Author Topic: Orbits Q&A  (Read 176179 times)

Offline edkyle99

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Re: Orbits Q&A
« Reply #360 on: 11/03/2023 11:27 am »
SLS is a bit of an oddity, though, as its core stage is a sustainer that takes it all the way into (an) orbit, and its upper stage is intended to mostly perform the departure from Earth.  I don't think I have seen any other rockets/missions that do it like that and inserts into orbits with a high apogee, while perigee is well inside the atmosphere.  (I fully expect to be corrected about that, though. :) )
SLS does that to safely dispose of the giant core stage.  Imagine that thing orbiting the Earth out of control for a few days while we wait to see where it reenters!  STS did roughly the same thing to dispose of the External Tank.

 - Ed Kyle
« Last Edit: 11/03/2023 11:27 am by edkyle99 »

Offline Jim

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Re: Orbits Q&A
« Reply #361 on: 11/03/2023 01:09 pm »
My grasp of orbital mechanics is limited, so I'm trying to understand the purpose of a perigee raise maneuver, in particular when heading to the Moon.  When an upper stage or spacecraft initially climbs into a high elliptical orbit, is the perigee of this orbit typically inside the Earth's atmosphere?  Thus, you fire the upper stage or spacecraft engine(s) at apogee to raise the perigee above the atmosphere?  I assume that launching into a high elliptical orbit where perigee is already above the atmosphere is not as fuel-efficient.  I hope I've properly stated the question here.
Thx

That is exactly what SLS does.  It is similar to the OMS-2 burn for Shuttle missions with Direct Insertion.   It is/was for SLS Core/ET disposal.

There are few to no other launch vehicles that do this.  Maybe Atlas E/F with NOAA spacecraft.
« Last Edit: 11/03/2023 01:11 pm by Jim »

Offline Jim

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Re: Orbits Q&A
« Reply #362 on: 11/03/2023 01:12 pm »

SLS is a bit of an oddity, though, as its core stage is a sustainer that takes it all the way into (an) orbit,

Can be thought of as suborbital.

Offline DanClemmensen

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Re: Orbits Q&A
« Reply #363 on: 11/03/2023 02:04 pm »
My grasp of orbital mechanics is limited, so I'm trying to understand the purpose of a perigee raise maneuver, in particular when heading to the Moon.  When an upper stage or spacecraft initially climbs into a high elliptical orbit, is the perigee of this orbit typically inside the Earth's atmosphere?  Thus, you fire the upper stage or spacecraft engine(s) at apogee to raise the perigee above the atmosphere?  I assume that launching into a high elliptical orbit where perigee is already above the atmosphere is not as fuel-efficient.  I hope I've properly stated the question here.
Thx

That is exactly what SLS does.  It is similar to the OMS-2 burn for Shuttle missions with Direct Insertion.   It is/was for SLS Core/ET disposal.

There are few to no other launch vehicles that do this.  Maybe Atlas E/F with NOAA spacecraft.
Apparently, Atlas V US remains (barely) suborbital when launching Starliner. Not really the same and for a different reason, but still suborbital.
    https://spaceflightnow.com/2022/05/19/boeings-starliner-crew-capsule-takes-off-on-long-awaited-test-flight/
Starliner itself provides the kick up to full orbit. I think they do this to provide a passive abort. Starliner will re-enter if its engines fail to fire.

Offline deltaV

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Re: Orbits Q&A
« Reply #364 on: 01/23/2024 04:01 am »
My question is why are the NASA LSP (https://elvperf.ksc.nasa.gov/Pages/Query.aspx) performance figures for high altitude circular LEO so low? For example NASA LSP has the following performance for Vulcan VC6 to circular 28.5 degree inclination LEO: 27.910 tonnes to 200 km and 13.680 tonnes to 2000 km. That 200 km figure approximately matches other sources but the 2000 km figure is surprisingly low. The delta vee of a Hohmann transfer between 200 km circular and 2000 km circular is only 883 m/s. Interpolating between VC6's LEO (27.2 tonnes) and GTO (14.5 tonnes) performance from https://www.ulalaunch.com/rockets/vulcan-centaur using the fact that GTO (assuming 1800 m/s from GEO) is 2.47 km/s from a 200 km 28.5 degree LEO I would expect the 2000 km performance figure to be around 27.2*(14.5/27.2)^(883/2470) = 21.7 tonnes. (That's equivalent to linear interpolation of log(payload) vs. delta vee. If you instead do linear interpolation of payload vs. delta vee the discrepancy is even worse.) The other Vulcan models, Falcon and New Glenn also show surprisingly low 2000 km performance (but by varying amounts) so this isn't just a VC6 thing.

Tags: inclination 
 

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