Deorbit ISS. A Job for Starship?

[JB47394]

NASA has put out a Request for Proposals to deorbit the ISS (in 2030, I assume). They apparently want to make sure that it drops into the spacecraft graveyard in the Pacific Ocean. It seems like good timing for a SpaceX bid to tackle that with a Starship. Just extend the tanks to turn the ship into a tug, go up and deorbit the thing.

I'd add that they should wire explosives all over the ISS so that when it is well into the atmosphere, it gets obliterated into a bunch of small pieces that more easily burn up. Send up the folks from Controlled Demolition. They'll make sure there are only little bits reaching the Pacific.

Then there's the idea of boosting the ISS to a higher orbit and do something with all that mass. There's 450 tons of stuff. It may not be useable right now, but it would certainly serve as a source of raw materials for on-orbit smelting and such. Then again, that's only three Starship launches.

Just thinking out loud...

 

[ecarfan]

Just extend the tanks to turn the ship into a tug, go up and deorbit the thing.

How would that work? Use a Starship to decelerate the ISS and then undock the ship and return it safely to its launch site?

 

[Mike1080i]

I'd add that they should wire explosives all over the ISS so that when it is well into the atmosphere, it gets obliterated into a bunch of small pieces that more easily burn up. Send up the folks from Controlled Demolition. They'll make sure there are only little bits reaching the Pacific.

Absolutely not required. What little does survive entry is the reason for targeting Point Nemo in the uninhabited South Pacific.

 

[JB47394]

How would that work? Use a Starship to decelerate the ISS and then undock the ship and return it safely to its launch site?

That's the idea, but I found the contract requirements on the government contract site. Let's see what that gives us.

1. The deorbit vehicle design and operations shall enable a contingency call-up for launch on-need to support an earlier than nominal plan de-orbit scenario. This call may be made as late as L-6 months prior to the needed de-orbit date.
2. The deorbit vehicle design and operations shall support a later than nominal plan de-orbit scenario due to the extension of the ISS EOL beyond 2030.
3. The deorbit vehicle shall remain on-orbit in a quiescent mode for a minimum of one year preceding the final deorbit burn.
4. The deorbit vehicle design and operations shall allow rendezvous and attach to the ISS at Node 2 Forward.
5. The deorbit vehicle design and operations shall allow for operational scenarios including both a crewed and uncrewed ISS.
6. The deorbit vehicle design may require complex software development including autonomous operations and failure response late in the deorbit phases.
7. The deorbit vehicle is a must-work function.
8. The deorbit vehicle shall be capable of providing at least 47 m/s of delta-v for the ISS at 450,000 kg mass.
9. The deorbit vehicle shall provide a minimum total thrust of 3236 N.
10. The deorbit vehicle shall provide attitude control for the mated stack throughout the deorbit sequence of events.

The ISS is a 450 ton object that needs a 47 m/s shove. NASA calls for a minimum thrust of 3,236 N. They also call for "a must-work function". That suggests that hypergolics would be the way to go. Draco thrusters produce 400 N, so they'd need 10 of those to fill the requirement - but they're designed for short bursts. A SuperDraco thruster produces 71,000 newtons, so just one of those would be more than enough to meet the requirement - except that they're designed/packaged for a 25 second burn.

Quick math says that a single SuperDraco firing for 5 minutes would impart the needed shove to a 450 ton object, producing 0.016g of sustained acceleration. Assuming it could run that long, it would burn through 17 tons of hypergolic propellants.

For comparison, a Merlin vacuum engine can throttle down to 626,000 N. It would fire for 34 seconds, producing 0.14g of acceleration.

A Raptor vacuum engine can throttle down to 1,000,000 N. It would fire for 21 seconds, producing 0.22g of acceleration.

The minimum thrust requirement would involve a 1 hour 45 minute burn, producing 0.0007g of acceleration.

Each of those numbers ignores the mass of the tug vehicle.

I have no idea what sort of accelerations the ISS structure can tolerate without fear of breaking up.

The challenge for SpaceX would be ensuring that they have "a must-work function" system. Hypergolic fuels are extremely reliable, but if SpaceX can get Starship reliable enough in seven years, then they could use a Starship tug variant and easily move the ISS wherever it needs to go.

Using Starship would be interesting because the mission would have to involve a time when the ISS is passing over the right area of the Pacific ocean and also passes over Starship's landing site. Starship would be deorbiting itself as it deorbits the ISS, so after detaching from the ISS it would have to boost to some degree if it's going to make it to its landing site.

Absolutely not required. What little does survive entry is the reason for targeting Point Nemo in the uninhabited South Pacific.

I'm aware. I prefer the idea of vaporizing the entire thing so that more crap doesn't reach the sea floor - or float around in the ocean. I would expect some sizeable pieces to make it to the surface. The Russians calculated that 20 tons of Mir would reach the ocean, most of it small stuff. The ISS is three times the mass of Mir, suggesting 60 tons. I would assert more would make it because the larger the object, the more it shields its own mass from heating. So some rather large chunks could make it down. Blasting it into smaller pieces during interface would aid in ensuring that very little makes it down. Ideally, none at all. Well, ideally we stop deorbiting mass that cost a fortune to get up there in the first place.

I'm just musing because I can't imagine anyone sending explosives up to the ISS.

 

[CapsEngineer]

T


I'm aware. I prefer the idea of vaporizing the entire thing so that more crap doesn't reach the sea floor - or float around in the ocean. I would expect some sizeable pieces to make it to the surface.

Wouldn't blowing it up just leave more debris at LEO?

 

[JB47394]

Wouldn't blowing it up just leave more debris at LEO?

Think of it as a controlled demolition using shaped charges while it is already in the atmosphere. No part involved in the demolition should have enough energy imparted to it to cause any problems. As soon as the detonation happens, the surface area exposed to heating would increase tremendously, instantly vaporizing any small particles, and accelerating the demise of larger pieces/sections.

 

[ecarfan]

Wouldn't blowing it up just leave more debris at LEO?

@JB47394 explained the physics involved very well, but just to be crystal clear, it would not be blown up while traveling at its LEO altitude; first the ISS would be decelerated by the docked Starship and then as it was descending in the upper reaches of the atmosphere the controlled explosions would be triggered, well after Starship undocked.

 

[ecarfan]

Just read this Scientific American article NASA May Pay $1 Billion to Destroy the International Space Station. Here’s Why which got me thinking about this issue again.

The agency has said it expects to pay nearly $1 billion for this service to avoid relying on multiple Russian vehicles. The brutal ending is scheduled for early next decade but is already proving a delicate matter for aerospace engineering and international diplomacy.

@JB47394 helpfully posted the NASA contract requirements upthread, which specify the deorbit vehicle requirements. As we know, the plan is to bring the ISS down. But then he noted:

Then there's the idea of boosting the ISS to a higher orbit and do something with all that mass.

What about this instead:

• Build a “sacrifice” Starship with no flaps or heat shield
• Boost it to the same orbit as the ISS (and recover the Super Heavy of course)
• Refuel it on-orbit as many times as would be needed to…
• Dock with the ISS and slowly (to keep the ISS intact) boost it free of Earth gravity to a safe parking orbit around the sun

So no need to figure how to recover the Starship. By the early 2030’s the cost of Starship launches will be so low that such a mission will likely cost less — maybe much less — than $50 million (in today’s dollars), far less than the $1 billion that NASA is supposedly envisioning.

Strangely, though the SA article was published today, there is no mention of Starship.

But if NASA wants a single deorbit vehicle with a design based on the current global space fleet, McDowell adds, it doesn’t have many options. “The things that seem obvious when you first start thinking about it—they just don’t have the oomph to do this big final burn in a short time,” McDowell says. The closest existing technology, he thinks, is the Artemis program’s European Service Module, which powered NASA’s uncrewed Orion capsule on a milestone trip around the moon last fall and is scheduled to help land humans on the lunar surface later this decade. Everything else, he says, is either far too weak or too forceful or simply unable to carry enough fuel for the task—hence NASA’s solicitation of commercial proposals for a new, custom-built deorbit vehicle.

I realize that paragraph is predicated on “the current global space fleet”, but the ISS deorbit project is stated to be “early next decade”.

 

[scaesare]

• Dock with the ISS and slowly (to keep the ISS intact) boost it free of Earth gravity to a safe parking orbit around the sun

Wouldn't simply boosting it to a safe graveyard orbit be much easier and less fuel-costly than breaking free of earth's orbit and inserting it in to orbit around the sun?

 

[bxr140]

Wouldn't simply boosting it to a safe graveyard orbit be much easier and less fuel-costly than breaking free of earth's orbit and inserting it in to orbit around the sun?

It’s far too low—it will just come back. Graveyards really only make “not my problem now!!!” sense above 1000km or so.

While not actually accurate math, you can 0th order that the deltaV required to go up 100km is the same to go down 100km. I don’t know how far down ISS would go before passivating, but it can’t be more than 200km.

From a SX perspective, this is generally just a job for Draco. Modify for longer burn, add thrusters for more total impulse…

 

[scaesare]

Ah ok, imakese sense. Then trying to put it in a solar orbit would seem to even less tenable....

 

[ecarfan]

Wouldn't simply boosting it to a safe graveyard orbit be much easier and less fuel-costly than breaking free of earth's orbit and inserting it in to orbit around the sun?

You are probably right. The ISS weighs about 420mt. Even Starship would find that a bit too much to handle.

 

[Grendal]

Off topic here, but the question comes up: What comes next? Starship is definitely the candidate for building a new and much better station. However, let's say they had to do it today. Can F9 build a new ISS and how long would it take? And how much would it cost?

The current ISS took a little over eleven years time and $150 billion. Shuttle launches were $15 to $20 billion alone.

Assembly of the International Space Station - Wikipedia

en.wikipedia.org

 

[JB47394]

Can F9 build a new ISS and how long would it take?

ISS-sized modules fit easily onto a Falcon 9. If they are designed to be docked to each other, then upper stages of Falcon 9s could deliver each one in turn, including truss sections and solar panels (docking everything). Crew would then arrive to complete connections, activate everything, verify operation and so on. As few spacewalks as possible. Ideally, zero.

If all modules have been built in advance, then I'd say it would take about a month to launch it all, get a crew up to configure it, and have them take it on its shakedown cruise.

The important part is keeping politicians and bureaucrats out of it. Everything about ISS was slow and expensive because of that. It would be nice to move up to inflatable modules for habitat, laboratory and storage.

Then there's the question of why we would build a space station. Is it a viable business venture?

 

[Ben W]

5. The deorbit vehicle design and operations shall allow for operational scenarios including both a crewed and uncrewed ISS.

I certainly hope it's uncrewed when it deorbits!

 

[ggr]

You are probably right. The ISS weighs about 420mt. Even Starship would find that a bit too much to handle.

Isn't a milli-ton just a kilogram? That's a bit light. /s

 

[ecarfan]

First time I’ve heard the word “milli-ton”.