Author Topic: Starship-Enabled Extrasolar Missions  (Read 60641 times)

Offline TheRadicalModerate

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Starship-Enabled Extrasolar Missions
« on: 01/06/2022 06:25 am »
The discussion about possible Starship third stages or deployable kick stages got me thinking about extremely high-C3 missions and what they'd be good for.  In terms of outer planets missions, the answer is probably that the StarKicker idea (a stripped-down Starship with a dry mass of maybe 50t) makes more sense than a deployable kick stage.

But things get more interesting if you want to do extrasolar missions, out past the heliopause, within the lifetime of a researcher.

Two rough classes of mission are interesting:

1) Go as fast as you can as far as you can, allowing a probe to gather data on the interstellar medium as you get farther and farther (much farther than the Voyagers will survive) away from the heliopause.  A full sample out to 1000AU would be very useful.  You can probably do this with a very small payload.

2) Go out to a dead stop at the solar gravitational focus (about 542AU), to do sub-100-km-resolution imaging of extrasolar planets.  This is currently science fiction, but my guess is that it would stop being science fiction rather abruptly if somebody could demonstrate a not-incredibly expensive way to get a payload to the right spot.  Note that you're going to need a lot of delta-v once you get there, because the focal plane of the targets you're attempting to image are going to be whizzing along at impressive speeds, because you're tracking orbiting planets, and even nano-arcseconds of motion will cover impressive distances.

Even a heavily-laddered StarKicker can't get close to the speeds needed to make this convenient (and therefore attractive) for researchers.  However, I'd guess that these missions become interesting if you can get to your targets in 20 years.  That goal may or may not be possible. 

Let's find out.  Some thoughts:

1) 1000AU in 20 years is an average speed 237km/s.  Can we do that?  (See StarKicker constraints below.)

2) To do a dead stop at the solar focus, you need to do 271AU in 10 years, and then another 271AU in 10 years to stop.

3) Chemical propulsion just ain't gonna cut it, but a StarKicker can get you to heliocentric escape speed, which is C3=148km/s, if I did the math right.  If we assume:

- a dry mass of 50t
- total prop of 2000t
- Isp=378s
- we can dock arbitrarily sized payloads together on its nose because it has no fairing, and
- it leaves from a C3=-2 HEEO (400x385,000km) after being completely refueled

...then it needs 5550m/s to get to heliocentric escape.  That's massRatio = exp(5550/378/9.8) = 4.47.  Max payload is therefore 526t.  That's your maximum "payload" gross mass.

4) From heliocentric escape speed, am I correct in assuming that low-thrust electric propulsion delta-v will have no further gravity losses?

5) I'm going to assume Isp=3000s in the arm-waves below.

6) For interstellar medium measurement missions, hanging a nuke off the back of your probe is problematic if you're trying to measure various kinds of GCR.  Therefore, there's an NEP constraint for these missions that burnout occur at about 90AU, so that the nuke can be jettisoned.  That constraint doesn't exist for solar focus missions, though.  That may yield two very different power systems.

7) For both interstellar medium and solar focus missions, SEP has a constraint that burnout occur no more than about 5AU out, simply because insolation becomes untenable beyond that distance.  I'm pretty sure that this rules out SEP as a propulsion method, because you'd need thrusts that would require gazillions of kW, and I'm not gonna go any further with it unless somebody can convince me that it's viable.

8) For NEP, I think that gives us the following constraints:
- Time to target constrains us to a certain delta-v, which implies a certain amount of propellant.
- Burnout distance constrains us to a certain mass flow, which in turn implies a certain thrust to use all the prop.
- Thrust constrains us to a certain amount of power from the nuke.
- The nuke power constrains us to a certain nuke+radiator mass, given some specific power. Prometheus, which was supposed to be a 15kWe system, seemed to be getting about 12W/kg if you add up the reactor and heat rejection masses.  I suspect this is highly non-linear, and my previous guesses on power have been hundreds of kW, so I'm gonna go with 50W/kg and see what happens.

Anybody want to take a crack at sizing the NEP system?

One thing that will probably be handy:  For the interstellar sampling mission, you're going to need to compute a mass flow in terms of burnout distance D from the Sun (I'm going to use 90AU).  Assuming that your StarKicker velocity has decayed to approximately zero by the time you get to that distance, I found a magic formula and tweaked it a bit:

mdot = (ve/D)*(M0 - Mf(ln(M0/Mf)+1))

mdot * Isp * 9.8 then gives you your thrust, which gives you your nuke power, based on thrust/kW (usually very close to 60mN/kW), and the nuke power will then give you the mass of the nuke and its radiators, based on your guess for specific power (I'm using 50).

I'm already well into TL;DR territory and haven't yet put together the magic spreadsheet that lets me estimate the rest of the mission parameters.

It's a nice meaty problem, isn't it?
« Last Edit: 01/06/2022 06:31 am by TheRadicalModerate »

Offline DreamyPickle

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Re: Starship-Enabled Extrasolar Missions
« Reply #1 on: 01/06/2022 10:35 am »
If you're assuming multiple decades of flight time and NEP why not just use orbital assembly of multiple NEP modules in LEO?

The sort of mission that makes the most sense for an starship-based kicker is a heavy lander for the moons of the outer planets.

Offline Slothman

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Re: Starship-Enabled Extrasolar Missions
« Reply #2 on: 01/06/2022 10:55 am »
I feel like the key to extrasolar exploration is continuous propulsion, not giving an impulse close to the launch point and letting nature(newton) take its course. This naturally points towards nuclear propulsion of some sort, for which of course a "third stage" of ST/SH wouldn't be really necessary as it would be incorporated in the payload of Starship already.

Offline edzieba

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Re: Starship-Enabled Extrasolar Missions
« Reply #3 on: 01/06/2022 12:49 pm »
Once you start getting into mission masses of several hundred tons with high-thrust engines, the Solar Oberth Manoeuvre (performing your solar system escape burn as close to the Sun as possible) starts looking more attractive again due to the smaller impact of the required heatshield.

Offline Robotbeat

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Re: Starship-Enabled Extrasolar Missions
« Reply #4 on: 01/06/2022 02:07 pm »
Solar Oberth burn is pretty good. Id do that. Scalable up to 100-200, maybe 300km/s (with nuclear thermal and extreme measures). But you can do 100km/s with just chemical rockets.

Beyond 300km/s, you need something better. Fission fragment could potentially get you up to several percent the speed of light. Im principle you could get to 10% with lots of staging. But it becomes probably intolerable (fuel costs) for large payloads. Who has like 1 million tons of fissionables lying around? Fusion is also possible but a bit too handwavey at the moment (but that could change in this decade).

I think beamed propulsion using macroscopic particles is the way to go, showed down by magnetobrake against the interstellar medium. But that requires a megaton to orbit to build. You need Starship for that.
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Offline Comga

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Re: Starship-Enabled Extrasolar Missions
« Reply #5 on: 01/06/2022 03:17 pm »
Much of this has been discussed in the Interstellar Probe thread. Their report is attached to post 49. 
That discussion includes targets and trajectories and, of course, launch.
In essence, the study found no practical alternative for C3>200 to a four stage SLS stack, with all the caveats that entails.
They explored and abandoned the Solar Oberth maneuver.
IIRC, the study did not get a response from SpaceX. (Or ULA or Blue Origin for that matter.)
So deep space missions are an appropriate nail to swing at with our favorite hammer.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline edzieba

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Re: Starship-Enabled Extrasolar Missions
« Reply #6 on: 01/06/2022 04:05 pm »
I agree that Starship is not a system really suited to high C3 missions (lots of mass to orbit and low C3) beyond being used to lift a high C3 vehicle. At the moment, achieving high C3 means either a gossamer Hydrolox stage, solar-electric, or nuclear electric.
A light Hydrolox earth departure stage is the most near-term feasible: very large balloon tanks that only receive a full propellant load in orbit, and a high ISP low thrust engine. NTR would be a potential substitute using the same stage structure, but for the current lack of NTR engines and the poor specific power of proposed engines.
Solar electric would be preferable for pure ballistic high C3, but for the lack of sunlight as the vehicle recedes from the Sun. Low thrusts (less mass spent on engines) gets you your propulsive efficiency, but also means longer burns which takes you further from the Sun and drops your power. By the time you trade a shorter higher thrust burn near the Sun, you use up the payload mass you gained from a more efficient engine.
Nuclear electric would be preferable for its consistent high power delivery, if lightweight reactors existed. They currently do not: Kilopower has a better specific power (kg/kW) than an RTG, but still too high to offset the large dry mass, and that's the only near-term option available.

The big problem is that for really high C3, more mass-to-orbit isn't really all that big of a help. You need an improvement in propulsive efficiency at low stage mass, which is not something existing or near-term propulsion technology offers.

Offline TheRadicalModerate

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Re: Starship-Enabled Extrasolar Missions
« Reply #7 on: 01/06/2022 04:13 pm »
If you're assuming multiple decades of flight time and NEP why not just use orbital assembly of multiple NEP modules in LEO?

The sort of mission that makes the most sense for an starship-based kicker is a heavy lander for the moons of the outer planets.

I am indeed using orbital assembly, but my assumption is that an impulsive departure to solar escape eliminates multiple km/s in gravity losses and a lot of time spent spiraling around going nowhere.

I agree that StarKicker is better suited for heavy outer planets missions, but those require about an order of magnitude less delta-v than the "fast" extrasolar missions.

Offline TheRadicalModerate

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Re: Starship-Enabled Extrasolar Missions
« Reply #8 on: 01/06/2022 04:36 pm »
Solar Oberth burn is pretty good. Id do that. Scalable up to 100-200, maybe 300km/s (with nuclear thermal and extreme measures). But you can do 100km/s with just chemical rockets.

Beyond 300km/s, you need something better. Fission fragment could potentially get you up to several percent the speed of light. Im principle you could get to 10% with lots of staging. But it becomes probably intolerable (fuel costs) for large payloads. Who has like 1 million tons of fissionables lying around? Fusion is also possible but a bit too handwavey at the moment (but that could change in this decade).

I think beamed propulsion using macroscopic particles is the way to go, showed down by magnetobrake against the interstellar medium. But that requires a megaton to orbit to build. You need Starship for that.

Even with a fully-fueled StarKicker with the same specs I used in the OP, a 0.05AU x 5AU Oberth burn with a 1t payload only gives you a v∞ of 69km/s.  And that ignores the extensive laddering you'd have to do to get it full, and the massive thermal weirdness you'd need to keep the prop from boiling off before perihelion.

I'm trying to do this as "off-the-shelf" as I can.  Granted, a 100kWe+ space nuke is anything but off the shelf, but it's a piece of tech that we know how to build, and the rest of the electric propulsion is high TRL.  (Design life is an issue, but if you just made the thruster packs triply redundant, I don't think it changes your dry mass by more than a tonne.)

Offline TheRadicalModerate

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Re: Starship-Enabled Extrasolar Missions
« Reply #9 on: 01/06/2022 04:50 pm »
I agree that Starship is not a system really suited to high C3 missions (lots of mass to orbit and low C3) beyond being used to lift a high C3 vehicle. At the moment, achieving high C3 means either a gossamer Hydrolox stage, solar-electric, or nuclear electric.
A light Hydrolox earth departure stage is the most near-term feasible: very large balloon tanks that only receive a full propellant load in orbit, and a high ISP low thrust engine. NTR would be a potential substitute using the same stage structure, but for the current lack of NTR engines and the poor specific power of proposed engines.
Solar electric would be preferable for pure ballistic high C3, but for the lack of sunlight as the vehicle recedes from the Sun. Low thrusts (less mass spent on engines) gets you your propulsive efficiency, but also means longer burns which takes you further from the Sun and drops your power. By the time you trade a shorter higher thrust burn near the Sun, you use up the payload mass you gained from a more efficient engine.
Nuclear electric would be preferable for its consistent high power delivery, if lightweight reactors existed. They currently do not: Kilopower has a better specific power (kg/kW) than an RTG, but still too high to offset the large dry mass, and that's the only near-term option available.

The big problem is that for really high C3, more mass-to-orbit isn't really all that big of a help. You need an improvement in propulsive efficiency at low stage mass, which is not something existing or near-term propulsion technology offers.

Do you know how the specific power curve scales with nuclear electric (in theory, of course)?  Very quick BOE I did from skimming the Prometheus report seemed like they were looking at 12W/kg for 15kWe system, which is almost triple the early SNAP attempts, and the 10kWe version of Kilopower looks like it was at about 6.7W/kg when fully burdened with radiators.  I'm using 50 because it seems like the curve should look vaguely learning-like, but it's a SWAG. Any info would be appreciated.

PS:  My plan for sizing NEP dry mass is to treat the power and heat rejection system as an add-on fixed mass, based on the power requirements to hit the appropriate velocities and burnout times, and then use an SMF of 4% or so for the rest of the tankage, the bus, and the thrusters.  Sound reasonable?
« Last Edit: 01/06/2022 04:55 pm by TheRadicalModerate »

Offline InterestedEngineer

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Re: Starship-Enabled Extrasolar Missions
« Reply #10 on: 01/06/2022 07:43 pm »
Much of this has been discussed in the Interstellar Probe thread. Their report is attached to post 49. 
That discussion includes targets and trajectories and, of course, launch.
In essence, the study found no practical alternative for C3>200 to a four stage SLS stack, with all the caveats that entails.
They explored and abandoned the Solar Oberth maneuver.
IIRC, the study did not get a response from SpaceX. (Or ULA or Blue Origin for that matter.)
So deep space missions are an appropriate nail to swing at with our favorite hammer.

I think all studies that include ancient tech such a SLS need to be read, understood, and thrown away.

We are on the cusp of a two orders of magnitude change in cost of fuel and other mass to LEO.   All plans that don't include this soon-to-be-fact are obsolete.

We can get 100T of cargo leaving earth at C3 >= 225 with HEEO Oberth burn at Earth and throwaway Starship (aka Vinf = 15km/sec).   

What can we do with that 100T?   Prior plans could barely get 10T with the same C3.  It's time to re-examine.

Offline InterestedEngineer

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Re: Starship-Enabled Extrasolar Missions
« Reply #11 on: 01/06/2022 08:06 pm »

I'm already well into TL;DR territory and haven't yet put together the magic spreadsheet that lets me estimate the rest of the mission parameters.

It's a nice meaty problem, isn't it?

Executive summary of requirements.

Radical Moderate's Missions.  This is clearly a stretch goal, but a good stretch goal:

1.1) 237km/sec (C3 = 57,000)  for 1000AU over 20 years, no stopping.  Let's call this the "Oort cloud mission"
1.2) 542 AU in 20 years with a dead stop at the end (at the solar focus).  RadicalModerate this needs a name.

From  from the already-obsolete JHUAPL proposal, Table 3-3.  This is the minimum viable goal to avoid STL "expensive by 3 orders of magnitude" agony in 2036
https://interstellarprobe.jhuapl.edu/Interstellar-Probe-MCR_V4_for-posting.pdf

JHUAPL "Heliopause in 15 years" mission:
2.1) C3 = 130 to get Jupiter boosts (table H3)
2.2) Launch August 2036 because of Jupiter alignment
2.3) 100 AU goal in < 15 years
2.4) Overall budget $1.7B in 2021 dollars


Note useful map of outer solar system

https://www.nasa.gov/mission_pages/voyager/multimedia/pia17046.html

« Last Edit: 01/06/2022 08:16 pm by InterestedEngineer »

Offline InterestedEngineer

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Re: Starship-Enabled Extrasolar Missions
« Reply #12 on: 01/06/2022 08:33 pm »


From  from the already-obsolete JHUAPL proposal, Table 3-3.  This is the minimum viable goal to avoid STL "expensive by 3 orders of magnitude" agony in 2036
https://interstellarprobe.jhuapl.edu/Interstellar-Probe-MCR_V4_for-posting.pdf

JHUAPL "Heliopause in 15 years" mission:
2.1) C3 = 130 to get Jupiter boosts (table H3)
2.2) Launch August 2036 because of Jupiter alignment
2.3) 100 AU goal in < 15 years
2.4) Overall budget $1.7B in 2021 dollars


Note useful map of outer solar system

https://www.nasa.gov/mission_pages/voyager/multimedia/pia17046.html

Some thoughts on the "JHUAPL Heliopause in 15 years" mission:

Quote
The sky map representing Option 1 trajectory possibilities with a wet mass of 860 kg
(C3 = 304.07 km2/s2) appears in Figure 3-

Here's how obsolete planning with 4 stages of SLS is.  All the fancy kick stages, Jupiter gravity slingshot, 860kg probe and throwaway Starship can do this from a HEEO on an  Earth Oberth maneuver with a C3 of 310 km2/s2, and a payload of *10 times* what they are proposing (8T probe), with a non-probe budget of about $150M including the throwaway Starship. ($60M in fuel to HEEO, and $90M for the throwaway Starship).

https://docs.google.com/spreadsheets/d/14zdF7bsbJnUbFYjkMOKjMll12SjMAKjP9buIXLU0w1U

One could use the 7T of the probe as an ion drive kicker stage for another 10 km/sec or C3 of 700-ish.  Have to work out the details of diminishing solar energy during acceleration, so TBD on that.


Offline gosnold

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Re: Starship-Enabled Extrasolar Missions
« Reply #13 on: 01/06/2022 09:46 pm »
I agree that Starship is not a system really suited to high C3 missions (lots of mass to orbit and low C3) beyond being used to lift a high C3 vehicle. At the moment, achieving high C3 means either a gossamer Hydrolox stage, solar-electric, or nuclear electric.
A light Hydrolox earth departure stage is the most near-term feasible: very large balloon tanks that only receive a full propellant load in orbit, and a high ISP low thrust engine. NTR would be a potential substitute using the same stage structure, but for the current lack of NTR engines and the poor specific power of proposed engines.

Let's do some math on a ~500t propellant LH2 stage.
You get for LOX 500*6/7=430t=390m3= a 9m diameter tank
You get for LH2 500*1/7=70t=1000m3= a 9x16m cylinder

Let's round that up to a 9x25m cylinder
Using 0.51mm stainless steel stages we have 830m2 of tank wall, so 0.4m3, so 3t
A single RL-10C-1-1 is 188kg
Let's say we have a 3t probe, so 3t of dry mass in total

Delta-v is 4.5*ln(506/6)=20km/s
Burn time is 5.5h
We can try to burn at perigee to get a ~40% Oberth boost.

Really shows the limits of chemical propulsion...

However if you want to exploit the 3000s Isp of electric propulsion, you'll need to keep the power generation mass super low to not increase the dry mass too much.
« Last Edit: 01/06/2022 09:54 pm by gosnold »

Offline Robotbeat

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Re: Starship-Enabled Extrasolar Missions
« Reply #14 on: 01/06/2022 11:12 pm »
Solar Oberth burn is pretty good. Id do that. Scalable up to 100-200, maybe 300km/s (with nuclear thermal and extreme measures). But you can do 100km/s with just chemical rockets.

Beyond 300km/s, you need something better. Fission fragment could potentially get you up to several percent the speed of light. Im principle you could get to 10% with lots of staging. But it becomes probably intolerable (fuel costs) for large payloads. Who has like 1 million tons of fissionables lying around? Fusion is also possible but a bit too handwavey at the moment (but that could change in this decade).

I think beamed propulsion using macroscopic particles is the way to go, showed down by magnetobrake against the interstellar medium. But that requires a megaton to orbit to build. You need Starship for that.

Even with a fully-fueled StarKicker with the same specs I used in the OP, a 0.05AU x 5AU Oberth burn with a 1t payload only gives you a v∞ of 69km/s.  And that ignores the extensive laddering you'd have to do to get it full, and the massive thermal weirdness you'd need to keep the prop from boiling off before perihelion.

I'm trying to do this as "off-the-shelf" as I can.  Granted, a 100kWe+ space nuke is anything but off the shelf, but it's a piece of tech that we know how to build, and the rest of the electric propulsion is high TRL.  (Design life is an issue, but if you just made the thruster packs triply redundant, I don't think it changes your dry mass by more than a tonne.)
Go closer. Scream past the surface of the Sun with an ablative heatshield.
« Last Edit: 01/06/2022 11:18 pm by Robotbeat »
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Offline InterestedEngineer

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Re: Starship-Enabled Extrasolar Missions
« Reply #15 on: 01/07/2022 06:03 am »
Go closer. Scream past the surface of the Sun with an ablative heatshield.

Screaming past Jupiter may be good enough. I've figured out how to get 100T to Jupiter, just working on the math of combining a flyby with 20:1 oberth burn.   I've got 40km/sec (heliocentric) just from the flyby, because the HEEO Starship burn off of Earth gives enough deltaV to give the optimal flyby speed for Jupiter. 

Very rough math with the Oberth burn using C3 calcs put the total heliocentric Velocity exiting Jupiter at 75 km/sec for a 5T probe + tanks + engine boost stage.  This includes gravity assist and assumes Isp of 375.   Possibly unrealistic, not sure how a liquid fueled rocket fires up after half a year in flight.

That is one AU every 23 days, or out to 100 AU in about 6 years.

It's still 61 years out to the Oort cloud.   Going to need a different strategy for the Oort cloud.

Spreadsheet is in rough draft mode.  Here's an interesting read on squeezing out maximum deltaV from gravity assist.  Somehow we can get 2 * orbital velocity of the planetary body in question, given a optimal speed and approach:

https://space.stackexchange.com/questions/6582/how-much-delta-v-can-we-squeeze-out-of-a-gravitational-slingshot-and-what-factor

I'm starting to wonder whether going down to the Sun might be better, not for the Oberth factor, but for longer time powering an ion engine with sunlight.   Starship certainly gives the deltaV to dive straight at the Sun without going out to Jupiter first (it's past midnight this comment is not thought out since you have to slow your heliocentric velocity to dive into the sun. Manana)
« Last Edit: 01/07/2022 07:32 am by InterestedEngineer »

Offline Robotbeat

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Re: Starship-Enabled Extrasolar Missions
« Reply #16 on: 01/07/2022 06:03 pm »
Starship has to be able to handle months to potentially years between Raptor engine firings in space, so the long quiescent period is kind of a requirement anyway.
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Offline envy887

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Re: Starship-Enabled Extrasolar Missions
« Reply #17 on: 01/07/2022 06:30 pm »
Go closer. Scream past the surface of the Sun with an ablative heatshield.

Screaming past Jupiter may be good enough. I've figured out how to get 100T to Jupiter, just working on the math of combining a flyby with 20:1 oberth burn.   I've got 40km/sec (heliocentric) just from the flyby, because the HEEO Starship burn off of Earth gives enough deltaV to give the optimal flyby speed for Jupiter. 

Very rough math with the Oberth burn using C3 calcs put the total heliocentric Velocity exiting Jupiter at 75 km/sec for a 5T probe + tanks + engine boost stage.  This includes gravity assist and assumes Isp of 375.   Possibly unrealistic, not sure how a liquid fueled rocket fires up after half a year in flight.

That is one AU every 23 days, or out to 100 AU in about 6 years.

It's still 61 years out to the Oort cloud.   Going to need a different strategy for the Oort cloud.

Spreadsheet is in rough draft mode.  Here's an interesting read on squeezing out maximum deltaV from gravity assist.  Somehow we can get 2 * orbital velocity of the planetary body in question, given a optimal speed and approach:

https://space.stackexchange.com/questions/6582/how-much-delta-v-can-we-squeeze-out-of-a-gravitational-slingshot-and-what-factor

I'm starting to wonder whether going down to the Sun might be better, not for the Oberth factor, but for longer time powering an ion engine with sunlight.   Starship certainly gives the deltaV to dive straight at the Sun without going out to Jupiter first (it's past midnight this comment is not thought out since you have to slow your heliocentric velocity to dive into the sun. Manana)

Why not both? You can probably get quite a kick by using the Jupiter gravity assist to lower the perihelion a lot. I'd be interested in numbers on the fry-by, if you want to crunch them :D

Offline wannamoonbase

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Re: Starship-Enabled Extrasolar Missions
« Reply #18 on: 01/07/2022 06:31 pm »
We need orbiter missions to Uranus and Neptune.

It's hard to beat the efficiency of gravitational sling shots.

Nuclear power with ion drive is maybe the way to get this done.  I don't think Starship itself is going anywhere that deep in space.  But it could haul the pieces and fuel to LEO. 

It'd be a big heavy spacecraft, but if Starship can fly frequently and deliver 250 tons to LEO, then lets think big.  Why not a 250-500 ton probe to Neptune.

I'm 48, I'd love to see before I go, a significant probe to deep in the Ort cloud and maybe planet X (if that gets found).  It will be hard to not go visit a world that big and that far away.

Superheavy + Starship the final push to launch commit!

Offline TheRadicalModerate

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Re: Starship-Enabled Extrasolar Missions
« Reply #19 on: 01/07/2022 08:15 pm »
OK, I think I got the NEP spreadsheet working, and it's not encouraging.  It doesn't appear possible to get even close to my 20-year mission targets.  See the link, or the snapshot attached below.

Notes:

1) Currently, this assumes heliocentric v∞=0 at startup.  An obvious enhancement is to look at how the various gravity assists (both unpowered planetary flybys and oberth maneuvers) can help.

2) I'm estimating power, propulsion, and heat rejection for the nuke module in terms of W/kg.  This is a sheer wild-ass guess.  Tankage is based on a structural mass fraction, and is more of a WAG than a SWAG.

3) The interstellar sampling mission (go as fast as you can and fly by 1000AU) has a jettison point you can set to keep the data free of artifacts from the nuke's radiation. This implies a certain mass flow to get to burnout before that point, which can increase the power requirement, and therefore the size of the nuke.  The nuke power is a user-provided parameter (that's the stuff in blue), but if you make it too small with a jettison limit, the spreadsheet will whine at you.

4) The trip is broken into three segments: prograde thrust, coast, and, if a rendezvous mission, as needed for solar focus, retrograde thrust.  The coast speed is assumed to be the delta-v after prograde shutdown.

5) You can fool with the gross mass of the vehicle+payload, but it's set at 526t because that's what I computed that a StarKicker with dry mass=50t, prop=2000t, Isp=378s could send to heliocentric v∞=0 from a C3=-2 (400x385,000km) staging orbit.  Other conops are obviously possible, but they're gonna get a lot more expensive and risky if you start laddering refueling at large C3's.

6) Another fairly serious deficiency is that I'm assuming that the tankage isn't staged.  This is dumb, because the vehicle is big enough to require multiple Starship launches to assemble, and you'd obviously design things to throw empty tanks away.  However, the nuke and the tankage are each close to 50% of the mass.  By and large, we're pretty far out on the logarithmic curve, and increasing the amount of prop doesn't provide a lot of improvement.

7) Increasing the nuke power always seems to improve mission times, but not by very much.  It looks to me that there's probably a sweet spot for both missions using about a 500kWe system.

As usual when it comes to anything involving an exponential, my intuition failed me.  This doesn't appear to be a way forward, unless coupled with something like the solar Oberth maneuver.  Bummer.

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