Author Topic: L1/2 spacestation with depot  (Read 64680 times)

Offline simon-th

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L1/2 spacestation with depot
« on: 08/07/2009 08:13 am »
Let's have a dedicated thread about a small spacestation (doesn't have to be occupied permanently at the beginning) at L1/2 with a depot considering all the discussion about depots and Flexible Path recently.

Benefits include:

 - Depots at L1/2 make more sense than depots in LEO - you don't have to store all the EDS propellant
 - You can go from L1/2 to a variety of destinations and come back to L1/2:
 e.g. Moon's surface: 2.5km/s delta-v to nearly everywhere and the same delta-v back to L1/2
       to a Mars transfer orbit, you require about 0.8km/s delta-v from that station and another 0.9km/s into a Mars capture orbit - the same thing back
 - Rather than having to take a 20mt Orion along on deep-space travel, you just leave Orion at the station (probably even gets used for station crew rotation and you take the next Orion back to Earth after you return) and only take a dedicated hab on your journey
 - you could have a dedicated depot for LH2/LOX or only LOX or at the beginning only hypergolics for lunar missions and a dedicated depot for argon for interplanetary missions using VASIMR (or derivatives)
 - rather than throwing away your perfectly good VASIMR stage, 1MW solar power generation unit and interplanetary hab module (aka your spaceship which you could give an inspiring name...) after one mission, you use it for several missions
 - at some point, if technology improves, even lunar landers could be made reusable (As suggested by many people already)
 - a sustainable Mars mission architecture could be construed this way after you have done several Flexible Path missions - rather than putting all your structure on Mars in different launches, you use 2 launches for your Mars surface mission modules (40t each with heatshield) to get them to your L1/2 station, dock with your spaceship which you have already been using for a previous Mars orbital mission, fuel up your VASIMR stage with quite some argon and off you go for a Mars mission with just one spacecraft in the say 300t range (fully fueled) - and rather than throwing all your infrastructure away after each mission you just reuse it on the next mission.

EDIT: VASIMR
« Last Edit: 08/07/2009 12:36 pm by simon-th »

Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #1 on: 08/07/2009 08:24 am »
Quote
- Depots at L1/2 make more sense than depots in LEO - you don't have to store all the EDS propellant

I don't think this is true. For one you couldn't refuel your upper stage in LEO, so you would need a bigger launcher to launch a fully fueled EDS. Now I'm actually in favour of doing it this way in the short run (since it gets cryogenic depots off the critical path), but in the slightly longer run it is a disadvantage. Another disadvantage is that L1 rules out participation in propellant launches by small launch vehicles, which negates most of the benefit of depots. There is a very good case for doing both LEO and L1 depots and gateway stations however.
« Last Edit: 08/07/2009 08:28 am by mmeijeri »
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Offline Archibald

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Re: L1/2 spacestation with depot
« Reply #2 on: 08/07/2009 11:21 am »
Quote

Another disadvantage is that L1 rules out participation in propellant launches by small launch vehicles


Unless you use Belbruno trajectories.

Another alternative would be SEP/ STP tugs build by private entrepreneurship ( when talking about private SEP tug I think about Dennis Wingo Orbital Recovery /Hubble rescue  proposals > a SEP tug build from existing satellite electric thrusters plus ISS solar arrays) 

Why couldn't these tugs  refuel a NASA L1 / L2 station through COTS-like contracts ?

To summarize, a private entity would first launch a fuel tank to LEO. Then, a second private entity would pick the tank and send it to the L1 station via a STP or SEP tug.

Now that would boost the number of launches to LEO. Lots of fuel tanks, lots of tugs to launch !
« Last Edit: 08/07/2009 11:22 am by Archibald »
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Offline simon-th

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Re: L1/2 spacestation with depot
« Reply #3 on: 08/07/2009 12:24 pm »
Quote
- Depots at L1/2 make more sense than depots in LEO - you don't have to store all the EDS propellant

I don't think this is true. For one you couldn't refuel your upper stage in LEO, so you would need a bigger launcher to launch a fully fueled EDS.

I am not entirely sure I understand what you are getting at. A L1/2 depot (with a small manned space station - permanently manned or just temporarily manned) would be supplied by commercial launch providers. Of course only rockets with a launch capacity of several tons to EML-1 or EML-2 make sense to use for this task. This would rule out small rockets such as Taurus II. I'd say you are in the 10-35mt to LEO launcher field with about 4 to 15mt payload capacity to EML-1/2 for depot supplying duties. Entirely ok, if you ask me.

And the big benefit is, your depot doesn't have to store 1000mt fuel or more, it just has to hold in the range of 250-350mt of fuel for the same purpose than a LEO depot (+ you get all the added advantages above from a mission architecture perspective that a LEO depot doesn't provide you with - reusing in-deep space flight hardware - VASIMIR stages, Orion only to EML-1 or EML-2 and not all the way to Mars etc.).

Quote
Now I'm actually in favour of doing it this way in the short run (since it gets cryogenic depots off the critical path), but in the slightly longer run it is a disadvantage.

How so? I'd say it's the other way around. For NEO and circumlunar precursor missions, you really don't need any fuel depots. But once you go for lunar surface missions and interplanetary missions all those nice advantages that I listed pop up for a EML-1/2 depot.

Quote
Another disadvantage is that L1 rules out participation in propellant launches by small launch vehicles, which negates most of the benefit of depots.

I addressed that above. I very much disagree. Small rockets (in the 5-10mt to LEO range) are inefficient in kg to cost anyway. Launching 20 rockets with a 25mt to LEO and 12mt to EML-1/2  capacity gets you a much better price per kg to LEO/EML-1/2 than 50 rockets with a 10mt to LEO launcher. That's because your variable cost start to lead your total expense line when you get into the 15-20 launches per year range.


Offline simon-th

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Re: L1/2 spacestation with depot
« Reply #4 on: 08/07/2009 12:26 pm »
I attach a flowchart of how I would envision a development and ops timeline from 2010-2030 with a L1/2 spacestation with depot capability, Flexible Path, directly shuttle derived HLV and lunar sorties added to Flexible Path with a long term goal of a human Mars surface mission in the late 2030s.

Of course, this would be an "above budget" option, in the 90-100 billion range to 2020 (in line with FY2009 budget guidance however...).
« Last Edit: 08/07/2009 08:54 pm by simon-th »

Offline iontyre

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Re: L1/2 spacestation with depot
« Reply #5 on: 08/07/2009 12:32 pm »
Before we go much further, let's get one thing straight:

It is VASIMR, not VASIMIR.

Variable Specific Impulse Magnetoplasma Rocket
« Last Edit: 08/07/2009 12:41 pm by iontyre »

Offline Arthur

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Re: L1/2 spacestation with depot
« Reply #6 on: 08/07/2009 12:38 pm »
- you could have a dedicated depot for LH2/LOX or only LOX or at the beginning only hypergolics for lunar missions and a dedicated depot for argon for interplanetary missions using VASIMIR (or derivatives)

If you will permit a question born out of ignorance:
I know that the vacuum of space is a terrific insulator.
I also know that cryogenic fuels (like LOX and LH2) like to boil off.

So how fast would a L1/2 depot loose LH2 to boil off?
Could I fill the depot and use the fuel a year later, or would it be empty in a couple months?

I am just attempting to get a feel for what the practical usefulness and limitations are for fuel depots.



With respect for Hypergolics vs Cryogenic fuels, I was under the impression that LH2 was overwhelmingly superior than Hypergolics for long range missions (like Mars).

What would the typical mission mass penalty be for using Hypergolics over LH2?

Any idea how long of a storage/mission length is required to make Hypergolics better than Cryogenics?
Earth to Mars?
Earth to Jupiter?
Earth to OORT cloud? (unmanned of course)
« Last Edit: 08/07/2009 12:40 pm by Arthur »

Offline DGH

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Re: L1/2 spacestation with depot
« Reply #7 on: 08/07/2009 12:45 pm »
The station has many other benefits as well.

A safe haven.
Solar sells to help power the Orion.
A check out point before lunar landing.

Also a depot does not have to be only fuel.
Food, water, spare parts and other equipment could be stored there.
A Delta IV heavy could deliver about 3-4 mt of supplies more if fuel to an EML station.
Most equipment and fuel could take the slow route.

The trick would be to keep the station simple.
Say a single Bigelow Sundancer with additional docking ports not an ISS.

Mixing flexible and Lunar base in this way seems to me most in line with the original VSE proposals.

Offline simon-th

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Re: L1/2 spacestation with depot
« Reply #8 on: 08/07/2009 12:47 pm »

So how fast would a L1/2 depot loose LH2 to boil off?
Could I fill the depot and use the fuel a year later, or would it be empty in a couple months?

I am just attempting to get a feel for what the practical usefulness and limitations are for fuel depots.

You need to develop the technology which deals with boil off, that is you have a cycle where you constantly use the "biol off propellant" after you bring it to the required low temperature again and add it back to the depot. Of course that means you need such a cooling cycle system and a power system which runs that system as well as shielding which mitigates the amount of propellant you need to get through your cycle constantly.

Quote
With respect for Hypergolics vs Cryogenic fuels, I was under the impression that LH2 was overwhelmingly superior than Hypergolics for long range missions (like Mars).
For interplanetary travel you should (not necessarily must) go for alternative propulsion system - e.g. VASIMR or similar high-isp, medium thrust engines.

Quote
What would the typical mission mass penalty be for using Hypergolics over LH2?

isp hypergolics 320
isp LH2/LOX 450
for a delta-v of 1000m/s a 20mt spacecraft (Orion) requires 5.5mt of fuel if you go for hypergolics, for the same delta-v with LH2/LOX you only need 4mt of fuel.

Quote

Any idea how long of a storage/mission length is required to make Hypergolics better than Cryogenics?

See above. For a depot to work you need to get rid of the boil off entirely through a regenerative cooling cycle.

Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #9 on: 08/07/2009 12:48 pm »
I am not entirely sure I understand what you are getting at. A L1/2 depot (with a small manned space station - permanently manned or just temporarily manned) would be supplied by commercial launch providers. Of course only rockets with a launch capacity of several tons to EML-1 or EML-2 make sense to use for this task. This would rule out small rockets such as Taurus II. I'd say you are in the 10-35mt to LEO launcher field with about 4 to 15mt payload capacity to EML-1/2 for depot supplying duties.

Agreement so far.

Quote
Entirely ok, if you ask me.

Here's my worry: one of the main benefits I see for depots is that they provide a way for new launchers to prove their worth. I'm assuming new entrants will likely intially field very small launchers. See Falcon and Taurus, see the Scorpius and Aquarius concepts, see what Masten is doing. One of the nice things about propellant is that it is essentially infinitely divisible, allowing even small vehicles to compete. Another aspect is RLVs. They need high flight rates, which is easier to achieve with small payloads.

Therefore I would prefer both a LEO depot and an L1 depot. Bigger launchers could launch directly to L1, smaller ones could launch to LEO. This also requires transport from LEO to L1, which could be done by larger launchers launching fully fueled upper stages as transport vehicles, or topping up existing ones.

Quote
And the big benefit is, your depot doesn't have to store 1000mt fuel or more, it just has to hold in the range of 250-350mt of fuel for the same purpose than a LEO depot (+ you get all the added advantages above from a mission architecture perspective that a LEO depot doesn't provide you with - reusing in-deep space flight hardware - VASIMIR stages, Orion only to EML-1 or EML-2 and not all the way to Mars etc.).

Yes, high energy orbits offer many advantages. The thing is, you still need to get your crew capsule to such a higher energy orbit. If you can top up an existing upper stage in LEO, you need a much smaller launcher to get there.

Quote
How so? I'd say it's the other way around. For NEO and circumlunar precursor missions, you really don't need any fuel depots. But once you go for lunar surface missions and interplanetary missions all those nice advantages that I listed pop up for a EML-1/2 depot.

I'm totally in favour of an L1 depot, not instead of a LEO depot but in addition to it. Remember, my plan was for a hypergolic depot which essentially requires it to be at L1, or it would be far too inefficient to make sense. An L1 depot would still be useful for NEO missions, since it would allow you to launch bigger payloads for a fixed volume available to your EDS. And if you want to use the very heavy Orion and avoid the need for bigger upper stages, you will still need propellant transfer even for cis-lunar or NEO missions.

Quote
I addressed that above. I very much disagree. Small rockets (in the 5-10mt to LEO range) are inefficient in kg to cost anyway. Launching 20 rockets with a 25mt to LEO and 12mt to EML-1/2  capacity gets you a much better price per kg to LEO/EML-1/2 than 50 rockets with a 10mt to LEO launcher. That's because your variable cost start to lead your total expense line when you get into the 15-20 launches per year range.

I was thinking of RLVs (reuse to reduce cost) or maybe things like Scorpius (reduced cost through mass production) or Aquarius (mass production + lowered reliability).
« Last Edit: 08/07/2009 12:49 pm by mmeijeri »
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Offline simon-th

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Re: L1/2 spacestation with depot
« Reply #10 on: 08/07/2009 12:56 pm »
I understand where you getting at in your posts. I just think we can't have both LEO depots and L1/2 depots from a budget perspective and rather opt for one that is geared towards lunar flights and interplenatary flights


Yes, high energy orbits offer many advantages. The thing is, you still need to get your crew capsule to such a higher energy orbit. If you can top up an existing upper stage in LEO, you need a much smaller launcher to get there.

Just a note on that point. Orion is a given. So we are stuck with a 20mt spacecraft that - once we have killed off the Ares I - can go back to a 6-crew capacity. The committee will opt for an HLV in any event which at least a 75mt to LEO and about 30mt to TLI capacity (or more if you develop J-246). That being said, any down-the line (of course not simple precursor missions) beyond-LEO mission would use Orion with some kind of MPLM/Hab or Lunar Lander (dry) (all depending on the mission) to the L1/2 space station architecture. Maxing out your HLV with Orion and that additional payload is enough for any conceivable beyond-LEO mission WITHOUT refueling your EDS in LEO.

Offline Bill White

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Re: L1/2 spacestation with depot
« Reply #11 on: 08/07/2009 01:02 pm »
Bridges are usually built in both directions simultaneously. A LEO depot extends one side of a bridge from Earth, but to where?

An EML-1 / EML-2 depot augmented with RLLs gives purpose to a LEO depot. Without a destination out there, a LEO only depot is merely a government funded "make work" project which some people hope will stimulate RLV development.

But without a pre-existing destination "out there" a government depot cannot possibly buy enough fuel (IMHO) to generate enough demand to change the economic realities of Earth-to-LEO launch.

Therefore, try something different. If someone deploys an EML Gateway to the Moon and supports it with existing launchers (Proton Block D can be clipped to any payload for delivery to EML-1 / EML-2 easily enough) that creates a MARKET for private developers of depots to sell to.

Seek to create markets, Martijn, not mandates from government.

Am I truly less "statist" than you? That would be ironic.  ;)
« Last Edit: 08/07/2009 01:03 pm by Bill White »
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Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #12 on: 08/07/2009 01:06 pm »
I understand where you getting at in your posts. I just think we can't have both LEO depots and L1/2 depots from a budget perspective and rather opt for one that is geared towards lunar flights and interplenatary flights

Ah, I see, that makes sense. If I had to choose, I too would choose the L1 depot if it were hypergolic. Otherwise it would be a very difficult choice, but I'd lean towards L1.

If cost is the obstacle, one thing to consider is using in-flight refueling instead of a dedicated depot. This is one reason I'm fond of a universal fully reusable lander: it can be used as a lander, a cislunar and translunar crew shuttle, a rescue vehicle, a depot and a mini-space station. It would have reduced capability in each of these functions compared to a specialised vehicle, but development cost would be much lower. In that sense I think it would be ideal for the initial phase of an exploration program, much like the shuttle was very early on.

Quote
Just a note on that point. Orion is a given. So we are stuck with a 20mt spacecraft that - once we have killed off the Ares I - can go back to a 6-crew capacity. The committee will opt for an HLV in any event which at least a 75mt to LEO and about 30mt to TLI capacity (or more if you develop J-246). That being said, any down-the line (of course not simple precursor missions) beyond-LEO mission would use Orion with some kind of MPLM/Hab or Lunar Lander (dry) (all depending on the mission) to the L1/2 space station architecture. Maxing out your HLV with Orion and that additional payload is enough for any conceivable beyond-LEO mission WITHOUT refueling your EDS in LEO.

If you accept HLV as a given this is true and I agree it is likely. My thinking was centered around demonstrating an HLV was not in fact necessary. Initially it was even intended to show J-120 + Delta upper stage would be enough so at least you would not need a new upper stage even if you were stuck with an ET-derived core.
« Last Edit: 08/07/2009 01:35 pm by mmeijeri »
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Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #13 on: 08/07/2009 01:10 pm »
Seek to create markets, Martijn, not mandates from government.

Absolutely, but that does not absolve government from the responsibility to procure services commercially where it can. I would say it has a fiduciary duty to its taxpayers to do that.

Quote
Am I truly less "statist" than you? That would be ironic.  ;)

Well, since you are a proponent of government-owned launchers that would seem to be impossible. If there were no exploration program, I would not be lobbying the government to create one or to create depots. If there's going to be an exploration program anyway, I say it ought to be geared towards commercial synergy. I think that makes me less statist than you. Not that I think there are any prizes for being the least statist or anything. ;)
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Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #14 on: 08/07/2009 01:25 pm »
With respect for Hypergolics vs Cryogenic fuels, I was under the impression that LH2 was overwhelmingly superior than Hypergolics for long range missions (like Mars).

There are some additional advantages to hypergolics, but those are not crucial. The main argument is a political one: hypergolic fluid transfer is:

1) good enough for full-blown exploration to the moon, Mars and beyond without HLV, provided you put your depot at L1 and use LOX/LH2 and efficient trajectories to get there, perhaps augmented with high Isp propulsion
2) proven technology. NASA is using the alleged difficulty of cryogenic depots as a pretext, but this cannot apply to a technology that has seen continuous operational use ever since Salyut-6 in 1978 and is used on the ISS today.

Minor advantages to hypergolics include higher density (more oomph by volume, which might be an issue if restricted by EELV fairings especially since the reduced oomph by mass can be fixed with depots), higher reliability, no need to develop boil-off mitigation systems and no new engine development. Higher delta-v missions would suffer more from lower Isp, but longer duration missions might benefit more from having no need to consider boiloff.

But I'd say it's a wash technically, with politics being the crucial reason why I'd support hypergolics first. You may be surprised to hear this. How can I say it's a wash with the lower Isp? The thing is, Isp doesn't matter so much if

1) you start from a high energy orbit like L1 and get there with LOX/LH2,
2) use an Earth swingby,
3) preposition propellant using high Isp propulsion and/or
4) use ISRU.
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Offline randomly

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Re: L1/2 spacestation with depot
« Reply #15 on: 08/07/2009 05:22 pm »
What about having a LEO Propellant depot so that it can be filled by a variety of smaller launch vehicles, then move that depot to EML1, either with SEP or just a standard engine using some of the available propellant.

Assuming a hypergolic depot how much fuel would you need to boost it to EML1 and deliver 100mt of propellant? Or if you had something like a 200 Kw Solar powered VASIMR, how long would it take to move it from LEO to EML1?

Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #16 on: 08/07/2009 05:25 pm »
Do you mean moving the depot itself back and forth? That sounds a bit impractical. It's not an option for hypergolics, since that would be very inefficient.
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Offline randomly

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Re: L1/2 spacestation with depot
« Reply #17 on: 08/07/2009 05:35 pm »
yes, I'm proposing moving the depot back and forth. If SEP is a feasible option it could be efficient. Even with chemical rockets it might be feasible.

If the mass fraction of the depot is pretty good it may be relatively easy to move it back down to LEO again once it's empty.

As to using hypergolics instead of Cryo to boost it from LEO to EML1 how bad would the mass to LEO penalty be?

The other option is to use a cryo EDS to move the depot to EML1. You could probably use hypergolics to move it back.

It might be more efficient to move the depot to EML1 than to send a bunch of small rockets with upper stages out to EML1 to dock with the depot there and fill it up. You avoid all those upper stages, and you avoid the communications delay if you are controlling docking real time from the ground (I don't know if that's preferred or not).

If you had several depots cycling between LEO and EML1 you might be able to tolerate the long transit times from using SEP. You would also have backup depots in case of failures.
« Last Edit: 08/07/2009 05:44 pm by randomly »

Offline Hop_David

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Re: L1/2 spacestation with depot
« Reply #18 on: 08/07/2009 05:46 pm »
With respect for Hypergolics vs Cryogenic fuels, I was under the impression that LH2 was overwhelmingly superior than Hypergolics for long range missions (like Mars). cloud? (unmanned of course)

When you have a big delta V budget, a high exhaust velocity is desirable. That's because both delta V and exhaust velocity are in the exponent of the rocket equation for the ratio of fuel mass to payload mass:

Mf/Mp = e(dV/Ve) - 1

e is Euler's number, about 2.72.

Delta V from earth surface to Mars is about 15 km/sec. With a number like 15 in the the above exponent you can see the Mf/Mp is big. Unless Ve is big.

LOX/LH2 has a higher exhaust velocity than LOX and kerosene, methane, propane etc.

However fuel depots could change the picture. Delta V between LEO and L1 is about 4 km/sec. Delta V between L1 and Deimos is about 3 km/sec.

Fuel depots at LEO, L1, and Deimos would break the delta V budget into smaller hops. You'd still need about 10 km/sec to get from earth to LEO. But moving between fuel depots could be done in 4 km/sec and 3 km/sec hops.

When your delta V budget is 4 to 3 km/sec, there's not the pressing need for a fuel with a high exhaust velocity.

Offline mmeijeri

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Re: L1/2 spacestation with depot
« Reply #19 on: 08/07/2009 06:09 pm »
David, also consider the effect of an Earth swingby. Look up the Oberth effect and hyperbolic trajectories on Wikipedia. The swingby amplifies the effect of your TMI. There's also a section on this in the documentation (in the "Deep Space Manual") of the freeware Orbiter flight simulator.
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