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#80 by edzieba on 30 Jun, 2020 17:21
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That's a very odd architecture. The MEPs already contain their own propulsion, power, and capture mechanism and are able to independently manoeuvre to match the target satellite orbit. Beyond needing to add the visual positioning system to the MEPs, the MRV seems to be entirely redundant. Even with the worlds most expensive dual-redundant time-of-flight imagers, it seems unlikely that adding that imaging system to all the MEPs would cost more than would be gained by eliminating the MRV entirely and adding one or more MEPs (and paying customers) to each launch, - and eliminating development of an additional spacecraft.
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#81 by TrevorMonty on 30 Jun, 2020 18:16
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That's a very odd architecture. The MEPs already contain their own propulsion, power, and capture mechanism and are able to independently manoeuvre to match the target satellite orbit. Beyond needing to add the visual positioning system to the MEPs, the MRV seems to be entirely redundant. Even with the worlds most expensive dual-redundant time-of-flight imagers, it seems unlikely that adding that imaging system to all the MEPs would cost more than would be gained by eliminating the MRV entirely and adding one or more MEPs (and paying customers) to each launch, - and eliminating development of an additional spacecraft.
The systems for capturing satellite are very expensive , heavy and require lot of extra manoeuvring thrusters. MEV is likely to have redundant systems while low cost expendable MEP may not have any redundancy. If it fails MEV removes it from satellite and delivers to disposal orbit.
Think of MEP as extra fuel tank and engine for satellite. Trying to keep it cheap and light as possible with only enough systems to deliver it to GEO.
NGIS may use same approach with Artemis cryogenic transfer element. Use cut down version minus service module as OTV, with MEV used to capture it and deliver payload to final destination. Much like tug is used for helping ship berth.
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#82 by TGMetsFan98 on 24 Jul, 2020 19:04
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#83 by TrevorMonty on 25 Jul, 2020 01:13
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https://www.nasaspaceflight.com/2020/07/mission-extension-vehicles-validate-lifeline/
MEV could extend Hubble space telescope life. May even be able to bring it within range of ISS so it could be serviced by crew vehicle.
https://twitter.com/TGMetsFan98/status/1286738291430174732 -
#84 by Steven Pietrobon on 25 Jul, 2020 06:05
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MEV could extend Hubble space telescope life. May even be able to bring it within range of ISS so it could be serviced by crew vehicle.
The inclination difference would require a huge change in delta-V, which makes that impractical. Reality is nothing like the movie "Gravity". -
#85 by Ronsmytheiii on 25 Jul, 2020 07:03
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MEV could extend Hubble space telescope life. May even be able to bring it within range of ISS so it could be serviced by crew vehicle.
The inclination difference would require a huge change in delta-V, which makes that impractical. Reality is nothing like the movie "Gravity".
It is interesting though that the MEV could eventually have precise enough control to take over pointing for Hubble, since that requires a very high degree of precision. Might also make it a good bus for future telescope payloads, or even earth observing payloads. -
#86 by Star One on 25 Jul, 2020 14:18
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It would be easier to bring it in range of a Dragon 2 capsule for example flying on a dedicated service mission.https://www.nasaspaceflight.com/2020/07/mission-extension-vehicles-validate-lifeline/
MEV could extend Hubble space telescope life. May even be able to bring it within range of ISS so it could be serviced by crew vehicle.
https://twitter.com/TGMetsFan98/status/1286738291430174732 -
#87 by gemmy0I on 25 Jul, 2020 17:22
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This got me thinking, though - how much delta-v would such a plane change require? (I don't really know how to do the calculation myself short of firing KSP in RO and trying it out, which I'm not presently set up to do...)MEV could extend Hubble space telescope life. May even be able to bring it within range of ISS so it could be serviced by crew vehicle.
The inclination difference would require a huge change in delta-V, which makes that impractical. Reality is nothing like the movie "Gravity".
MEV is designed to provide a lot of delta-v over its many-year lifespan, including raising itself from GTO to GEO (at least 1500 m/s in the "ideal" case of launching from an Ariane 5 to a 0-degree GTO, but more like 1800-2000 m/s if launched from the Cape or from Baikonur, as the first one did), then providing station keeping service for several years with a heavy satellite attached.
IIRC, GEO comsats typically budget about half of their delta-v for the raise from GTO->GEO and the other half for stationkeeping over their typically 10-15 year lifespan. That means in the ballpark of ~2000 m/s to station-keep a GEO satellite for as long as MEV is designed to (accounting for the fact that it's to do multiple 5-year job increments on different satellites).
Electric propulsion complicates delta-v calculations a bit since it normally forces the satellite to take a less efficient trajectory, but going off my Kerbal experience at least, I don't think that would matter as much for an inclination change (not if it's done as a straight inclination change, without raising/lowering the orbit), since the Oberth effect doesn't really help with that anyway.
On the other hand, Hubble is pretty heavy (11,110 kg per Wikipedia) compared to GEO comsats, especially comparing to the dry mass of the comsat which is more representative of what it looks like at the end of its inbuilt propulsive life when MEV takes over.
All of this is to say, moving Hubble to the ISS's inclination would certainly be a prodigious delta-v expenditure, but MEV should have a decent amount of that in stock, so I'm not sure which would "win out". I imagine the MEV would be fully or nearly depleted in such a case, meaning it does the inclination change and then is done (perhaps requiring a second vehicle to be sent to maintain ongoing station-keeping).
This would definitely be expensive, but it would be small potatoes compared to the costs of past Hubble servicing missions - and the benefits of moving it to an inclination where crew missions can be staged from the ISS or commercial successors could be quite substantial. Besides allowing servicing to be performed cheaply "on the side" of ISS crew missions (vs. requiring dedicated launches), it would allow crew to take refuge at the ISS in the event of a contingency - a major concern for the last Shuttle Hubble mission (STS-125 which required the STS-400 rescue shuttle to stand by on the pad). That may be less of a concern for e.g. Dragon since it doesn't have the Shuttle's heat shield/foam strike woes, but NASA is once burned, twice shy, so I could see them really appreciating the flexibility.
Doing this with Northrop's more advanced modular/pod-based successors to MEV would make the equation even more attractive by permitting the vehicle's delta-v to be easily scaled up. MEV is a relatively small bird relative to the capabilities of today's launchers (especially if launched solo, and to LEO as it would be for Hubble), so it'd be smart to increase the fuel load at launch. Modular fuel pods would also allow it to be refueled again and again to provide Hubble station-keeping for decades. (And at the ISS's inclination, those pods could even be piggybacked as unpressurized cargo on an ISS cargo launch.)
I do feel obliged to point out (because if I don't, someone else will
) that this could all be moot in an era of cheap daily reusable Starship launches, which could readily be performed to any inclination. The rescue/staging advantages of being in the ISS's inclination are far less if another Starship can be called up on short notice to Hubble's inclination. Still, the plane change would be a one-time expense, and not a huge one at that by Hubble standards. 51.6o is likely "the place to be" for future successors to the ISS (for several reasons*), i.e. anything in that same inclination is "more valuable real estate" than elsewhere in terms of human spaceflight and EVA servicing potential.
*The reasons I suspect the ISS's commercial successors will end up sharing its 51.6o inclination include:
1. International accessibility - a high inclination is required to reach it from Baikonur. (This is why the ISS is at that inclination in the first place, as was Mir before it.) Russia right now seems to be more interested in cooperating with the Chinese going forward but they're still involved at ISS and I can't imagine NASA would want to "freeze them out" from future participation in any case.
2. Easy/cheap travel between stations. ISS will likely have some overlap with its commercial successor(s) before it's retired, and I'm sure NASA would really, really like to be able to ferry people and supplies between them during that overlap without requiring a new launch. Historically, this was done during the Salyut 7/Mir overlap and was planned for Mir/ISS (but forestalled by ISS delays which kept it from launching until after Mir's reentry).
3. Permitting land touchdowns of capsules in the continental U.S. When Space Station Freedom was in the works prior to becoming the ISS, it would have been at the 28.5o Canaveral inclination. NASA contemplated buying Soyuz capsules from the Russians for escape pods, but would've had to launch them (unmanned, two at a time) in the Shuttle's payload bay since Soyuz couldn't reach that inclination (at least pre-Kourou); and because the orbit's ground track only passes over the southern tips of the continental U.S., they had to consider landing in the Australian Outback because a parachuting capsule like Soyuz needs a wide open space to target. (Trying to land in Florida or South Texas would have risked a splashdown if it went at all off course.) This isn't a problem for Dragon or Orion, but it would be a problem for Starliner. (Dream Chaser, like the Shuttle, does precise runway landings so it doesn't have this problem.) Having the ISS at 51.6o opens up a lot more options for land landings, and that would remain true for commercial successors - probably outweighing the delta-v cost vs. 28.5o, which can be easily paid by today's powerful rockets (unlike Shuttle which had to undergo extensive lightening). -
#88 by TrevorMonty on 25 Jul, 2020 19:49
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I didn't really look into Hubble orbit inclination when suggesting idea.
They are planning for MEV to be refuelled, wth MEP being used to resupply MEVs. -
#89 by Coastal Ron on 25 Jul, 2020 20:07
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Just read the NSF article and got up to date with what has been happening. I remember when it was proposed I was thrilled we'd finally be able to test out this concept, and it looks like it is a winner!
Of course we're talking about peaceful uses for this technology, but there are non-peaceful ones too that hopefully never come to pass.
There is a negative side effect, which is that the market for new satellites might see a decline. Oh well, TINSTAAFL.
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#90 by primer_black on 26 Jul, 2020 02:06
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I didn't really look into Hubble orbit inclination when suggesting idea.
They are planning for MEV to be refuelled, wth MEP being used to resupply MEVs.
Not quite, the plan is for a more capable ‘MRV’ (built in partnership with DARPA RSGS) to install inexpensive, mini ‘MEPs’ on client vehicles at a much lower price point than affected When tying up the entire attention of an MEV for the duration of a client life extension. -
#91 by Steven Pietrobon on 26 Jul, 2020 09:10
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A single burn delta-V from 28.5 to 51.6 degrees is 2*7.7*sin((51.6-28.5)/2) = 3.1 km/s, the same as that required for trans Lunar injection!
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#92 by Hog on 26 Jul, 2020 15:44
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A single burn delta-V from 28.5 to 51.6 degrees is 2*7.7*sin((51.6-28.5)/2) = 3.1 km/s, the same as that required for trans Lunar injection!
Excellent factoid!
A shuttle Orbiter Vehicle with a full OMS pods fuel load had a total delta V capacity of 300 m/s(1000 fps) with the OV carrying a 29,484kg/65,000 pound payload. While STS-107 used a 39º inclination and not a 28.5º orbit, your maths helps to illustrate just how unattainable reaching the safe-haven of the ISS for safe was for the STS-107 mission, even if anyone had indeed known that there was a problem with the TPS of Columbia. -
#93 by TrevorMonty on 26 Jul, 2020 19:52
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Their range of inspace products and services will only increase if TE for landers is built. This will give them high performance hydrolox OTV with option of adding MRV robotics.I didn't really look into Hubble orbit inclination when suggesting idea.
They are planning for MEV to be refuelled, wth MEP being used to resupply MEVs.
Not quite, the plan is for a more capable ‘MRV’ (built in partnership with DARPA RSGS) to install inexpensive, mini ‘MEPs’ on client vehicles at a much lower price point than affected When tying up the entire attention of an MEV for the duration of a client life extension.
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#94 by gemmy0I on 27 Jul, 2020 17:30
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A single burn delta-V from 28.5 to 51.6 degrees is 2*7.7*sin((51.6-28.5)/2) = 3.1 km/s, the same as that required for trans Lunar injection!
Good to know, thanks!
Especially given Hubble weighs so much more than the average GEO comsat, I would guess that this puts the plane change out of the range of a standard MEV. I do wonder if it would be any more feasible with the "pods" architecture NG is developing, though. If the pods essentially represent dockable fuel tanks, then several of them could be aggregated to provide enough cumulative delta-v for the whole plane change.
The trick would be ensuring that all the fuel/delta-v can be launched in a single launch - because the telescope/MEV would be a "moving target" during the plane change, eliminating the logistical advantages MEV relies on in GEO, namely, frequent launches to that destination and the ability to "hop" from satellite to satellite with minimal delta-v. So it would be easiest to just launch MEV pre-loaded with all the propellant needed to do the whole plane change. Since xenon ion propulsion is so efficient, I suspect this might actually be feasible. Let's do the math...
Specific impulse for ion thrusters seems to vary considerably across different designs, and I don't know what MEV's is, so I'll pick 2000 s out of thin air as a (hopefully) conservative ballpark estimate. Hubble masses 11,110 kg (all dry mass since it has no propulsion). I couldn't find any public numbers for MEV's dry mass (or even launch mass), but it's probably fair to assume its dry mass is in the ballpark of 3000 kg (conservatively), given the configurations NG has been launching them in. Let's round up and say that comes to a total of roughly 14,500 kg dry mass for the combined Hubble+MEV complex. (That would mean 3390 kg dry for just MEV. That's probably a lot heavier than a real MEV, but it's safer to round up here since this would be a "heavy" MEV loaded with extra fuel, requiring extra tankage.)
Now we can solve for the propellant mass of xenon needed to achieve a 3.1 km/s plane change:
delta-v = Isp * 9.8 m/s^2 * ln(wet mass / dry mass)
3100 m/s = 2000 s * 9.8 m/s^2 * ln((14,500 kg + x)/14,500 kg)
x = 2484.7 kg
...which makes for a total launch mass of (3390 kg + 2485 kg) = 5875 kg.
In other words, to launch an MEV with sufficient propellant to complete the entire plane change without refueling, "all" we need is to modify MEV with some extra/bigger fuel tanks (which shouldn't be a major change given how modular the architecture seems to be; modularly adjusting propellant mass per-mission seems to be de rigeur for GEO comsat manufacturers), and find a launcher which can put ~6 tonnes into LEO. Nearly all medium-class launchers on the market can do this. A Falcon 9 could do it with over 10 tonnes capacity to spare, i.e. it could rideshare with other payloads.
In fact, adding another tonne or so of xenon would give the vehicle the ability to maintain normal Hubble station-keeping for years to come after the plane change, and would still leave loads of headroom for a rideshare on Falcon 9.
All told, the mission could probably be done for less than $250 million (including launch), accounting for the fact that this would be a bespoke MEV capable of docking to Hubble's unique docking port (LIDS, which is basically a "beta" spec of IDS and thus could use the docking system NG is developing for HALO with minimal modifications) and meeting any other requirements for taking over Hubble's precise station-keeping needs. This is assuming launch on a Falcon 9 for ~$50 million as a solo payload, which could be decreased further with a rideshare.
This is...surprisingly feasible. (Ion propulsion is cool!)
Edit: OK, maybe $250 million is a bit optimistic. I don't think there are any public figures on what a single MEV costs but since NG is planning to amortize the cost of each one over several customer servicing missions, perhaps it could be as high as $500 million for a bespoke one capable of meeting Hubble's unique needs. Still, that's cheap by Hubble servicing standards. I don't see the cost of adding the propellant for the plane change as factoring significantly into that cost vs. "just" making an MEV to take over Hubble station keeping in its current 28o orbit. -
#95 by edzieba on 27 Jul, 2020 17:56
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Would 'just' moving Hubble to the ISS' vicinity in order to repair and refurbish be sufficient for it to continue providing useful science, or would you then need to move it back to its prior orbit again? That would double mission costs for sending a second MEV (or at the very least, add several additional tons of Xenon to the super-MEV).
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#96 by gemmy0I on 27 Jul, 2020 18:42
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Would 'just' moving Hubble to the ISS' vicinity in order to repair and refurbish be sufficient for it to continue providing useful science, or would you then need to move it back to its prior orbit again? That would double mission costs for sending a second MEV (or at the very least, add several additional tons of Xenon to the super-MEV).
My understanding is that Hubble isn't in its current inclination because it's specifically desirable for science, but rather because it's the "cheapest" inclination to launch to from Canaveral. The Shuttle had so much dry mass that it was extremely limited in the orbits it could reach; back when Hubble was launched, it simply couldn't reach the ISS's 51.6o orbit. Several of the orbiters had to undergo extensive lightening, and the external tank had to be redesigned to use light-weight alloys, in order for them to reach that inclination for Shuttle-Mir (and subsequently ISS). IIRC, Challenger, the heaviest of the orbiters (having begun its life as a structural test article, as indicated by its low serial number, OV-099) wouldn't have been able to reach ISS/Mir if it had survived that long.
Today's vehicles being much more flexible in the orbits they can reach, these issues no longer dominate the equation, and I suspect if Hubble were launched today it would be put in a different orbit, probably 51.6o for easy servicing from ISS. The penalty for that orbit vs. 28.5o isn't that bad - it was just that the Shuttle was always riding the edge of its mass-to-orbit capacity and every m/s of delta-v counted dearly. At the time, the ISS didn't exist yet, so 28.5o was "the place to be" for optimal serviceability, as the Shuttle was the only game in town for that. That's no longer the case now that the Shuttle is retired and the ISS is now the focal point of LEO human spaceflight activity.
It's worth noting that when Hubble was under development, Space Station Freedom, before it became the ISS, was envisioned to be at 28.5o, since there was no need to reach it from Baikonur. Bringing Hubble to 51.6o would restore it to its originally envisioned accessibility.
Beyond that, I'm not really sure what would make a particular orbit more or less "desirable" for Hubble, but I'm pretty sure that changing its inclination to 51.6o, without changing any other orbital parameters, wouldn't change that.
The modern trend seems to be to send big space telescopes out to Earth-Sun L2 (e.g. JWST, Spektr-RG) so that they remain in a fixed position relative to the Sun and Earth, which apparently (my personal knowledge on this is fairly limited - this is per Wikipedia) can make "shielding and calibration...much simpler" because "an object around L2 will maintain the same relative position with respect to the Sun and Earth". But L2 is much more expensive to reach than LEO (JWST weighs ~6500 kg vs. Hubble's ~11000, and it's launching on a rocket better suited to high orbits than Shuttle, which had to rely on solid upper stages for that), so it wouldn't have been practical for something as big and heavy as Hubble (not to mention that it would have precluded Shuttle servicing flights). I get the impression that Hubble's current orbit was chosen as a practical compromise rather than for primarily scientific reasons. -
#97 by Hog on 28 Jul, 2020 15:11
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Columbia was the heaviest of the orbital Orbiter Vehicles, all weights without SSMEWould 'just' moving Hubble to the ISS' vicinity in order to repair and refurbish be sufficient for it to continue providing useful science, or would you then need to move it back to its prior orbit again? That would double mission costs for sending a second MEV (or at the very least, add several additional tons of Xenon to the super-MEV).
My understanding is that Hubble isn't in its current inclination because it's specifically desirable for science, but rather because it's the "cheapest" inclination to launch to from Canaveral. The Shuttle had so much dry mass that it was extremely limited in the orbits it could reach; back when Hubble was launched, it simply couldn't reach the ISS's 51.6o orbit. Several of the orbiters had to undergo extensive lightening, and the external tank had to be redesigned to use light-weight alloys, in order for them to reach that inclination for Shuttle-Mir (and subsequently ISS). IIRC, Challenger, the heaviest of the orbiters (having begun its life as a structural test article, as indicated by its low serial number, OV-099) wouldn't have been able to reach ISS/Mir if it had survived that long.
Today's vehicles being much more flexible in the orbits they can reach, these issues no longer dominate the equation, and I suspect if Hubble were launched today it would be put in a different orbit, probably 51.6o for easy servicing from ISS. The penalty for that orbit vs. 28.5o isn't that bad - it was just that the Shuttle was always riding the edge of its mass-to-orbit capacity and every m/s of delta-v counted dearly. At the time, the ISS didn't exist yet, so 28.5o was "the place to be" for optimal serviceability, as the Shuttle was the only game in town for that. That's no longer the case now that the Shuttle is retired and the ISS is now the focal point of LEO human spaceflight activity.
It's worth noting that when Hubble was under development, Space Station Freedom, before it became the ISS, was envisioned to be at 28.5o, since there was no need to reach it from Baikonur. Bringing Hubble to 51.6o would restore it to its originally envisioned accessibility.
Beyond that, I'm not really sure what would make a particular orbit more or less "desirable" for Hubble, but I'm pretty sure that changing its inclination to 51.6o, without changing any other orbital parameters, wouldn't change that.
The modern trend seems to be to send big space telescopes out to Earth-Sun L2 (e.g. JWST, Spektr-RG) so that they remain in a fixed position relative to the Sun and Earth, which apparently (my personal knowledge on this is fairly limited - this is per Wikipedia) can make "shielding and calibration...much simpler" because "an object around L2 will maintain the same relative position with respect to the Sun and Earth". But L2 is much more expensive to reach than LEO (JWST weighs ~6500 kg vs. Hubble's ~11000, and it's launching on a rocket better suited to high orbits than Shuttle, which had to rely on solid upper stages for that), so it wouldn't have been practical for something as big and heavy as Hubble (not to mention that it would have precluded Shuttle servicing flights). I get the impression that Hubble's current orbit was chosen as a practical compromise rather than for primarily scientific reasons.
OV-099 Challenger 155,400 pounds
OV-101 Enterprise 143,600 pounds (weight as used for Approach and Landing Tests)
OV-102 Columbia 158,289 pounds
OV-103 Discovery 151,419 pounds
OV-104 Atlantis 151,315 pounds
OV-105 Endeavour 151,205 pounds
Reaching ISS was never an issue, reaching ISS with an appreciable payload was. SLWT, 5 segment boosters and 106% RPL on the SSME were looked at to claw back the losses to 51.6º. SLWT alone accounted for over 1/2 of the payload clawback. It was a big deal the day that NASA was informed that they would be working with Russia and that Freedom at 28.5º became ISS at 51.6º.
28.5º allowed for the highest orbital altitude with an apogee of approx 600 km. The HST service missions were the "highest" STS missions. Increasing inclination or orbital altitude BOTH reduce the max payload of the orbiter, or any launch vehicle. 28.5 vs 51.6º certainly is a major difference, no matter the launch/space vehicle.
Modern spacecraft have nowhere near the payload capacity that STS did. 28.5º to 51.6º most certainly IS a major payload hit. Some modern launchers do and the fact that they have various launch facilities that allow pretty much any desired orbit.
OV-102 Columbia, the heaviest orbiter, was destined for ISS missions following her next OMDP(Orbiter Maintenance Down Period) which would have removed her internal airlock for STS-118 usage.
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#98 by gemmy0I on 28 Jul, 2020 17:10
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Columbia was the heaviest of the orbital Orbiter Vehicles, all weights without SSME
Good to know, thanks for the detailed information! I stand corrected.
OV-099 Challenger 155,400 pounds
OV-101 Enterprise 143,600 pounds (weight as used for Approach and Landing Tests)
OV-102 Columbia 158,289 pounds
OV-103 Discovery 151,419 pounds
OV-104 Atlantis 151,315 pounds
OV-105 Endeavour 151,205 pounds
Reaching ISS was never an issue, reaching ISS with an appreciable payload was. SLWT, 5 segment boosters and 106% RPL on the SSME were looked at to claw back the losses to 51.6º. SLWT alone accounted for over 1/2 of the payload clawback. It was a big deal the day that NASA was informed that they would be working with Russia and that Freedom at 28.5º became ISS at 51.6º.
28.5º allowed for the highest orbital altitude with an apogee of approx 600 km. The HST service missions were the "highest" STS missions. Increasing inclination or orbital altitude BOTH reduce the max payload of the orbiter, or any launch vehicle. 28.5 vs 51.6º certainly is a major difference, no matter the launch/space vehicle.
Modern spacecraft have nowhere near the payload capacity that STS did. 28.5º to 51.6º most certainly IS a major payload hit. Some modern launchers do and the fact that they have various launch facilities that allow pretty much any desired orbit.
OV-102 Columbia, the heaviest orbiter, was destined for ISS missions following her next OMDP(Orbiter Maintenance Down Period) which would have removed her internal airlock for STS-118 usage.
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#99 by TrevorMonty on 14 Aug, 2020 20:25
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Satellite servicing industry seeks interface standards
https://spacenews.com/satellite-servicing-industry-seeks-interface-standards/
NGSS are particularly interested in having this standard in place as their mission extension architecture can offer more services. A USB type interface that provides power and data means they could attach reaction module to replace faulty one, or extra payloads that would upgrade satellite helping to future proof if. In case of MEP (mission extension pod) satellite would have direct control of it, at present MEP is controlled from ground and doesn't communicate with satellite.
Refuelling ports is another on NGSS wish list, their preference is attaching external fuel tank instead of refuelling. Plus side is it contains pressurisation gas as well. Satellite just needs valves to switch between internal and external tank.
NGSS plan to upgrade the MEV to MRV(mission robot vehicle) ie MEV with robotic arms, this will stay on orbit indefinitely and be used to attach much simpler and cheaper MEP to satellite. MEP will supply MRV with fuel, pods and upgrades as part of their delivery mission. The MEP could then be attached to satellite. Unlike MEV which is attached satellite, MRV would be free most if time allowing it to be on call for satellites in trouble. This could be case of doing inspection, moving satellite to graveyard orbit or providing station keeping till MEP can be deployed.
) that this could all be moot in an era of cheap daily reusable Starship launches, which could readily be performed to any inclination. The rescue/staging advantages of being in the ISS's inclination are far less if another Starship can be called up on short notice to Hubble's inclination. Still, the plane change would be a one-time expense, and not a huge one at that by Hubble standards. 51.6o is likely "the place to be" for future successors to the ISS (for several reasons*), i.e. anything in that same inclination is "more valuable real estate" than elsewhere in terms of human spaceflight and EVA servicing potential.