How To Intercept Cheaply Versus Quickly In LEO

[Spacescifi]

Obviously this is known.

Astronauts take hours to catch up and dock with the ISS.

Playing Space War (old gravity space sim) shows that slowing will make a vessel orbit a world faster until it falls right into it. Speeding up will slow orbital speed but also put you on course for deep space or a very high but slow orbit.

So to intercept I presume means slowing to below LEO speed, while still not enough to fall into the atmosphere.

Or perhaps they launch timed so that the ISS is passing by and they launch in its wake to catch up by lower orbits?

What is most cost effective?

[cubinator]

What's the goal in 'intercepting'? To get a new batch of astronauts to the space station safely and comfortably so they can complete a mission? To destroy the space station from the ground? From a higher orbit? To attach a Space Kraken to it to eat the space station? I'm going to focus on the first option because I like it when people work together for constructive projects and don't try to solve their problems by hurting others.

Right now the Progress and (I think) the Soyuz have pulled off rendezvous within a couple hours of launch, which lets the poor astronauts get some leg room quickly. Dragon could probably do the same with some planning. The fastest you could probably technically rendezvous would be 10-15 minutes, but that is unsafe for the station as they don't want vessels approaching at high speed and conducting large burns nearby.

The difference between these methods of docking is entirely procedural - which second you launch at to put yourself a certain distance in front of or behind the station - this can be a very small distance if you don't mind blowing on the solar panels with rocket exhaust or potentially plowing through them at a hundred meters/second - and how far above/below the station you orbit so that you have a particular relative speed closing on it. Again, in some cases you could get a close approach which puts your periapsis inside the atmosphere or inside the Earth, but you'll have to perform a big rocket burn right next to the station to get out of that situation, and that puts the station at risk. In Kerbal we don't usually worry about such things and are happy to perform grazing high-speed maneuvers, but in real life we like to be far more careful with our prized ISS.

Edited January 9, 2021 by cubinator

[Spacescifi]

6 minutes ago, cubinator said:

What's the goal in 'intercepting'? To get a new batch of astronauts to the space station safely and comfortably so they can complete a mission? To destroy the space station from the ground? From a higher orbit? To attach a Space Kraken to it to eat the space station? I'm going to focus on the first option because I like it when people work together for constructive projects and don't try to solve their problems by hurting others.

Right now the Progress and (I think) the Soyuz have pulled off rendezvous within a couple hours of launch, which lets the poor astronauts get some leg room quickly. Dragon could probably do the same with some planning. The fastest you could probably technically rendezvous would be 10-15 minutes, but that is unsafe for the station as they don't want vessels approaching at high speed and conducting large burns nearby.

The difference between these methods of docking is entirely procedural - which second you launch at to put yourself a certain distance in front of or behind the station - this can be a very small distance if you don't mind blowing on the solar panels with rocket exhaust or potentially plowing through them at a hundred meters/second - and how far above/below the station you orbit so that you have a particular relative speed closing on it. Again, in some cases you could get a close approach which puts your periapsis inside the atmosphere or inside the Earth, but you'll have to perform a big rocket burn right next to the station to get out of that situation, and that puts the station at risk. In Kerbal we don't usually worry about such things and are happy to perform grazing high-speed maneuvers, but in real life we like to be far more careful with our prized ISS.

 

The first one LOL!

But yeah....it is easy to forget that rocket exhaust does more damage in vacuum to solar panels since there is no air to slow it down.

Thanks for the time tables...the possibilities in scifi could be some sort pf rescue mission without much time (because they are bring shot at).

[cubinator]

3 minutes ago, Spacescifi said:

The first one LOL!

But yeah....it is easy to forget that rocket exhaust does more damage in vacuum to solar panels since there is no air to slow it down.

Yeah there is also the possibility of slowly coating the panels in soot if your rocket has certain fuels, like RP-1. (That's why the James Webb telescope isn't planned to be repaired by another spaceship - it's simply not built to have other ships flying around it.) Those solar panels are ridiculously thin and light, there was an EVA once to repair one that tore as it was being deployed and they had to be extremely careful to not damage it any further or hurt themselves. Even little RCS puffs can blow on them pretty strongly if too close. I think there might be some videos of the Apollo puffs blowing on the Skylab sun shield and knocking it about quite a bit.

3 minutes ago, Spacescifi said:

Thanks for the time tables...the possibilities in scifi could be some sort pf rescue mission without much time (because they are bring shot at).

On the ISS, there are always enough vessels docked for the crew to use as escape pods in case there is a need to immediately evacuate the space station. A situation where there might be more need for rescue missions might be translunar ships or more distant space stations.

[wumpus]

Intercepting an orbit ("first one") means not only being in the same spot,  but having the same orbit (otherwise you'd be moving at different velocities).

A fast ISS intercept has the advantage in that you are actually waiting arbitrarily long for the ISS to be in the right position for you launch pad before you "start the clock" at liftoff.  Fastest possible intercept is flying toward the ISS at full speed (whatever that is, obviously less than c), followed by a "suicide burn" that corrects your orbital velocity and hopefully doesn't spray the ISS or ISS orbital path with exhaust particles.  In KSP, you typically trade fuel for quick intercepts where make significant changes in orbital periods for one spacecraft to catch up to the other and then burn off (or re-apply) said speed.  I don't think this is the case with NASA vs. ROSCOSMOS: the delta-v is almost cm/s for the NASA approach.  More likely that is how they did it the first time with the shuttle, and that is how they know how to do it.

[kerbiloid]

8 hours ago, Spacescifi said:

below LEO speed, while still not enough to fall into the atmosphere.

While it's still not enough to fall into the atmosphere it's what they call LEO.

[Shpaget]

There is this awesome game you could try out where you can perform rendezvous with all sorts of different orbits. It simulates the "slow down to speed up" mechanics very well and is very intuitive in what maneuvers cause what changes in orbital parameters.

It's called Kerbal Space Program, check it out, it's awesome.

[kerbiloid]

The rendez-vouses in KSP are so tricky, while in Elite I was just running right into the rectangular airlock of a quickly rotating space station right from fly-by.

Edited January 9, 2021 by kerbiloid

[Gargamel]

7 hours ago, cubinator said:

but that is unsafe for the station as they don't want vessels approaching at high speed and conducting large burns nearby.

In KSP, I have done a launch to rendezvous, and I did get too close once.   I called it "Docking, with emphasis".    

There is no revert in real life.  You can't just reset the universe to previous save.  So taking your time and apporaching slowly is the only way they will do it.  I would imagine given some emergency they could do it, but that would also require a suitable launch vehicle to already prepped and on the pad, and in the right position to launch into the same orbit.   The chances of very time critical emergency happening  while all these other criteria are in place are so slim to be unimaginable.   Even then, if the right items aren't in the payload already, short of running up to an astronaut already in the capsule and saying "Here, hold onto this wrench!", everything had to have been planned for a long time before then. 

The only time critical emergencies that would happen on the ISS are ones that would require them to leave, not stay.   Even then they have isolation doors, and capsules that can return to the ground rather quickly.  Any medical emergency that is time critical can be dealt with on the station already.  Anything else, the patient should be able to survive the 12 hours or so it would require to get them to the proper facilities.   Having spent a significant part of my career in emergency medicine, I cannot foresee any time critical medical situation not being handled (stabilized at least) on the station. 

So, I see no benefits at all to a rapid launch to rendezvous profile.   Even cost.   Yeah a direct ascent might save a few bucks, but the chances of damage to the station itself far outweigh those meager savings. 

[KSK]

11 hours ago, Spacescifi said:

Obviously this is known.

Astronauts take hours to catch up and dock with the ISS.

Playing Space War (old gravity space sim) shows that slowing will make a vessel orbit a world faster until it falls right into it. Speeding up will slow orbital speed but also put you on course for deep space or a very high but slow orbit.

So to intercept I presume means slowing to below LEO speed, while still not enough to fall into the atmosphere.

Or perhaps they launch timed so that the ISS is passing by and they launch in its wake to catch up by lower orbits?

What is most cost effective?

Other way around.  A craft launching from Earth will launch to  a lower orbit than the ISS, then gradually raise its orbit until it makes rendezvous. Lots of technical detail for non-rocket scientists and a useful diagram (near the bottom of the page) here.

In a bit more detail, the simplest way to raise your orbit is to carry out a Hohmann transfer. For simplicity, consider a spacecraft in a circular orbit. It fires its engine once to accelerate which converts its circular orbit to an elliptical orbit. The longer the burn, the longer the ellipse. Then, at the new highest point on its orbit (the far end of that ellipse), it fires the engine again to accelerate. That raises the lowest point of the ellipse and, if the burn is timed right, converts the elliptical orbit back into a (now higher) circular orbit.

I know other folks have made the point already but if you want to play around with this sort of thing in a very visually intuitive way, you really can't do better than to grab a copy of Kerbal Space Program. 

So, in principle, a spacecraft wanting to rendezvous with the ISS could launch into a low earth orbit that's below the ISS orbit, carry out a single, perfectly timed Hohmann transfer, and arrive at a point in  the ISS orbit just as the ISS reaches that point. In practice (again, as other folks have pointed out), that involves a big rocket burn right next to the ISS, which isn't a great plan. It's also a one-shot deal and if you mess it up, smacking into the ISS is a distinct possibility - also not a good a plan.

So a rendezvousing spacecraft will carry out a number of smaller Hohmann transfers to intermediate orbits (phasing orbits) before making the final transfer to the ISS orbit.  Takes longer but it's a lot safer.

 

 

 

Edited January 9, 2021 by KSK

[YNM]

"Rendezvous" is not the same as "Intercept". ASAT missiles intercept satellites the same way torpedoes intercept ships. But rendezvous is more akin to trying to berth/dock a ship to a seaport (hence the term "docking" and "berthing" also being used in space).

There are a few basic considerations to be given when doing a rendezvous :

1. The likelihood for the velocity vector equalization to not happen, either due to cancellation of the whole manoeuvre or due to failure when doing the manoeuvre

2. The effect of exhaust on the vicinity of the object we're doing rendezvous with.

There are often other reasons as well, such as timing your operations with the most favourable window (say, you want to try to have the docking when comms are clear and it's "daytime"), or wanting to have more assurances (think this was fairly more urgent on the Shuttle, at the very least after loss of Columbia), and also the amount of fuel you're willing to throw away.

If you're referring to the fact that Soyuz have "expedited" rendezvous procedure, think this has to do partly with how fairly cramped the Soyuz capsule is, and reaching the station with it's more spacious pressurized modules would be a nice thing to have. Shuttle on the other hand was perfectly capable of properly destroying the entire ISS if it hits even at low speeds, as well as the larger amount of exhaust produced (due to larger mass and as such larger propellant usage), combined with the manual operations, so you don't really have any other way but to be very sure of everything along the way.

[kerbiloid]

The orbital station should be equipped with a protective field to stop meteoroids, which are 15..70 km/s fast.

Then any quick rendezvous/intercept is trivial: aim the station, stick in the field, and get carefully berthed.

[Spacescifi]

7 hours ago, Gargamel said:

In KSP, I have done a launch to rendezvous, and I did get too close once.   I called it "Docking, with emphasis".    

There is no revert in real life.  You can't just reset the universe to previous save.  So taking your time and apporaching slowly is the only way they will do it.  I would imagine given some emergency they could do it, but that would also require a suitable launch vehicle to already prepped and on the pad, and in the right position to launch into the same orbit.   The chances of very time critical emergency happening  while all these other criteria are in place are so slim to be unimaginable.   Even then, if the right items aren't in the payload already, short of running up to an astronaut already in the capsule and saying "Here, hold onto this wrench!", everything had to have been planned for a long time before then. 

The only time critical emergencies that would happen on the ISS are ones that would require them to leave, not stay.   Even then they have isolation doors, and capsules that can return to the ground rather quickly.  Any medical emergency that is time critical can be dealt with on the station already.  Anything else, the patient should be able to survive the 12 hours or so it would require to get them to the proper facilities.   Having spent a significant part of my career in emergency medicine, I cannot foresee any time critical medical situation not being handled (stabilized at least) on the station. 

So, I see no benefits at all to a rapid launch to rendezvous profile.   Even cost.   Yeah a direct ascent might save a few bucks, but the chances of damage to the station itself far outweigh those meager savings. 

 

It would mainly be valuable in a scifi story setting where a ship does not have the delta v to leave orbit and a planet is trying to shoot it down (missiles/lasers). A rescue vessel will attempt to rendezvous with it and then use a jump drive to escape.

 

IRL landing is always an option, and I suppose even in scifi, but if that makes you a fugitive and it is not even your homeworld...

Edited January 9, 2021 by Spacescifi

[cubinator]

6 hours ago, YNM said:

There are often other reasons as well, such as timing your operations with the most favourable window (say, you want to try to have the docking when comms are clear and it's "daytime")

Yes - I recall that the Dragon currently docked to the station had the opportunity to hang around a few extra minutes near the station until the sun set - as it was blinding the cameras looking towards the station. Then they docked in the night.

[YNM]

44 minutes ago, cubinator said:

the Dragon currently docked to the station

You know that there're currently two of them docked to the station, right ? XD

1 hour ago, Spacescifi said:

It would mainly be valuable in a scifi story setting where a ship does not have the delta v to leave orbit and a planet is trying to shoot it down (missiles/lasers). A rescue vessel will attempt to rendezvous with it and then use a jump drive to escape.

We actually have real-life proposals to do this, although for launching from and returning to the Earth...

 

Edited January 9, 2021 by YNM

[paul_c]

20 hours ago, Spacescifi said:

Playing Space War (old gravity space sim) shows that slowing will make a vessel orbit a world faster until it falls right into it. Speeding up will slow orbital speed but also put you on course for deep space or a very high but slow orbit.

Ummmmm.......no. Slowing down in a LEO can put you into a lower LEO but you won't necessarily "fall right into it" until you enter its atmosphere* Similarly speeding up doesn't "put you into deep space" unless its enough to escape the SOI. It puts you in a higher orbit, sure. But in between there is a massive range of orbits.

20 hours ago, Spacescifi said:

 

So to intercept I presume means slowing to below LEO speed, while still not enough to fall into the atmosphere.

Or perhaps they launch timed so that the ISS is passing by and they launch in its wake to catch up by lower orbits?

What is most cost effective?

There is not one "LEO speed". 

Of course, the launch is timed to intercept nicely. In the real world, the longer a spaceship stays in space, the more expensive and heavy it is, due to life support systems etc. Hence why they would optimise for a reasonable travel time too. In KSP we have the luxury of time warp and everlasting Kerbals who don't need to eat or visit the loo too much.

It sounds like you're not actually used KSP because there is a whole world of things to find out about orbits, rendezvous, intercepting etc etc which is full of subtlety and detail.

 

 

* Or if its a body with no atmosphere, so low as to hit terrain

[Spacescifi]

7 minutes ago, paul_c said:

Ummmmm.......no. Slowing down in a LEO can put you into a lower LEO but you won't necessarily "fall right into it" until you enter its atmosphere* Similarly speeding up doesn't "put you into deep space" unless its enough to escape the SOI. It puts you in a higher orbit, sure. But in between there is a massive range of orbits.

There is not one "LEO speed". 

Of course, the launch is timed to intercept nicely. In the real world, the longer a spaceship stays in space, the more expensive and heavy it is, due to life support systems etc. Hence why they would optimise for a reasonable travel time too. In KSP we have the luxury of time warp and everlasting Kerbals who don't need to eat or visit the loo too much.

It sounds like you're not actually used KSP because there is a whole world of things to find out about orbits, rendezvous, intercepting etc etc which is full of subtlety and detail.

 

 

* Or if its a body with no atmosphere, so low as to hit terrain

 

I assumed there was a range of LEO orbits, otherwise vessels would have a harder time doing rendezvous with the ISS. Plus the game Space War shows as much. You can slow as much as you want,  but it will only make you curve hard into the planet.

If you have enough thrust you can even stop orbiting and fall straight down.

I may not have KSP,  but I was aware of extremely low orbits capable on airless bodies like moons.

[paul_c]

This is the KSP forum. 

[Spacescifi]

9 minutes ago, paul_c said:

This is the KSP forum. 

 

Yes...highly informative too.

For me I started with Scott Manley vids, but later came here.

I assume for the ISS that it is put in a rather stable high speed, so as to prevent premature deorbits.

Faster orbiters would deorbit faster, but that would take longer than it takes to rendezvous if they do it right.

Obviously one would be a fool to orbit a spacestation as close to the atmosphere as possible, so I am sure distance also plays a role, as atmosphere tends to deorbit stuff in flames.

Edited January 9, 2021 by Spacescifi

[YNM]

6 minutes ago, Spacescifi said:

I may not have KSP,  but I was aware of extremely low orbits capable on airless bodies like moons.

At least try the demos... they're more than enough to tell a lot of things, although it might push you to actually buy the thing... XD

[magnemoe]

In KSP I tend to do one orbit and dock, launch and set an circulation burn, adjust this for intercept. 
At kerbin I tend to launch early and raise Ap up to 90-100 km because the 80 km orbit of stations and the 70 km atmosphere. 
On other bodies I launch late and simply let don't raise Pe to circulation to get the intercept. 
Then an adjustment burn and an velocity matching one closest to the station. 

Now this will not work RL, however real launches are far more accurate and Soyuz just do a few orbits. Aproaching ISS also take time as you have to go slow and indirect so you don't crash if you loose control during approach. 
As I understand they tend to use over an day simply as the astronaut has to get up long before launch so they have an long day before getting into orbit so simply sleeping in the capsule make them fresh for docking and leaves more time for adjustments of trajectory. 

 

[cubinator]

The ISS orbits at about 400 kilometers altitude, and does periodic boost burns to keep itself at that height. Without that, it would eventually spiral into the atmosphere.

If you were orbiting at about 120 kilometers, you might be able to go around at least a couple times depending on the aerodynamics of your ship. Obviously that's not very practical for a space station that's going to be up there for years and years. The lowest that we tend to put things in orbit is about 200 kilometers. 

[KSK]

23 minutes ago, Spacescifi said:

Yes...highly informative too.

For me I started with Scott Manley vids, but later came here.

I assume for the ISS that it is put in a rather stable high speed, so as to prevent premature deorbits.

Faster orbiters would deorbit faster, but that would take longer than it takes to rendezvous if they do it right.

Obviously one would be a fool to orbit a spacestation as close to the atmosphere as possible, so I am sure distance also plays a role, as atmosphere tends to deorbit stuff in flames.

Sort of. The ISS orbit is a compromise which both US and Russian spacecraft could reach whilst being above enough of the atmosphere that it doesn't need to be re-boosted to a higher orbit too often.

The Space Shuttle was capable of around 200m/s delta-V once on orbit and it needed around 70m/s to deorbit.  Not sure what the specs for Soyuz or Progress are.  That limits/limited how much maneuvering they can/could do and limited how much higher the ISS orbit could be compared to the initial parking orbit for Soyuz or Shuttle (recall that a spacecraft will launch into a lower orbit before starting its rendezvous with ISS).

The ISS was also at a somewhat awkward inclination for US spacecraft as I recall, again set by the needs of Russian spacecraft. In this case, the latitude they were launching from placed limits on the orbital inclinations they could reach.