tell the NASA engineers to build something.

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Sergio_Morozov t1_j5y2d4y wrote

Did you mean aerobraking and aerobreaking? If so, are YOU a NASA enigeer perchance?

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gandraw t1_j5y2mk6 wrote

Ares is the greek name of Mars, so "areobraking" is the equivalent of "geobraking" and technically means "braking using the Mars surface"

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anomalous_cowherd t1_j60adde wrote

Nice, I'll use that when I want to outnerd somebody. (Said in an affectionate way, I like outnerding people!)

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tdmonkeypoop t1_j63v923 wrote

Ok.. Socrates!! Dudes over here playing 4D chess like he's dust in the wind or something

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LionST1 t1_j5xbury wrote

Using elastic deformation of structural materials and the landing zone to rapidly dissipate kinetic energy, very clever.

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Equoniz t1_j5xh962 wrote

I’m pretty sure lithobraking often entails deformation significantly past the elastic limits of the materials involved.

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LetterBoxSnatch t1_j5z15vc wrote

I suppose it depends on just how elastic your definition of “elastic” is…the materials may still be capable of reattaining their prior size and shape, given enough energy and engineering dollars

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oz6702 t1_j5xl67o wrote

This is the kind of engineering we're trying to perfect over at /r/KerbalSpaceProgram

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Iz-kan-reddit t1_j5xlwoe wrote

> Does that mean letting the ground do the breaking for free?

I'd correct you with braking, but breaking is also correct in it's own way.

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NetworkLlama t1_j5xeh9a wrote

That actually happened with the Genesis mission to collect samples from the solar wind. It hit the atmosphere at 11 km/s, but after slowing down, the parachutes never deployed. It impacted the ground at 86 m/s, contaminating most but not all of the collectors, and some of the science was salvaged.

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overlydelicioustea t1_j5y8rvv wrote

mars sample return mission will not do this in a pinch, its the planned reentry mode to just have the container be sturdy enough and , well, just let it hit the deck.

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BaldBear_13 t1_j5ww4tg wrote

If Sci Fi movies taught me anything, it's that when a probe returns from uncharted reaches of deep space, lack of survivability is a plus, and is called sterilization.

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dWintermut3 t1_j5xt71k wrote

NASA has a specialist for that actually, with the most badass job title in history of "planetary protection officer".

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SirCB85 t1_j5xv9yr wrote

Wait, my understanding of their job description was that they actually responsible for making sure WE don't contaminate let's say Mars before we are really really really sure that us contaminating it doesn't destroy any indigenous life we might like to study before we annihilate it?

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dWintermut3 t1_j5xw7in wrote

yes, that's a very important job of the PPO. but my understanding is as the staff's highest-ranked microbiologist control of potential alien lifeform risks to earth is also in their purview.

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b33r_engineer t1_j5yifcj wrote

That’s what they want you to think, yes…

(It’s also true, but they do both)

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BaldBear_13 t1_j5yi0tj wrote

are they issued a flamethrower? A cool suit? A memory wipe device?

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PromptCritical725 OP t1_j5v47hy wrote

Really good point about the delta-V. I hadn't thought of that.

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fishling t1_j5v6yas wrote

Note that the problem is larger than they made it sound, as those are vectors as well. The sample return mission is almost certainly not going to be coming on a path aligned with the ISS orbit that only needs to slow 4 km/s to meet it.

Also, I would think that it would increase the risk to ISS, some of the return capacity is already booked, and that doing the load/transfer of samples and ensuring everything is balanced and secured appropriately for reentry is hard.

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CountingMyDick t1_j5x2tjs wrote

To be slightly annoyingly pedantic, the actual return trajectory is most likely nowhere near the ISS, but if they were planning to dock with the ISS, they would presumably cheaply adjust their incoming trajectory to be as close to the ISS orbit as possible while still far away. If they were rather good at it, presumably they could get pretty close to only that 4km/s of total DeltaV to match orbits.

Of course that's still a hell of a lot of DeltaV versus aerobraking.

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cjameshuff t1_j5yfrbi wrote

> they would presumably cheaply adjust their incoming trajectory to be as close to the ISS orbit as possible

That's a pretty major presumption. It can be hard enough just intercepting Earth, requiring that interception to also occur with the probe trajectory aligned with the orbital plane of the ISS would greatly restrict the set of targets we could retrieve samples from.

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UnamedStreamNumber9 t1_j5zb3na wrote

They can be precise enough to know where it’s coming down. That said, Osiris ain’t stopping at the earth. It’s just dropping off a package and heading back out to intercept Apopthis

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KingZarkon t1_j5z8y96 wrote

Isn't it just Newtonian math? I mean, we know the weight of the craft, how much thrust it has etc, so why is it so hard to calculate?

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rabidferret t1_j5zfqvx wrote

Measuring a spacecraft's position and trajectory has a margin of uncertainty. Engines are not devices that can produce a known exact amount of thrust for an exact amount of time.

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Beetin t1_j5zutlb wrote

I mean, when you have a nice long trip like these missions, we get REALLY accurate pretty quick, and there are smaller more reliable thrusters we can use to make small course corrections once we get the data on the initial thrust errors.

We've got really good computers compared to even 10 years ago.

For example, the dart mission accurately hit a 530 foot object orbiting another 2500 foot object which was 11 million kilometers from earth (1/10th of the distance from earth to mars). All were travelling at several km/s. While that isn't the type of rendezvous the ISS is looking for :) it shows the extreme accuracy we are able to achieve aligning with objects and doing orbital mechanics.

There are no technological limitations on docking with the ISS, but huge practical disadvantages as talked about above. We aren't going to spend the money designing a return ship that can slow down into a stable orbit near the ISS and then correct into a docking procedure when we can just slam into the atmosphere with a heat shield and get the data back faster, easier, and WAY cheaper.

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cjameshuff t1_j602f2y wrote

Yeah, the issue isn't accuracy. It wouldn't be that difficult to hit the ISS. The solution space for a rendezvous with near-zero relative velocity is rather more restrictive.

For Earth, there's vast areas suitable as landing locations, where it doesn't really matter what direction we approach them from. We just need atmospheric entry to happen at a reasonable angle and velocity.

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cjameshuff t1_j600x1g wrote

It's not a matter of it being hard to calculate, it's a matter of the solar system not being physically arranged to conveniently allow it.

A minimum-energy transit will come at Earth roughly aligned with its orbital motion. To match planes with the ISS, the return must happen at one of the two times a year where the ISS orbit is also aligned with that motion, which means the trip must have started on the opposite side of the sun from that point, half a transfer orbit earlier. But we don't control where other solar system objects are or what their motions are. Windows to/from Mars occur every 26 months. If by chance things are properly aligned one year, it will be 60 degrees off the next time, and most Earth-Mars windows will be unusable: it will be 3 launch windows, 6.5 years, before they align again. And they in reality don't line up in such nice whole numbers, so in reality you're going to have a substantial plane correction to make on arrival, even with such limited windows.

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fishling t1_j5xmq9a wrote

You're still assuming that the path is coming up from behind the ISS, in the same direction ISS is moving. If it's moving in the opposite direction, it would have to come to a stop and then accelerate to catch up to the ISS. Or, if it coming in at a right angle, it would have to shed all that extra perpendicular velocity and add all the parallel velocity. Only in the most perfectly aligned case could it be 4 km/s.

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jinxbob t1_j5xtox4 wrote

Choosing whether to enter prograde or retrograde is relatively easy if you're far enough away from earth

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Mispunt t1_j5xnwyd wrote

There isn't really an opposite direction or a perpendicular direction though. If you circularize on the 'right' side you go left around the planet, on the 'left' side right. Edit: once you match the orbital plane of the ISS you can catch up or slow down for a rendezvous by changing orbit height.

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Nemisis_the_2nd t1_j5y7igp wrote

That's really not a big problem. Coming back fro somewhere like Mars, you'd need to alter the tragectory by a fraction of a degree to flip the reentry trajectory 180^0.

From there, you could bleed off speed like in early space missions with rounds of aerobraking.

The original commenter makes a good point about fuel weight, but its also got less to go wrong if you just slam the vehicle into the atmosphere one time.

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Skipp_To_My_Lou t1_j5vra6n wrote

Even if the sample tube attaches to the exterior of the space station, at some point they will have to bring it inside to transfer it to the reentry vehicle & every set of hands touching it is another opportunity to contanimate the sample.

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oz6702 t1_j5xlhp5 wrote

> The sample return mission is almost certainly not going to be coming on a path aligned with the ISS orbit that only needs to slow 4 km/s to meet it.

To be fair, this is a trivially easy problem to solve. A change of a few cm/s when you're a million clicks away can result in huge differences in your destination, so setting things up such that your incoming deep space probe lines up with the ISS' direction of orbit and plane of orbit and whatnot would be quite easy, and pretty cheap as far as dV is concerned. Still, slowing down to match orbit with the ISS is something that's gonna cost you a ton of fuel either way - unless you aerobrake, in which case you might as well just do that instead of bring the fuel along to begin with.

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Nemisis_the_2nd t1_j5y7nt7 wrote

> Still, slowing down to match orbit with the ISS is something that's gonna cost you a ton of fuel either way

Aerobraking, as you suggest, is the answer. It's how we used to do it for a long time before more effective technology and better understanding of orbital trajectories came along.

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sinspawn1024 t1_j5wep8o wrote

also, don't forget that if something fails to work on slow-down, you have an 11.5 km/s projectile heading straight for the ISS... Full of astronauts...

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notjordansime t1_j5wtq6o wrote

If it didn't slow down at exactly the correct rate, it'd more likely miss than anything

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FolkSong t1_j5x2jme wrote

Yeah, the risk scenario would probably be more like everything goes perfectly until the last second and then it explodes.

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sinspawn1024 t1_j5wyzal wrote

Even if the probability of collision was very low, do you think Congress will fund a NASA mission where there was a small chance the craft might smash the International Space Station, all its active experiments, and the astronauts of multiple countries into the Pacific Ocean for all the world to see?

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FriendlyDespot t1_j5x8bme wrote

The risk would be substantially lower than any number of other risks that are accepted daily for the ISS mission. With the maneuvers required to match an orbit, any failure would put the intercepting vehicle somewhere other than where the ISS is.

Consider that the scenario you're describing is a risk that's faced every single time a crew or supply mission is launched to the station.

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paaaaatrick t1_j5xztog wrote

You are overestimating what “very low” looks like for something like this

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R3lay0 t1_j5z7t18 wrote

The risk of it crashing into the ISS is just as high when just going directly into the atmosphere

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WeDrinkSquirrels t1_j5zc0p8 wrote

You mean like every resupply mission they send up?

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sinspawn1024 t1_j6kpdgy wrote

Resupply missions are moving up earth's gravity well. If an engine malfunctions, the craft will lose velocity and altitude due to Earth's gravity. Return missions are falling into Earth's gravity well, so engine malfunction results in continued acceleration. Also, retropropulsion is fundamentally unstable (the force balance is the same as balancing a ruler vertically on your finger), which means that if a system loses attitude control, the craft will much more likely enter a tumbling condition, which if not arrested, will dramatically widen the cone of possible collision.

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lethal_rads t1_j5x2p6x wrote

It wouldn’t go straight at the iss. It’d have to go into a different orbit first, and then do the standard approach for capture/docking.

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LordOverThis t1_j5wz0uy wrote

It’d be pretty unlikely to collide with the ISS, since it’s already like trying to shoot a bullet with a smaller bullet on a completely different trajectory. Just guesstimating that it’s probably a margin of error of like a milliarcsecond between “intercepted successfully” and “it flew by so far away it couldn’t be seen”, which would put the difference between “intercepted successfully” and “everyone aboard was killed” at even smaller.

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dWintermut3 t1_j5xtmlg wrote

now my knowledge of orbital mechanics comes entirely from video games, but couldn't you use a partial aerobrake?

one tactic I often use is to plan a trajectory that has a periapsis that is barely in the atmosphere, just brushing it such that there is only a tiny bit of drag from very thin atmosphere. this acts to lower my periapsis just a bit, the effect being exaggerated by the fact the relative orbital velocity is at maximum at periapsis and minimum at apoapsis. over a few successive orbits I can bleed a significant amount of velocity with minimal heat and stress. I never used this to attempt a rendezvous, though I want to try now, but I do use it to bleed enough speed that a drogue chute is viable to go the rest of the way

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Kantrh t1_j5xwy1v wrote

If you're partially aerobraking you might as well do it fully rather than trying to catch up to the ISS

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Alblaka t1_j5y6ikr wrote

I'd suggest that performing a precise aerobrake in reality might be slightly more difficult than in an abstracted simulation, i.e. due to the simulation always being 100% accurate, whilst any modelling of our actual atmosphere might be less precise.

It's easy to do a full aerobrake, and it's relatively easy to avoid aerobraking. But treading that fine line between might be a bit unfeasible in an unsimulated environment.

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gdshaffe t1_j5ydp8g wrote

I see someone other than me has spent the last drop of their fuel getting that Kerbin periapsis down to 50km or so and just used the "aerobrake 200 times" maneuver to get back home.

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ninthtale t1_j5wwnrm wrote

>If you did not have atmosphere to slow you down on you way back you would need nearly as big of a rocket to land

So missions from a lunar post would still need quite a lot of braking fuel?

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fyrstormer t1_j5xdz36 wrote

The Moon has a tiny amount of gravity compared to the Earth, so lander modules falling towards the Moon don't speed up nearly as much and don't need nearly as much fuel to slow them down before they land. The Apollo Lunar Module was a single-stage-to-land/single-stage-to-orbit aluminum box with a little rocket motor strapped to the underside; the same setup on the surface of the Earth wouldn't even be able to lift its own weight.

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FolkSong t1_j5x2ytl wrote

A similar amount as they use for takeoff, but it's still very little compared to getting out of Earth's gravity well.

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cjameshuff t1_j5ygqwh wrote

A landing on an airless Earth, launched from the moon? It would be a bit more efficient than the moon landing, because the spacecraft would be at its heaviest (with a full stack of stages fully loaded with propellant) in low lunar gravity and would be doing its final braking in Earth's heavy gravity after burning most of its propellant and discarding most of its stages, but gravity losses are fairly small in comparison to the overall acceleration/deceleration requirements. You'd need something of similar size, just with fewer first stage engines to get it off the moon.

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LeatherCode2624 t1_j5yx36j wrote

Why did sci-fi lie to me about the ease of which zipping around in space would be when I got older?

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TryingNot2BeToxic t1_j5xew3p wrote

Oh this brings up an interesting problem when it comes to an intermediary moon base/launch platform. Would the propellant needed in order to reduce speed to land back on the moon offset the propellant saved from launching in lower gravity/no atmosphere?

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waylandsmith t1_j5xtcqe wrote

Naively, on paper, yes. Launching from the Moon vs Earth saves you about 9km/s in delta-v, more than making up for the 4km/s to slow down, and then another 2.4km/s to actually land on the moon. But the problem is a lot more complicated than that. Without aerobraking, the propellant needed to land would need to be sent with the spacecraft on its entire journey. Fuels that are stable for long, long journeys are typically significantly less efficient than those that can be refreshed/topped-up until the moment of launch, so more of that stable fuel is needed, requiring more launch fuel to get it into space. The delta-v budget would be turned on its head and the vast majority of it would be needed to be spent right at the end, instead of at the beginning. And this, of course, ignores the problem of how to get the spacecraft onto the moon and the payload back to Earth.

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TryingNot2BeToxic t1_j5zwnns wrote

Aight, new question. Would the space elevator concept be viable on the moon? Like we'd have a moon base, an elevator, and then a station in orbit connected.

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waylandsmith t1_j60w9pt wrote

A lunar space elevator is possible, but would be quite different than one that could be built on Earth. But the moon's lower gravity makes a lot of the benefit of a space elevator moot. A lack of atmosphere on the moon also makes some sort of railgun launch possible and economical, at least for cargo.

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electric_ionland t1_j5xte0y wrote

Yes stopping on the surface of the moon is a pretty terrible idea. However you could envision missions where lunar ice is mined to create propellant and that fuel is sent to a convenient orbit that is more "on the way".

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Alerith t1_j5yopiy wrote

I assume there are a bunch of calculations accounting for the Earth moving around the sun and through space in general? I can't imagine the math that goes into hitting Earth from deep space.

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UnamedStreamNumber9 t1_j5ytgex wrote

To add to that, the osris-Rex probe itself isn’t going to end its mission on return to earth. It’s just dropping off the reentry vehicle and heading back off into space where it will be targeted to flyby the asteroid apophis (the one that will have a very close earth encounter in the late 2020’s). This wouldn’t be possible if it slowed down to earth orbital speeds

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electric_ionland t1_j5yukz7 wrote

Sure but AFAIK this was never a primary mission goal. It's just a nice bonus because they had the margins.

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LazyLizzy t1_j5za8bk wrote

Your answer has made me wonder. In the future if we have surface to space aircraft would they theoretically be able to enter Earth's atmosphere at a slow enough rate (with some form of propulsion assistance) to enter with little to no friction?

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odddutchman t1_j5w958r wrote

Strictly speaking, they don't get the atmospheric braking for free...still needs a heatsheild and guidance system; but they don't have to drag extra mass of fuel, rocket engines, etc with them during the ascent part of the flight...which adds up REALLY fast. Look up "the tyranny of the rocket equation".

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17934658793495046509 t1_j5wpij1 wrote

Also you would need a heat shield to slow down in the atmosphere. It’s heavy but weighs less than the fuel you would need to do the same thing.

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Hokulewa t1_j5x1h2j wrote

You could really improve your payload mass margin by using aerobraking to match velocity with the Earth, but you would need a shielding device of some kind to protect you from the heat. That would weigh a lot less than the propellant you'd need to use, otherwise.

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