Commercial space's role in Exploration Gateway station

[rklaehn]

Just a note on trajectories: the propulsive lunar gravity-assist trajectory (two impulse after TLI) is a good combination of transfer time and delta-v. It is not, however, a free-return trajectory. The single-impulse lunar gravity-assist can be free-return, and so may be preferable for crew missions to L2. Two-impulse would still be the best option for return to Earth, though.

What is the advantage of a free return trajectory? Increased safety? At some point you will have to rely on your propulsion system.

But the single-impulse trajectory might be preferable for crew exchange because it is a bit quicker as well.

[Jason1701]

Just a note on trajectories: the propulsive lunar gravity-assist trajectory (two impulse after TLI) is a good combination of transfer time and delta-v. It is not, however, a free-return trajectory. The single-impulse lunar gravity-assist can be free-return, and so may be preferable for crew missions to L2. Two-impulse would still be the best option for return to Earth, though.

For getting the station to L2, a standard low-thrust spiral is more likely. It's by far the most optimized for a solar-electric transfer stage, and naturally cancels out any phasing and inclination effects from assembling the station at ISS. Even for a WSB trajectory, the mass of propellant required to send a proper station to L2 is probably large enough to justify the SEP stage (especially considering that they are apparently scaling the gateway to SLS).

WSB-type trajectories may be useful for cargo runs, though, and would need to be traded with the extra cost of a SEP stage. I would be curious what a Cygnus with extra solar arrays and an ion engine could haul to L2...

I have a trajectory question relating to this - if we wanted to land on the Moon from the Gateway, would there be a delta-v penalty to going to LLO before landing? How much?

Also, is the delta-v from each L-point to the lunar surface similar?

[rklaehn]

I have a trajectory question relating to this - if we wanted to land on the Moon from the Gateway, would there be a delta-v penalty to going to LLO before landing? How much?

Also, is the delta-v from each L-point to the lunar surface similar?

I think there is no penalty for entering LLO. But you don't want to stay in LLO for long because it takes some delta-v to stay in a predefined LLO because of the inhomogenous lunar gravity field.

Regarding the delta-v: L1 and L2 should be roughly similar. With L3, L4 and L5 there is a tradeoff between delta-v and transit time. You can get e.g. from L5 to LLO very cheaply (almost as cheap as from L1/L2), but it will take a long time (weeks).

[Jason1701]

I have a trajectory question relating to this - if we wanted to land on the Moon from the Gateway, would there be a delta-v penalty to going to LLO before landing? How much?

Also, is the delta-v from each L-point to the lunar surface similar?

I think there is no penalty for entering LLO. But you don't want to stay in LLO for long because it takes some delta-v to stay in a predefined LLO because of the inhomogenous lunar gravity field.

Regarding the delta-v: L1 and L2 should be roughly similar. With L3, L4 and L5 there is a tradeoff between delta-v and transit time. You can get e.g. from L5 to LLO very cheaply (almost as cheap as from L1/L2), but it will take a long time (weeks).

Thanks! Do you think there would be some optimal halo orbit radius for L1/2 that minimizes the delta-v to LLO? Or maybe the optimum would depend on LEO transfer too.

[rklaehn]

I have a trajectory question relating to this - if we wanted to land on the Moon from the Gateway, would there be a delta-v penalty to going to LLO before landing? How much?

Also, is the delta-v from each L-point to the lunar surface similar?

I think there is no penalty for entering LLO. But you don't want to stay in LLO for long because it takes some delta-v to stay in a predefined LLO because of the inhomogenous lunar gravity field.

Regarding the delta-v: L1 and L2 should be roughly similar. With L3, L4 and L5 there is a tradeoff between delta-v and transit time. You can get e.g. from L5 to LLO very cheaply (almost as cheap as from L1/L2), but it will take a long time (weeks).

Thanks! Do you think there would be some optimal halo orbit radius for L1/2 that minimizes the delta-v to LLO? Or maybe the optimum would depend on LEO transfer too.

You can't really answer this without doing a simulation and knowing a lot of details about the proposed station. A larger halo orbit radius should help with getting from EML1/2 to LLO. But if you increase it too much you need more stationkeeping delta-v. On the other hand the stationkeeping could be done using electric propulsion, which would make it cheaper than delta-v that has to be impulsive.

I think that since there is no particular benefit of being precisely at EML1 or 2, you would probably start at, say, EML2 and then increase the halo orbit radius gradually until the stationkeeping cost becomes to high.

You might even move the station to a larger halo orbit for arrival and departure of vehicles from earth or the moon, and move it to a tighter halo orbit for long periods without visiting vehicles.

[Robotbeat]

...I'm not sure about Crew, because transit times might get too long for Crewed Dragon....

Why is that? Crewed Dragon volume (10m^3) is not much less than the combined volume of the Apollo LM (~6m^3) and Apollo Command Module (~6m^3). Heck, the two-person Gemini spacecraft was only barely more than 2m^3 for two astronauts, and it demonstrated a ~14 day mission. 10m^3 for 4 astronauts would be positively roomy by comparison.

That's for cislunar transport, not for other beyond-LEO missions.

EDIT: That applies for CST-100, too, though it might want to add solar arrays and may have a little harder time finding a suitable launch vehicle, though I doubt that's a deal breaker.

[baldusi]

...I'm not sure about Crew, because transit times might get too long for Crewed Dragon....

Why is that? Crewed Dragon volume (10m^3) is not much less than the combined volume of the Apollo LM (~6m^3) and Apollo Command Module (~6m^3). Heck, the two-person Gemini spacecraft was only barely more than 2m^3 for two astronauts, and it demonstrated a ~14 day mission. 10m^3 for 4 astronauts would be positively roomy by comparison.

That's for cislunar transport, not for other beyond-LEO missions.

EDIT: That applies for CST-100, too, though it might want to add solar arrays and may have a little harder time finding a suitable launch vehicle, though I doubt that's a deal breaker.

Once you add the necessary support (ECLSS, potty, etc.) equipment and radiation reduction layers, you start eating your volume. A Lunar Dragon would have less than 10m³. Might be doable, though. But it would definitely be Dragon 3.0 (2.0 being the crewed version). Please note that they will probably move cargo to Dragon 2.0, to test it before putting a crew on it.
That's also why the CST-100 is designed the way it is. Most of the complications of a BEO design would go into a new and improved Service Module and heat shield.
And wrt the LV, they could always man rate the Delta IV Heavy, do an Atlas V Heavy or even better, do an Atlas V Phase 2. Why would NASA pay for such a development when they have the SLS/Orion stack, is beyond me, thou.
I do see them doing Cargo to EML2, and letting the Crewed capsule to EML2 as an exercise to the supplier.

[Robotbeat]

...I'm not sure about Crew, because transit times might get too long for Crewed Dragon....

Why is that? Crewed Dragon volume (10m^3) is not much less than the combined volume of the Apollo LM (~6m^3) and Apollo Command Module (~6m^3). Heck, the two-person Gemini spacecraft was only barely more than 2m^3 for two astronauts, and it demonstrated a ~14 day mission. 10m^3 for 4 astronauts would be positively roomy by comparison.

That's for cislunar transport, not for other beyond-LEO missions.

EDIT: That applies for CST-100, too, though it might want to add solar arrays and may have a little harder time finding a suitable launch vehicle, though I doubt that's a deal breaker.

Once you add the necessary support (ECLSS, potty, etc.) equipment and radiation reduction layers, you start eating your volume. A Lunar Dragon would have less than 10m³. Might be doable, though. But it would definitely be Dragon 3.0 (2.0 being the crewed version). Please note that they will probably move cargo to Dragon 2.0, to test it before putting a crew on it.
That's also why the CST-100 is designed the way it is. Most of the complications of a BEO design would go into a new and improved Service Module and heat shield.
And wrt the LV, they could always man rate the Delta IV Heavy, do an Atlas V Heavy or even better, do an Atlas V Phase 2. Why would NASA pay for such a development when they have the SLS/Orion stack, is beyond me, thou.
I do see them doing Cargo to EML2, and letting the Crewed capsule to EML2 as an exercise to the supplier.

Apollo and Gemini didn't have potties. Soyuz has a small one that would be appropriate for the task. ECLSS can be open-loop for the most part. There's plenty of space in there for a trip lasting a week or two at most.

Suck it up and get on the rocket.

[krytek]

...I'm not sure about Crew, because transit times might get too long for Crewed Dragon....

Why is that? Crewed Dragon volume (10m^3) is not much less than the combined volume of the Apollo LM (~6m^3) and Apollo Command Module (~6m^3). Heck, the two-person Gemini spacecraft was only barely more than 2m^3 for two astronauts, and it demonstrated a ~14 day mission. 10m^3 for 4 astronauts would be positively roomy by comparison.

That's for cislunar transport, not for other beyond-LEO missions.

EDIT: That applies for CST-100, too, though it might want to add solar arrays and may have a little harder time finding a suitable launch vehicle, though I doubt that's a deal breaker.

Once you add the necessary support (ECLSS, potty, etc.) equipment and radiation reduction layers, you start eating your volume. A Lunar Dragon would have less than 10m³. Might be doable, though. But it would definitely be Dragon 3.0 (2.0 being the crewed version). Please note that they will probably move cargo to Dragon 2.0, to test it before putting a crew on it.
That's also why the CST-100 is designed the way it is. Most of the complications of a BEO design would go into a new and improved Service Module and heat shield.
And wrt the LV, they could always man rate the Delta IV Heavy, do an Atlas V Heavy or even better, do an Atlas V Phase 2. Why would NASA pay for such a development when they have the SLS/Orion stack, is beyond me, thou.
I do see them doing Cargo to EML2, and letting the Crewed capsule to EML2 as an exercise to the supplier.

Apollo and Gemini didn't have potties. Soyuz has a small one that would be appropriate for the task. ECLSS can be open-loop for the most part. There's plenty of space in there for a trip lasting a week or two at most.

Suck it up and get on the rocket.

A potty for BEO is a worthwhile compromise. The alternatives may save mass, but I don't think the hassle is worth it.