Axiom's private space station is coming sooner than we thought

[u/spacedotc0m]

https://www.space.com/space-exploration/private-spaceflight/axioms-private-space-station-is-coming-sooner-than-we-thought

[u/Isnotanumber]

Semi-random thought. Is it really necessary to send part of this to ISS first? And if so, can its orbit be modified significantly once it becomes a free flying vehicle. As I recall, the orbit for ISS was dictated to give Russians access, and while doable from KSC it kinda sucks. Limits the windows to launch, made the amount payload deliverable a little more limited. At least for Shuttle. Why not just optimize it for launches from the US? Is there a hope of keeping Russia as a partner?

[u/mfb- → u/Isnotanumber]

The payload increase with a lower inclination orbit isn't that large. Launching to the ISS inclination leads to a launch trajectory that follows the US coast, which is nice for launch abort scenarios.

[u/jimhillhouse → u/mfb-]

Doesn’t following the U.S. Easter seaboard, a densely populated area, as opposed to going over the Atlantic when targeting lower inclination orbits of say 28°, present higher risk from a launch abort point of view?

As I recall from an aerospace engineering orbital mechanics lecture at the time ISS’s Harmony module was launched, ISS’s current inclination was solely to accommodate the former Soviet Union’s participation in ISS.

The loss of delta-V from a nearly 24° change in launch azimuth from 28° to 52° isn’t significant, but the impact also and largely hinges on the launch vehicle’s performance, or specific impulse. Here’s an example.

At the equator, the velocity contribution from the earth’s rotation is 464 m/s. The cosine at 28° is 0.88 and that at 52° is 0.62, ~30% less than 28°. So, at 28°, that velocity contribution is ~409.6 m/s; at 52° it’s ~279 m/s.

If we say orbital velocity is 7,800 m/s, at 28° the delta-V required of a launch vehicle is 7,390 m/s and at 52° it is 7,514.4 m/s or so, about an additional 124 m/s more than a vehicle launch at 28° latitude.

Using the Rocket Equation to look at launch vehicle performance at differing launch azimuth and assuming-away its imprecision due to not including various drag components can be useful. For a launch vehicle with a propulsion system performance of 450 s, the mass ratio, which is massWet/massDry, is 5.33 at 28° and 5.49 at 52°, a nearly 30% difference. Higher is worse because the difference between wet and dry mass of a launch vehicle is propellant mass, at least for purposes of calculating performance using the rocket equation.

The payload mass fraction is (MR-1)/MR. At 28° payload mass fraction is 0.812 and at 52° is 0.817. A lower payload mass fraction is better because it means less propellant needed to launch a given payload. So, for a high performance launch vehicle with an ISP of 450s, the change from 28° to 53°represents a 2.77% payload penalty.

For a less performant system like the latest Falcon 9 with an ISP of ~230s, that at 28° the payload fraction is 3.78% and at 52° it’s 3.57%, a difference of 5.35%.

[u/snoo-boop → u/jimhillhouse]

the launch vehicle’s performance, or specific impulse.

I think performance analysis is more sophisticated these days.

You use 450s ISP as "high performance" ISP, can you let us know which vehicle has that ISP at sea level?