Replacing SLS/Orion using Starship HLS and Crew Dragon (AI data allowed)

[Roy_H]

The Oberth effect is easy to explain - why spend all that energy to raise your fuel to a 900km orbit when you can raise it to a 150km orbit for a lot less energy?

If your goal is just to reach any orbit, yes it takes less fuel to reach a lower orbit. In this case the goal is to reach Moon orbit.

The lower altitude you throw stuff out the back, the less work you have to do to raise that stuff to a higher orbit.

Please define higher orbit. I haven't attempted to make any calculations, so don't take this example too literally.
I would be willing to agree that it takes less fuel to go from a 150km orbit to a 350km orbit than it would to go from a 400km orbit to a 600km orbit. Both raising by 200km. But it does not take less work to go from 150km orbit to 600km orbit than it does starting at 400km. Please provide proof that it takes less work to go from 150km to 600km than from 400 km to 600km.

[Roy_H]

To reduce TLI fuel requirement I changed the LEO launch altitude to 900Km

Your AI is apparently too sycophantic to bring this to your attention — human to the rescue!!  — but using a higher parking orbit is less fuel-efficient, not more.

One of the ways NASA fit the lunar rover into the mass budget was by lowering the parking orbit from 100 nm to 90 nm. This increased the Oberth effect and made the trajectory more efficient.

Indeed, the charts show you want to be lower than 500 km, not higher.

I agree, there is no advantage to the 900km orbit with regards to debris. I will cease to make such a claim. But it is out of the area with large numbers of satellites. The reason to go to the 900km orbit is to save fuel for TLI, not to avoid debris.

No the Oberth effect doesn't just apply to elliptical orbits. Yes the Oberth effect is counterintuitive, but I'm sure AI can explain it better than me. 

Attached explanation by Grok. It explains how raising the initial orbit saves fuel. Not costing more as you claim.

[Twark_Main]

First, let's recognize that this is a textbook example of the limitations of this human/AI hybrid approach. Sycophancy means AI will avoid — and even act against — real fact-checking.

With that out of the way let's rescue our friend Roy here.

The Oberth effect is easy to explain - why spend all that energy to raise your fuel to a 900km orbit when you can raise it to a 150km orbit for a lot less energy?

If your goal is just to reach any orbit, yes it takes less fuel to reach a lower orbit. In this case the goal is to reach Moon orbit.

In this case too, starting in a lower Earth orbit is more efficient.

I haven't attempted to make any calculations, so don't take this example too literally.
 ... Please provide proof that it takes less work to go from 150km to 600km than from 400 km to 600km.

The correct question is: does it take more fuel to go from 150 to 900 and then from 900 to 500,000, or straight from 150 to 500,000?

I'm lazy and don't want to do the Hohmann transfer math, so I just Googled "Hohmann transfer calculator" and found this.

150-900: 413.26 m/s

900-500,000: 2991.01 m/s

150-500,000: 3166.17 m/s


So that's 3404.27 m/s versus 3166.17 m/s, or a 238.10 m/s reduction in delta-v thanks to Oberth.

[Roy_H]

The correct question is: does it take more fuel to go from 150 to 900 and then from 900 to 500,000, or straight from 150 to 500,000?

I'm lazy and don't want to do the Hohmann transfer math, so I just Googled "Hohmann transfer calculator" and found this.

150-900: 413.26 m/s

900-500,000: 2991.01 m/s

150-500,000: 3166.17 m/s

So that's 3404.27 m/s versus 3166.17 m/s, or a 238.10 m/s reduction in delta-v thanks to Oberth.

Ah, So you change the question in order to prove your point.

Your own data shows that the delta V is greater going from 150 - 500,000: of 3166.17m/s than 900 - 500,000: of 2991.01 m/s for a savings of 175.16 m/s.

My goal was to use the smallest possible tanker ship to carry fuel to get to the moon and back. Not to use the least amount of total fuel. And you have proven Grok was right.

[InterestedEngineer]

The correct question is: does it take more fuel to go from 150 to 900 and then from 900 to 500,000, or straight from 150 to 500,000?

I'm lazy and don't want to do the Hohmann transfer math, so I just Googled "Hohmann transfer calculator" and found this.

150-900: 413.26 m/s

900-500,000: 2991.01 m/s

150-500,000: 3166.17 m/s

So that's 3404.27 m/s versus 3166.17 m/s, or a 238.10 m/s reduction in delta-v thanks to Oberth.

Ah, So you change the question in order to prove your point.

Your own data shows that the delta V is greater going from 150 - 500,000: of 3166.17m/s than 900 - 500,000: of 2991.01 m/s for a savings of 175.16 m/s.

My goal was to use the smallest possible tanker ship to carry fuel to get to the moon and back. Not to use the least amount of total fuel. And you have proven Grok was right.

How many hours have you put into playing KSP?

Your intuitions appear wrong, so my guess is next to none.

If you want really good intuition on how to plan a space mission, then KSP is your friend.  It's cheap, go play it for a while, you intuition will be updated.

Grok cannot compete with KSP enforced human intuition.  Though I suspect if you added the prompt "let's plan something just like we would in KSP" it might spit out better answers, since there are many KSP forum posts on the topic.

[Twark_Main]

The correct question is: does it take more fuel to go from 150 to 900 and then from 900 to 500,000, or straight from 150 to 500,000?

I'm lazy and don't want to do the Hohmann transfer math, so I just Googled "Hohmann transfer calculator" and found this.

150-900: 413.26 m/s

900-500,000: 2991.01 m/s

150-500,000: 3166.17 m/s

So that's 3404.27 m/s versus 3166.17 m/s, or a 238.10 m/s reduction in delta-v thanks to Oberth.

Ah, So you change the question in order to prove your point.

Your own data shows that the delta V is greater going from 150 - 500,000: of 3166.17m/s than 900 - 500,000: of 2991.01 m/s for a savings of 175.16 m/s.

My goal was to use the smallest possible tanker ship to carry fuel to get to the moon and back. Not to use the least amount of total fuel.

I anticipated that reply, lol.

If that's your goal, you and Grok are still doing it wrong. You'd be better off (both in smallest tanker size and least amount of total fuel) by using an intermediate parking orbit that's an elliptical orbit with a 150 km perigee rather than a circular 900 km orbit.

Again, a human would've immediately recognized this problem, but Grok consistently shows it doesn't know the right question to ask (a more valuable skill than just being able to answer the question when someone has asked the wrong question).


This is gold, keep going. Don't let humans and facts ruin the glorious AI future! You're doing a marvelous job showing the pitfalls of AI, don't stop now. 

[Roy_H]

I anticipated that reply, lol.

If that's your goal, you and Grok are still doing it wrong. You'd be better off (both in smallest tanker size and least amount of total fuel) by using an intermediate parking orbit that's an elliptical orbit with a 150 km perigee rather than a circular 900 km orbit.

Again, a human would've immediately recognized this problem, but Grok consistently shows it doesn't know the right question to ask (a more valuable skill than just being able to answer the question when someone has asked the wrong question).

This is gold, keep going. Don't let humans and facts ruin the glorious AI future! You're doing a marvelous job showing the pitfalls of AI, don't stop now. 

Again I feel compelled to clarify my goals. In your elliptical orbit there are issues I wish to avoid, one is that it swings into the Van Allan Belt and two, makes multiple passes through the greatest number of satellite orbits where course adjustments might have to be made, and three, it is more difficult to launch other ships to the same elliptical orbit vs a circular orbit. I did not want to send the Dragon crew into the Val Allan Belt in order to match orbit with the HLS. If you confine the elliptical orbit to a 900km apogee, then the Oberth benefit is less than the gravity losses.

However, I agree that technically you are right. Just doesn't align with my goals. And Grok did not ask the wrong question, I posed my questions to Grok to find the most optimal plan to meet all my criteria.

[Twark_Main]

I anticipated that reply, lol.

If that's your goal, you and Grok are still doing it wrong. You'd be better off (both in smallest tanker size and least amount of total fuel) by using an intermediate parking orbit that's an elliptical orbit with a 150 km perigee rather than a circular 900 km orbit.

Again, a human would've immediately recognized this problem, but Grok consistently shows it doesn't know the right question to ask (a more valuable skill than just being able to answer the question when someone has asked the wrong question).

This is gold, keep going. Don't let humans and facts ruin the glorious AI future! You're doing a marvelous job showing the pitfalls of AI, don't stop now. 

Again I feel compelled to clarify my goals. In your elliptical orbit there are issues I wish to avoid, one is that it swings into the Van Allan Belt

Depends on the apogee height. You can still get the same delta-v reduction on the final burn without reaching the inner belt.

and two, makes multiple passes through the greatest number of satellite orbits where course adjustments might have to be made

Most of the object density (and collision risk) is in a ring near the poles, whereas this would be passing through those altitudes closer to the equator, so it's not actually as concerning as the simple altitude plot would suggest.

and three, it is more difficult to launch other ships to the same elliptical orbit vs a circular orbit.

Despite TheRadicalModerate's fearmongering, this isn't really true.

I did not want to send the Dragon crew into the Val Allan Belt in order to match orbit with the HLS. If you confine the elliptical orbit to a 900km apogee, then the Oberth benefit is less than the gravity losses.

Can I see the math? 

The inner belt starts at around 5,000 km, so there's plenty of margin here.

However, I agree that technically you are right. Just doesn't align with my goals. And Grok did not ask the wrong question, I posed my questions to Grok to find the most optimal plan to meet all my criteria.

Right. The problem is that AI doesn't push back. It's incapable of doing so. It prioritizes sycophancy over questioning whether "the requirements are wrong" (to borrow an Elon Musk phrase).

AI is seductive because it promises access to a smart person, but without the annoying part where the smart person is ever smarter than you. Problem is, that's the most important part. 

[xvel]

LLMs are perfectly capable of pushing back, commercial models are just not trained to do so as such a product would not sell well to a wide audience.

[Roy_H]

I anticipated that reply, lol.

If that's your goal, you and Grok are still doing it wrong. You'd be better off (both in smallest tanker size and least amount of total fuel) by using an intermediate parking orbit that's an elliptical orbit with a 150 km perigee rather than a circular 900 km orbit.

Again, a human would've immediately recognized this problem, but Grok consistently shows it doesn't know the right question to ask (a more valuable skill than just being able to answer the question when someone has asked the wrong question).

This is gold, keep going. Don't let humans and facts ruin the glorious AI future! You're doing a marvelous job showing the pitfalls of AI, don't stop now. 

Again I feel compelled to clarify my goals. In your elliptical orbit there are issues I wish to avoid, one is that it swings into the Van Allan Belt

Depends on the apogee height. You can still get the same delta-v reduction on the final without reaching the inner belt.

and two, makes multiple passes through the greatest number of satellite orbits where course adjustments might have to be made

Most of the object density (and collision risk) is in a ring near the poles, whereas this would be passing through those altitudes closer to the equator, so it's not actually as concerning as the simple altitude plot would suggest.

and three, it is more difficult to launch other ships to the same elliptical orbit vs a circular orbit.

Despite TheRadicalModerate's fearmongering, this isn't really true.

I did not want to send the Dragon crew into the Val Allan Belt in order to match orbit with the HLS. If you confine the elliptical orbit to a 900km apogee, then the Oberth benefit is less than the gravity losses.

Can I see the math? 

The inner belt starts at around 5,000 km, so there's plenty of margin here.

However, I agree that technically you are right. Just doesn't align with my goals. And Grok did not ask the wrong question, I posed my questions to Grok to find the most optimal plan to meet all my criteria.

Right. The problem is that AI doesn't push back. It's incapable of doing so. It prioritizes sycophancy over questioning whether "the requirements are wrong" (to borrow an Elon Musk phrase).

AI is seductive because it promises access to a smart person, but without the annoying part where the smart person is smarter than you. Problem is, that's the most important part. 

Ah, thank you. Now I feel this discussion is finally addressing my real problems. I was under the impression that the lower limit of Van Allan Belt was closer to 1,200km and wanted to be conservatively below that. If it is acceptable to send the Dragon and crew to higher orbits then I view this as beneficial.

Also for the issue of elliptical orbit through the main belt of satellites does merit re-consideration. I guess I am intuitively apposed to the idea, but probably not rational.

If I accept your recommendations, then this plan becomes even more desirable. What would you suggest as the highest apogee for Dragon crew to fly and avoid extra radiation exposure? In order to firm up this plan, I would need more accurate estimates on the dry mass of HLS and my proposed Lifeboat Tanker. But I was worried that the tanker size would be too large to be practical (as was my giant tanker in the previous proposal) and looked for solutions to minimize that. Also, what is the practical limit of Dragon launched on Falcon 9 with recovered booster? How high of an elliptical orbit could it reach with a perigee of 150km? I have finally come around to your approach and believe with those improvements this mission profile would be very attractive.

[Twark_Main]

Reaching a 900 km circular orbit takes 413.26 m/s, so if we use the same delta-v that ensures that Dragon has enough fuel to reach the rendezvous point. That works out to 150x1740 km.


If you just want to achieve the same final burn delta-v as the original 900x900 circular orbit (ie saving 175.16 m/s for a final TLI burn of 2991.01 m/s), that works out to 150x770 km. Surprisingly it's entirely below 900 km!

Polaris Dawn hit 1,500 km, but even at that altitude I seem to recall they were expecting (like ISS) most radiation would be encountered when crossing over the South Atlantic Anomaly. I have not yet seen the radiation data from that mission.

[Roy_H]

Grok tells me that Dragon can achieve a 5,000km HEEO with booster landing at sea instead of land. Using a 5,000km HEEO the Lifeboat Tanker can be 1,360 ton capacity and total Starship Tanker flights reduced from 25 to 24. Not as much savings as I had hoped, but still an improvement.

[Greg Hullender]

The Oberth effect is easy to explain - why spend all that energy to raise your fuel to a 900km orbit when you can raise it to a 150km orbit for a lot less energy?

If your goal is just to reach any orbit, yes it takes less fuel to reach a lower orbit. In this case the goal is to reach Moon orbit.

The lower altitude you throw stuff out the back, the less work you have to do to raise that stuff to a higher orbit.

Please define higher orbit. I haven't attempted to make any calculations, so don't take this example too literally.
I would be willing to agree that it takes less fuel to go from a 150km orbit to a 350km orbit than it would to go from a 400km orbit to a 600km orbit. Both raising by 200km. But it does not take less work to go from 150km orbit to 600km orbit than it does starting at 400km. Please provide proof that it takes less work to go from 150km to 600km than from 400 km to 600km.

This one puzzled me for quite a while too (and I can do orbital calculations), so maybe I can make it clear what's happening here.

First, the higher up you place your depot, the more expensive it's going to be to put fuel in it. But because the tanker is only about 50% fuel by weight even in the very lowest usable orbit, that means half or more of the fuel you burn lifting it to a higher orbit is wasted moving the mass of the vehicle--not the fuel in it. So if you really wanted a depot at 900 km, you'd do best to fill up two depos in very low Earth orbit, boost them both up to 900 km, and have one of them top the other one off. Perhaps you already understand this point. (Apologies if so.)

But the second point is that the faster your rocket is going, the better a burn you get. This is the Oberth effect everyone is talking about. Intuitively, this is because the rocket has to put energy into moving both the vehicle and the propellant. You're paying to send the rocket to the moon, but you're also paying to send the exhaust on its own trip. You'd like to avoid that, if possible.

So if your rocket flies past the Earth at the exact same speed as the exhaust, then all the energy (relative to the Earth) goes into the rocket; the exhaust, which comes out at zero velocity relative to the Earth, simply falls to the ground. This turns out to be such a big effect, that the best way to get to the moon from your 900 km-high orbit is probably to a) retrofire to move to a 900 x 200 km orbit b) wait until closest approach to the Earth c) fire enough to boost into TLI. That is, the Oberth effect is so big that it's worth it to burn fuel to drop from a higher orbit into a lower (elliptical) orbit just so you can maximize the effect.

However, if you kept all your fuel in LEO all along then you don't have to do this. On the other hand, some proposals do call for using tandem depots such that you'd fuel up two depots, then fill a starship from one of them and have the starship plus the other depot go into a highly elliptical orbit. (Of a few days duration.) The second depot tops off the starship, and at the next perigee, it does a big Oberth burn. It can actually achieve solar escape velocity this way.

Another phenomenon you might not know about is that an elliptical orbit is faster than a circular one at perigee and slower at apogee. That is, a 900 x 200 km elliptical orbit is faster than a 200 km circular orbit at perigee and slower than a 900 km circular orbit at apogee. In the case of twin depots, that's all to the good, since you get that extra speed for your biggest burn.

If you like, I'll work out the math for when the best way to reach escape velocity from Earth orbit at altitude h is to use Oberth and when it's not. It's not hard, but I have to be in the right mood, and dinner is almost ready. :-)

[Robotbeat]

Grok tells me that Dragon can achieve a 5,000km HEEO with booster landing at sea instead of land. Using a 5,000km HEEO the Lifeboat Tanker can be 1,360 ton capacity and total Starship Tanker flights reduced from 25 to 24. Not as much savings as I had hoped, but still an improvement.

Are you sure? What’s the actual calculation?

BTW, there’s a big difference between V3 and V4. Twice the capacity.

[Roy_H]

Grok tells me that Dragon can achieve a 5,000km HEEO with booster landing at sea instead of land. Using a 5,000km HEEO the Lifeboat Tanker can be 1,360 ton capacity and total Starship Tanker flights reduced from 25 to 24. Not as much savings as I had hoped, but still an improvement.

Are you sure? What’s the actual calculation?

BTW, there’s a big difference between V3 and V4. Twice the capacity.

it's actually a lot of work to show the calculation, I did it for the last post showing the latest mission diagram. Copy and paste doesn't work because all the formatting gets lost, so I resorted to screen captures. But I can certainly to it for this one. You can look at that one to get the general idea, obviously a new one would show different values.

My mission profile assumes V4 for 150 ton payload to LEO.

[Roy_H]

So if your rocket flies past the Earth at the exact same speed as the exhaust, then all the energy (relative to the Earth) goes into the rocket; the exhaust, which comes out at zero velocity relative to the Earth, simply falls to the ground. This turns out to be such a big effect, that the best way to get to the moon from your 900 km-high orbit is probably to a) retrofire to move to a 900 x 200 km orbit b) wait until closest approach to the Earth c) fire enough to boost into TLI. That is, the Oberth effect is so big that it's worth it to burn fuel to drop from a higher orbit into a lower (elliptical) orbit just so you can maximize the effect.

However, if you kept all your fuel in LEO all along then you don't have to do this. On the other hand, some proposals do call for using tandem depots such that you'd fuel up two depots, then fill a starship from one of them and have the starship plus the other depot go into a highly elliptical orbit. (Of a few days duration.) The second depot tops off the starship, and at the next perigee, it does a big Oberth burn. It can actually achieve solar escape velocity this way.

Another phenomenon you might not know about is that an elliptical orbit is faster than a circular one at perigee and slower at apogee. That is, a 900 x 200 km elliptical orbit is faster than a 200 km circular orbit at perigee and slower than a 900 km circular orbit at apogee. In the case of twin depots, that's all to the good, since you get that extra speed for your biggest burn.

If you like, I'll work out the math for when the best way to reach escape velocity from Earth orbit at altitude h is to use Oberth and when it's not. It's not hard, but I have to be in the right mood, and dinner is almost ready. :-)

Thank you for your clear explanation. My goal wasn't primarily lowest total fuel, just smallest possible supporting tanker. And returning HLS and Lifeboat Tanker to an orbit accessible by Dragon launched on Falcon 9, booster recovered. I consider hardware is expensive and fuel is cheap because SpaceX plans on re-using the same Starship Tankers with recovery of both the booster and tanker. In previous example the Lifeboat Tanker is significantly larger than the HLS, in the last case using HEEO it is about the same size.

[Roy_H]

Another iteration using elliptical orbit to take advantage of Oberth effect.

What I asked of Grok:
 Ok, here we go again determining minimum Lifeboat Tanker fuel capacity necessary for the mission profile. HLS dry mass in increased by 50t PE radiation shielding for crew (covering all crew area, not just a small shelter) and likewise 10t PE shielding on Lifeboat Tanker with crew quarters equivalent to Orion and of course all necessary crew support equipment and food necessary to return to LEO from LLO. HLS carries 100t payload to be deposited on Moon south pole per Artemis destination. HLS and Lifeboat tanker are equipped with 3 Vac Raptors (zero sl Raptors), 50 layer MLI and appropriate sized pair of solar panels (dual axis control allowing always pointing at Sun) with active cooling to keep fuel at 100°K. Fuel tanks sized for this temperature and CH4 at 1 bar and O2 at 6 bar. HLS remains as designed with the exception of additional PE. 5% fuel reserve for HLS returning from LLO to LEO and a smaller 1% fuel reserve for Lifeboat Tanker returning from LLO to LEO. New mission profile: Launch Lifeboat Tanker and HLS to 200km circular orbit with HLS having 100t payload included. Both loaded with the maximum fuel still allowing them to achieve orbit. Starship and Dragon meet up to install MLI and solar panels. Tanker Starships as required to fill Lifeboat tanker and HLS. Lifeboat tanker and HLS go to elliptical orbit, on plane suitable for TLI, 3000km x 200km. More Starship Tanker missions to top off Lifeboat Tanker and HLS in their elliptical orbit. But to save a little, instead of the previous 6 Starship Tankers flying to HEEO, one Starship tankers remains in 200km orbit, another Starship tanker makes flights to partially fill the first Starship tanker, and then only one Starship tanker has to fly to the HEEO to top off Lifeboat Tanker and HLS. Dragon crew arrives and boards HLS, Dragon performs EDL. Lifeboat Tanker and HLS perform TLI to LLO (100km). Tanker adds minimum fuel to bring HLS up to fuel required to land on Moon, deposit 100t payload, and return to LLO with 5% reserve, and remain on Moon surface for about 2 weeks (until moon is in desirable position for efficient TEI). Add fuel to HLS for return to LEO elliptical orbit. Please state amount of fuel transferred both before and after lunar landing. Dragon launched to meet HLS in elliptical orbit and return astronauts to Earth. Lifeboat Tanker remain in orbit until next mission, 6 -12 months. As stated before the main question is how large the Lifeboat Tanker has to be and how many Starship tanker flights required.

What Grok responded:
Minimum Lifeboat Tanker Fuel Capacity and Mission Refinements
Based on the updated mission profile with the 3,000 km × 200 km elliptical orbit (semi-major axis ≈7,971 km), 3 Raptor Vacuum engines (I_sp = 380 s, v_e ≈ 3.727 km/s), 50-layer MLI + active cooling/solar panels (dual-axis Sun-pointing) for 100 K fuel (CH₄ at 1 bar, O₂ at 6 bar), dry masses (HLS: 135 t including full crew PE shielding + support; LT: 95 t including Orion-equivalent quarters PE shielding + return support/crew supplies), 100 t HLS payload (includes 2-week supplies), 5% HLS return reserve, 1% LT return reserve, and 1.5% boil-off uplift on LLO transfers, the minimum LT fuel capacity is 1,558 metric tons.
This enables the single LT to handle outbound to LLO (Δv = 3.30 km/s total), transfers to HLS, and return to elliptical (Δv = 3.30 km/s total with reserve), assuming full load before TLI from the elliptical perigee.
The HLS retains its 1,200 metric ton fuel capacity without derating at 100 K—the tanks support the subcooled densities (~440 kg/m³ CH₄, ~1,120 kg/m³ O₂) with no volume loss, and active cooling ensures stability during the 2-week surface stay and transits.

Key delta V values:
Raise from 200 km circular to elliptical: 0.656 km/s.
TLI (elliptical perigee to translunar): 2.477 km/s (v_inf ≈ 0.83 km/s at Moon).
LOI to 100 km polar LLO: 0.82 km/s.
Outbound total: 3.30 km/s.
Landing (LLO to South Pole): 1.88 km/s.
Ascent (surface to LLO): 1.88 km/s.
TEI: 0.82 km/s.
Propulsive insertion (to elliptical): 2.477 km/s.
Return total: 3.30 km/s.

Propellant transfers in LLO:
246 metric tons pre landing.
205 tons post accent.

Tanker Flights Overview

Initial Fill at 200 km Circular: Vehicles launch with max allowable propellant (~200-300 t each, including payload for HLS). Tankers top off to full (1,558 t LT + 1,200 t HLS = 2,758 t total) → 19 flights (150 t delivery/flight; last partial).
Top-Off at Elliptical (Pre-TLI): Raise consumes 499 t total (232 t HLS + 267 t LT). Direct delivery to elliptical requires 1 flight (ST delivers 499 t net; max capacity ~971 t per reusable round-trip ST, assuming 100 t dry/1,200 t capacity—well within limits). 5 ST flights, 4 to fill the first one.

Diagram looks pretty much the same as previous. Mostly just text changes to reflect the 1,560 t capacity of Lifeboat Tanker and HEEO staging orbit.

This does reduce the tanker flights by 1 vs previous 900km circular orbit, 24 vs 25 flights, and reduced size of Lifeboat Tanker.

Greg, if you would like to verify the delta V figures it would be appreciated.

[InterestedEngineer]

If there's one thing that LLMs are good at it's formatting text.  So please stop pasting Grok's wall of text, it's practically unreadable.

Here's an example from chatGPT on how to format the above so it's human readable and you can actually follow the math.

ChatGPT link there so you can emulate the prompts:   https://chatgpt.com/share/68f10b1d-eb64-8013-8697-01e02014e774


Summary (keep-orig assumptions)

* Orbit staging: 200 km circular → raise to 3000×200 km elliptical (Oberth at perigee used for TLI).
* Engines: 3×Vac Raptors, Isp = 380 s (vₑ ≈ 3.7265 km/s).
* Temperatures / tankting: fuel at 100 K, CH₄ @1 bar, O₂ @6 bar, 50-layer MLI, active cooling.
* Masses (Grok): HLS dry = 135 t (plus 100 t payload), LifeboatTanker (LT) dry = 95 t.
* Reserves: HLS return reserve 5%, LT return reserve 1%.
* Key Δv (Grok): outbound total = 3.30 km/s; landing = 1.88 km/s; ascent = 1.88 km/s; LOI/TEI legs ≈0.82/2.477 km/s as listed.
* Grok final headline: LT fuel capacity ≈ 1,558 t; HLS fuel capacity ≈ 1,200 t.
* Prop transfers (Grok): 246 t pre-landing, 205 t post-ascent.

Essential equations (use direct LaTeX image links)

Rocket equation: 


Mass ratio: 


Propellant mass: 


Quick sample checks (minimal work to verify numbers)

Constants: 


1) For Δv = 3.30 km/s: 
 
Dry 95 t → 
 


2) For landing+ascent Δv = 1.88+1.88 = 3.76 km/s: 
 
Dry+payload = 235 t → 
 


Interpretation: single-stage surface round trip needs ≈410 t prop (order of magnitude matches 1,200 t tank).

Short logical notes

1. LT 1,558 t derives from system requirement (topping HLS, own Δv, reserves, losses). Plausible scale. 
2. Verify per-burn sequencing (Oberth perigee burn first, etc.). 
3. Tanker flights: Grok assumes ~150 t net delivery/flight → 19–24 flights total.

Deliverables (paste to NSF post)

Lifeboat Tanker (LT) dry 95 t, HLS dry 135 t + 100 t payload. 
Isp=380 s (vₑ≈3726.53 m/s). 
Key Δv: outbound 3.30 km/s; land 1.88 km/s; ascent 1.88 km/s. 
Grok result: LT fuel ≈ 1,558 t; HLS fuel ≈ 1,200 t. 
Prop transfers LLO: 246 t pre-landing, 205 t post-ascent.

Equations: 
 
 
Sample (Δv=3.30 km/s): → 95 t dry →  
Sample (land+ascent 3.76 km/s): → 235 t dry+payload →

One-line conclusion: 
Grok’s LT ≈1,558 t and HLS ≈1,200 t are order-of-magnitude consistent with single-stage rocket-eq checks when accounting for HLS surface round-trip Δv (≈3.76 km/s) and reserves; verify per-burn sequencing and Starship net delivery per tanker flight to confirm 19–25 flight count.

[crandles57]

The inner belt starts at around 5,000 km, so there's plenty of margin here.

Hmm confusing?

https://en.wikipedia.org/wiki/Van_Allen_radiation_belt

Earth's two main belts extend from an altitude of about 640 to 58,000 km (400 to 36,040 mi)[3] above the surface, in which region radiation levels vary.
...
The inner Van Allen Belt extends typically from an altitude of 0.2 to 2 Earth radii (L values of 1.2 to 3) or 1,000 km (620 mi) to 12,000 km (7,500 mi) above the Earth.[4][23]

Polaris Dawn reached 1400Km and that was ok, but they didn't stay that high long.

So what is the efficient way to do Roy's plan without too much radiation exposure? Maybe using something like 1000 * 125 km orbit for final fuelling and then Dragon dock for astronaut transfer?

Mission sequence is now:
1. launch HLS, Lifeboat tanker, cargo Starship and crewed Dragon into 200km LEO.
2. Astronauts perform EVA and assemble solar panels, radiator, and MLI shielding on HLS and Lifeboat Tanker.
3. Lifeboat tanker and HLS are fully fueled by 22 Starship Tankers. Assuming 150t fuel delivered per flight.
4. Lifeboat tanker and HLS fly to 900km circular orbit.
5. 3 more Starship tankers fly to 900km orbit and top off HLS and Lifeboat tanker.
6. Crew arrive via Dragon and board HLS.
7. HLD and Lifeboat tanker fly to LLO at 100km altitude. Polar orbit.
8. Lifeboat Tanker adds 247 tons fuel to HLS.
9. HLS performs Moon landing remains for about 2 weeks and unloads 100t cargo.
10. HLS launches to LLO.
11. Lifeboat tanker adds 253 tons fuel to HLS.
12. Lifeboat tanker and HLS return to 900km LEO
13. Astronauts transfer to Dragon for EDL to Earth.

Lifeboat tanker and HLS remain in orbit for future missions. When required they will return to 200km altitude for fuel and cargo for next mission.

Re Number 5 above: I don't think you send 3 tankers to 1000*125 km for final fuelling. I think it is more like one depot filled by 2 tankers in 150*150 km orbit then send the depot to 1000*125 km to do one final fuelling. Maybe with 1000*125 km orbit we get away with a smaller tank and only need one tank filling the depot before going to 1000*125 for final fuelling?

Is fuelling and astronaut transfer in 1000*125 km orbit a lot more difficult than in circular orbits? Is 125km too low a perigee?

[Roy_H]

Re Number 5 above: I don't think you send 3 tankers to 1000*125 km for final fuelling. I think it is more like one depot filled by 2 tankers in 150*150 km orbit then send the depot to 1000*125 km to do one final fuelling. Maybe with 1000*125 km orbit we get away with a smaller tank and only need one tank filling the depot before going to 1000*125 for final fuelling?

Is fuelling and astronaut transfer in 1000*125 km orbit a lot more difficult than in circular orbits? Is 125km too low a perigee?

Yes, it is definitely better to add fuel to one ST to deliver fuel to the HEEO via multiple flights. Did this in my latest proposal. I can't answer the question about the viability of 125km perigee. I chose a more conservative 200km because I was concerned about leaving the HLS and Lifeboat Tanker in orbit for long periods of time, waiting for the next mission, which could be 6 to 12 months. How much would the orbit decay in that time? If 125km is acceptable, then better. I was originally concerned about matching elliptical orbits, but several people here have assured me that is not a problem.