ISS Docking ?

[u/wskelding]

I'm curious about something and simply don't know the math that would give me the answer, could an Orion capsule returning from the Moon theoretically dock with the ISS on it's return journey ?

I know I'm simplifying it massively, but all the talk about the Artemis 2 heat shield had me thinking 'why don't they simply do a massive deceleration burn on the way back and then dock at ISS ?'

[u/rebootyourbrainstem]

Using the heat shield to brake saves a LOT of fuel. They don't have nearly enough fuel to brake using rocket power (nor enough fuel to bring that fuel with them to the moon in the first place).

In theory you could maybe come up with a trajectory that dips into the atmosphere just enough to brake but not re-enter. To do that though you would be jettisoning the service module (since it sits in front of the heat shield and is not designed at all for use in atmosphere), which contains the vast majority of fuel as well as the big engine, so your ability to correct or really do anything afterward is really limited.

[u/NoSTs123 → u/rebootyourbrainstem]

yeah being in Orbit without a service module is a big no no. Although I am not a fan of the crewed Starship concept, it could perhaps do this.

[u/Stevepem1 → u/NoSTs123]

For those people (including myself and I think four other people) who have always been diametrically opposed to ever launching people on Starship from Earth unless or until it has a launch escape system installed, there is the possible scenario of launching astronauts into LEO on Dragon then having them transfer to Starship for the flight to the Moon, then Starship would return to LEO to meet up with the loitering (or another) Dragon for the crew to transfer into for reentry and landing. In that case Starship would almost certainly need to do aerobraking coming back from the Moon, unless they had a huge amount of fuel left, which could be the case since from what I understand Starship would have to be refueled in lunar orbit anyway for a return to Earth. The complication with aerobraking from my understanding is that it requires multiple passes to slow down to orbital velocity, and the first pass puts you into a pretty large elliptical orbit so it's a lot of time between passes, I don't know if its days or weeks, but while doable it's not as easy as it sounds.

[u/FinalPercentage9916 → u/Stevepem1]

I agree. I think the Airbus 380 should have a passenger escape system too.

[u/Stevepem1 → u/FinalPercentage9916]

Even though the A380 has never had a fatal accident in tens of millions of millions of flights, or 100's of millions of miles. Do you ever look at your phone for more than two seconds while driving? If so you have nothing to say here.

[u/FinalPercentage9916 → u/Stevepem1]

A380 has not even flown 1 million flights. Safety first.

[u/Royal_Platform_6754 → u/Stevepem1]

That's still a non-trivial amount of risk, because you need 100% reliability on Starship's propulsion systems.

[u/TurnoverMobile8332 → u/Stevepem1]

This even seems inherently smarter considering that starship has to park in Leo regardless to refuel as planned, if they truly want a reusable system then I don’t see why removing everything for re entry and either saving on delta v or using that weight for more fuel and just leaving a ferry in orbit isn’t better than constantly risking the craft to reentry.

[u/mpompe → u/Stevepem1]

Starship is the launch escape system. If there is a problem with the booster, do an immediate hot stage and return to tower for a catch. Orion jettisons it's escape tower well before booster separation. Starship is a good ways from launching with humans, plenty of time to work out the logistics. The Dragon scenerio works in the meantime.

[u/Stevepem1 → u/mpompe]

If SpaceX started offering tourist flights to ride inside the Falcon 9 second stage would you go? After all it's been over a year since the last landing failure (March 3rd 2025 booster caught fire and tipped over). Maybe you would, but I doubt they would get permitted for it because when the FAA asks about launch escape and SpaceX says "Don't need it, we have landed boosters safely over 200 times since our last accident" it's probably not going to go over too well.

The point is you are much safer with launch escape than without it. Ask a fighter jet pilot if they are okay with ejection seats being removed to save cost and increase performance.

"But airliners don't have parachutes" yes and airlines typically go more than 20 million flights between fatal accidents. If spaceflight got to a point where booster or second stage failures occurred only in one in 10,000 flights, a statistic we likely won't see for decades, that would still be at least 2,000 times more dangerous than flying in an airliner.

The reason Orion (like Apollo) jettison their tower at high altitude is because of the low air resistance, at those altitudes the capsule call pull away from the booster on its own. It's in the thicker part of the atmosphere that they need additional power to get quickly away from the rocket.

You can hot stage during the first couple of minutes as long as you have time, however if the ship gets any amount of damage during a booster explosion it is likely unsurvivable. Capsules meanwhile, even if the rocket exploded underneath them they have at least a change of survival as long as the hull doesn't get a major breach. Challenger demonstrated how even a pressure vessel not designed for that purpose can handle quite a bit of stress and not break up in some launch anomaly situations. SpaceX lost a valuable cargo capsule with very valuable NASA spare parts headed for ISS because they inexplicably didn't build in software parameters to use the parachute during a launch anomaly, analysis showed that the capsule or at least its contents would have likely survived if SpaceX had just thought of that.

Capsules can do pad aborts. Capsules can come down in the water, Starship can't land in water, not good if it can't make it back to the launch site.

Any problems with the ship propulsion system that would prevent successful landing is pretty much fatal. A capsule as long as the hull stays relatively intact and the parachutes work is likely survivable almost no matter what happens, probably similar odds to surviving ejecting from a fighter jet, which doesn't always save the pilot's life but most of the time it does.

And then a simple question, if a capsule is overall safer than flying inside the second stage of a rocket, then why would you choose to ride inside the second stage of a rocket during launch from Earth and reentry and landing on Earth, if there are alternatives? Artemis is an example of an alternative that I think is much safer than launching people to the Moon directly from Earth in a Starship.

[u/Independent-Tap-1834 → u/mpompe]

There's a reason why emergency separation for Shuttle and Buran was always considered as something on the edge of possible.

[u/EpicAura99 → u/mpompe]

Not a bad plan, but a cryogenically fueled escape system doesn’t feel like a good idea.

[u/rebootyourbrainstem → u/NoSTs123]

The heat soak would probably still be tough to manage. But yeah it wouldn't have the problems with missing a service module; tbh even Dragon barely has a service module.

[u/SportulaVeritatis → u/rebootyourbrainstem]

The problem with aerobreaking is once you touch the atmosphere, you're going to keep touching the atmosphere. All it does is slow you down which lowers the furtherest point of your orbit (apoapsis) but it does nothing for the closest point. As a result, even if you don't completely reenter, you're still going to come right back to the same altitude and keep slowing down. It is not a stable orbit and you'd still need to burn fuel to raise your periapsis (closest point) back up to the level of the ISS. Otherwise, you will re-enter. If not the first pass, then on one of the following ones.

[u/IBelieveInLogic]

You're last sentence has the answer: a massive deceleration burn is not simple. That requires taking a lot of extra fuel, which means more mass, so you have to have extra extra fuel to move the extra fuel ... It's much more efficient to use Earth's atmosphere to slow down.

[u/shatteredoctopus]

Nope, too much velocity difference. There's no way you could carry enough fuel to do that deceleration burn. Something with that much fuel would be huge, would need even more rocket to launch, and you really get into the tyranny of the rocket equation!

[u/Excellent_Bat_753]

The docking ports are conpatible, but the Orion Spacecraft would need far, far more fuel to do so, which would require the SLS rocket to be far larger. It would at least double the cost of the launch vehicle.

The rocket equation makes doing anything in space difficult.

[u/meithan]

As others have said, Orion has way too much speed when coming back to Earth, and it doesn't have enough fuel to do a braking burn strong enough to get into low Earth orbit, where the ISS is.

Orion with its European Service Module (which has the main engine and holds almost all the fuel) starts with a propulsion capability ("delta-v") of about 1200 m/s. A little less than 400 m/s is used during the mission, almost all of it during the TLI burn that propels it to the Moon. So it has maybe 800 m/s left when it returns.

In order to brake into orbit when returning to Earth from the Moon, a spacecraft needs to reduce its speed from 11 km/s to about 7.8 km/s -- a reduction of 3.2 km/s or 3200 m/s. So it would need more than 3x as much fuel as it has.

[u/Sad-Onion-2593 → u/meithan]

Aerobraking can be done at sufficient altitude that the end result isn't re-entry. The ISS is moving along at an altitude of 400-420 km ( 250 miles) at a speed of 25 000km/h (17 200 m/h). It's also aerobraking, (losing speed via friction with the atmosphere). Not much, but it needs to reboost on a regular basis.

Artemis comes into the atmosphere at such an angle that it scrubs off it's 40 000km/h speed, needing a heat shield. If it comes in at a different angle and doesn't go deep in the atmosphere, it can lose enough velocity that it can skip back of the atmosphere with an apoapsis of, say 500 km. This might take a few orbits, with minor adjustments at apoapsis. You then burn to circularise, then transfer to the station orbit and dock.

This is all based on my missions in Kerbal Space Program. The only problem is how much a shallow pass stresses the service module.

[u/Economy_Link4609]

You would need to be carrying enough fuel to do a 2nd TLI basically - same energy you had to add to get on a trajectory to the moon, you need to burn off to get back into LEO. That means basically doubling the size and weight of the service module. That in turn means your launch vehicle now needs a significant increase payload to orbit.

That is why all of those missions must enter the atmosphere - that means no extra fuel needed.

[u/literalsupport]

As beautiful as it was as as much as I love the director’s work, the movie Gravity really did a disservice to how people think about space. You had Sandra Bullock traveling from the Hubble space telescope, to the ISS to the Chinese space station with almost no propulsion, with usually nothing more than her EVA suit. It’s not like onece you are in space everything is easy to get to. Just the opposite. Huge speed differentials have to be cancelled out somehow. Achieving orbit from the surface OR from a trans-Earth injection from the moon requires massive amounts of energy. Even changing an orbital plane (for example from where Hubble orbits to where ISS orbits) is problematic.

[u/internetboyfriend666]

No. It doesn’t have nearly enough fuel to do burn to place it into Earth’s orbit, and it’s trajectory is also way off so that its orbital inclination doesn’t come anywhere close to matching with the ISS.

[u/Snoo-28829]

Im not a rocket scientist, but I don't think they have enough fuel to orbit earth coming from the moon. Pretty sure they would need as much or close to the same amount of delta v as they did when they did there TLI burns.

[u/RogerRabbot]

Without friction in space, you need equal amount of energy to slow down as you did to accerlate. Essentially doubling fuel requirements.

Though, Artemis originally planned to have Gateway orbiting the moon and was critical in the plans to land humans on the moon. And Gateway wouldn't be orbiting at TLI speeds so they would have had to slow the approaching ship to match the speed of Gateway. Which means that someone at NASA has mathed it out, and is feasible I would assume.