Author Topic: Why are the stated performance figures for Vulcan Heavy Centaur not better?  (Read 5006 times)

Offline LouScheffer

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This train of thought was inspired by the possibility of the Vulcan Heavy launching Europa Clipper (6065 kg to C3~42). At first glance this seems unworkable.  Vulcan Heavy is rated at 7200 kg to GEO, which requires about 4100 m/s more than LEO (2300 to get to GTO, 1800 more to circularize).  C3=42 is about 5000 m/s more than LEO.  Reducing the mass from 7200 to 6065 kg means you could do a GEO burn (4100 m/s) and still have 1135 kg of fuel left.  But that's not enough to get 900 m/s more, given any plausible second stage mass (which we don't know, but must be a few tons just from the stainless steel and known dimensions.)

But then I wondered why the Vulcan Heavy numbers are so wimpy.  The GEO mass (with the same 6 solid booster) only goes from 6500kg to 7200kg.  Likewise the other numbers (LEO, GTO) are only slightly (less an 10%) higher.  But a simple scaling indicates they should be a lot bigger.  The propellant mass (at least as widely reported) goes from 55t to 77t, a ratio of 7/5 or a 40% increase.  The dry mass increase should be less than 7/5 (only the sidewalls change dimensions, and they only account for half the area (the rest is the domes and bulkhead) so that's only a 20% increase in steel area).  So the payload should go up by at least 40% for the same delta-V, or up to 7/5*6500 = 9100 kg to GEO (neglecting the decrease in booster delta-V from pushing 20 more tonnes of Centaur fuel).  That should be plenty of margin for Europa Clipper, given any reasonable second stage mass.

So what's going on here?  I can think of two possibilities.  One is that 7200 kg to GEO is the maximum they think they will ever need.  (The public DOD orbits max out at 6590 kg to GEO).  So maybe they set the number conservatively, as it covers the DOD needs with plenty of margin.  They could even state this number without doing any detailed design at all - it's that conservative.

Alternatively, maybe the Centaur Heavy is just a brute force design.  Since they will only make a few, perhaps they tried to minimize design and test costs.  So make it out of non-thinned stainless steel, add lots of margin, don't worry about the last few percent of mass optimization, and so on.  Maybe it's a lot heavier than it might seem.

Whatever the reason, the small increase in payload mass (10%) seems at odds with the large increase in fuel mass (40%).  Does anyone know what's going on here, or are the Heavy numbers just wildly sandbagged?



« Last Edit: 02/01/2021 11:27 pm by zubenelgenubi »

Offline RotoSequence

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Re: The unreasonable wimpiness of the Vulcan Heavy Centaur
« Reply #1 on: 02/01/2021 10:58 pm »
Are they still using ultra thin stainless steel balloon tanks for Vulcan Centaur?

Offline LouScheffer

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Re: The unreasonable wimpiness of the Vulcan Heavy Centaur
« Reply #2 on: 02/01/2021 11:01 pm »
Are they still using ultra thin stainless steel balloon tanks for Vulcan Centaur?
For the regular Vulcan Centaur, yes.  Even thinner than the Atlas Centaur. 
EDIT: Not sure about thinner than the Atlas Centaur.  Maybe the thickness in the first panel of the ULA illustration refers only to the dime, and not the Atlas Centaur walls.
« Last Edit: 02/01/2021 11:08 pm by LouScheffer »

Offline russianhalo117

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Re: The unreasonable wimpiness of the Vulcan Heavy Centaur
« Reply #3 on: 02/01/2021 11:02 pm »
Are they still using ultra thin stainless steel balloon tanks for Vulcan Centaur?
similar but different mm thickness reference Centaur-G/G'/T variable diameter versions.

Offline ncb1397

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Re: The unreasonable wimpiness of the Vulcan Heavy Centaur
« Reply #4 on: 02/01/2021 11:18 pm »
Quote
The dry mass increase should be less than 7/5 (only the sidewalls change dimensions, and they only account for half the area (the rest is the domes and bulkhead) so that's only a 20% increase in steel area).  So the payload should go up by at least 40% for the same delta-V...

You aren't counting that the upper stage weight will cause the first stage and boosters to stage slower. If you increased all stages by 40%, yes, you should get a 40% increase in throw weight.

Offline LouScheffer

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Quote
The dry mass increase should be less than 7/5 (only the sidewalls change dimensions, and they only account for half the area (the rest is the domes and bulkhead) so that's only a 20% increase in steel area).  So the payload should go up by at least 40% for the same delta-V...

You aren't counting that the upper stage weight will cause the first stage and boosters to stage slower. If you increased all stages by 40%, yes, you should get a 40% increase in throw weight.
You are correct.  I ignored this (and said so) in my rough guess, but it's very significant, enough to explain why the Heavy gets only a 10% or so performance increase.

Almost all of this effect will happen after the solid booster burnout.  (The solids have a much higher initial and burnout mass, minimizing this effect).  So I derate the fuel to the amount remaining after SRB burnout, about 250t.   Then using some rough values (booster mass = 30t, fuel=250t, isp=312) and two cases for the second stage, regular (55t fuel, 4t empty, 6.5t payload, isp=453) and the heavy (77t fuel, 5.2t empty (+30%), payload 7.2t, isp=453), then the second stage indeed supplies much more dV (+643 m/s) but the first stage losses eat up much of this (-478m/s).   And if you increase the second stage burnout mass to 5.7t, then the two effects cancel completely, and the 7.2t payload for the Heavy has the same margins as 6.5t for the regular, as on the ULA chart.

So my intuition that the effect on the first stage was small was wrong.  The only excuse I can offer is that when I did similar calculations for stretches of the F9 second stage, the effect on the first stage was smaller (because the first stage needs to reserve fuel for landing, which can more than double the mass at burnout, making this effect less).

 

Offline AstroWare

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Did I miss something?

I was under the impression that ULA was now using only one Centaur size upper stage. The difference between the Vulcan variant with 6 SRBs and the Vulcan Heavy variant with 6 SRBs  was just the RL10 variant (RL10C and RL10CX respectively)


From ULAs website:
https://www.ulalaunch.com/docs/default-source/rockets/vulcancentaur.pdf?sfvrsn=10d7f58f_2

1. You can observe on that the side-by-side view that the centaurs are the same length.

2. You can also see the RL10 nozzle extensions on the Heavy variant (right)

3. The description of the heavy variant says, "Centaur is a liquid hydrogen/liquid oxygen-fueled vehicle, with two RL10C engines. The Vulcan Centaur Heavy vehicle, flies the upgraded Centaur using RL10CX engines with nozzle extensions. "

Offline ncb1397

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Did I miss something?

I was under the impression that ULA was now using only one Centaur size upper stage. The difference between the Vulcan variant with 6 SRBs and the Vulcan Heavy variant with 6 SRBs  was just the RL10 variant (RL10C and RL10CX respectively)


From ULAs website:
https://www.ulalaunch.com/docs/default-source/rockets/vulcancentaur.pdf?sfvrsn=10d7f58f_2

1. You can observe on that the side-by-side view that the centaurs are the same length.

2. You can also see the RL10 nozzle extensions on the Heavy variant (right)

3. The description of the heavy variant says, "Centaur is a liquid hydrogen/liquid oxygen-fueled vehicle, with two RL10C engines. The Vulcan Centaur Heavy vehicle, flies the upgraded Centaur using RL10CX engines with nozzle extensions. "

Yes, this was puzzling me as well. If you look at the rocket rundown, there is no apparent length increase of the rocket either.

https://www.ulalaunch.com/docs/default-source/rockets/atlas-v-and-delta-iv-technical-summary.pdf

But then you have, somewhat dated information from Tory Bruno like this...

https://twitter.com/torybruno/status/987484411691122688

All information might be 100% accurate, even when contradictory, if you account for changing plans.

Quote
The very first Centaur we fly will be called Centaur 5. It will already have twice the propellant that Centaur 3 has. Centaur 3 (which flies on the Atlas V rocket) is 3.8 meters in diameter. The very first Centaur we fly on Vulcan will go straight to 5.4 meters in diameter. Then, what we will do in the second upgrade to Centaur is upgrade the thrust in the RL10 engines and make it even longer, to stretch the propellant tanks to give it even that much more energy. That’s what takes it all the way to the Vulcan Heavy configuration.
https://arstechnica.com/science/2018/12/talking-rockets-with-tory-bruno-vulcan-the-moon-and-hat-condiments/

If you compare the chart on Ars Technica to the current chart on ulalaunch.com. You get discrepencies like the GEO payload for the 2 solid configuration is 2600 kg while the one on Ars is 2050 kg. This 27% increase could be due to the stretched upper stage flying on the 2 solid version from the get go.

Consider this as well

6 solid Vulcan Centaur (GEO, old ) =  6000 kg
6 solid Vulcan Centaur Heavy (GEO, old ) =  7200 kg
6 solid Vulcan Centaur (GEO, new) = 6500 kg
6 solid Vulcan Centaur Heavy (GEO, new) = 7200 kg

It looks like they are talking about the same Vulcan Centaur Heavy configuration (same payload), but the base 6 solid version is 6500 kg vs 6000 kg. So, it looks like the stretched upper stage really is a +1200 kg improvement (20%), the base upper stage just includes part of that upgrade (which looks like tank sizing).
« Last Edit: 02/02/2021 05:58 am by ncb1397 »

Offline MATTBLAK

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There has been no official talk of a version of Vulcan that would use a load of 8x or even 10x GEM-63XL solid strap on boosters - I once asked Tory Bruno if such a thing was possible - he would neither confirm nor deny that such a thing had been studied...

...And if looked at in cross-section; it looks as though 12x solids would fit on that thing! A booster like that with an enhanced Centaur V could get a fair payload into LEO in a 29 degree inclination orbit. Cheaper than SLS, but starting to approach Block 1 SLS in capability?! ;)
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Offline sdsds

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Is there a chance the Centaur V that flies on the zero-solids configuration will have smaller tanks than those for the other configurations? Put differently, are all the non-zero solids configurations using a stretched Centaur V?
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Offline TrevorMonty

Did I miss something?

I was under the impression that ULA was now using only one Centaur size upper stage. The difference between the Vulcan variant with 6 SRBs and the Vulcan Heavy variant with 6 SRBs  was just the RL10 variant (RL10C and RL10CX respectively)


From ULAs website:
https://www.ulalaunch.com/docs/default-source/rockets/vulcancentaur.pdf?sfvrsn=10d7f58f_2

1. You can observe on that the side-by-side view that the centaurs are the same length.

2. You can also see the RL10 nozzle extensions on the Heavy variant (right)

3. The description of the heavy variant says, "Centaur is a liquid hydrogen/liquid oxygen-fueled vehicle, with two RL10C engines. The Vulcan Centaur Heavy vehicle, flies the upgraded Centaur using RL10CX engines with nozzle extensions. "
Heavy has 75t Centuar compared to Centuar V 55t. These are rough values going off memory.

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Offline Robotbeat

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This train of thought was inspired by the possibility of the Vulcan Heavy launching Europa Clipper (6065 kg to C3~42). At first glance this seems unworkable.  Vulcan Heavy is rated at 7200 kg to GEO, which requires about 4100 m/s more than LEO (2300 to get to GTO, 1800 more to circularize).  C3=42 is about 5000 m/s more than LEO.  Reducing the mass from 7200 to 6065 kg means you could do a GEO burn (4100 m/s) and still have 1135 kg of fuel left.  But that's not enough to get 900 m/s more, given any plausible second stage mass (which we don't know, but must be a few tons just from the stainless steel and known dimensions.)

But then I wondered why the Vulcan Heavy numbers are so wimpy.  The GEO mass (with the same 6 solid booster) only goes from 6500kg to 7200kg.  Likewise the other numbers (LEO, GTO) are only slightly (less an 10%) higher.  But a simple scaling indicates they should be a lot bigger.  The propellant mass (at least as widely reported) goes from 55t to 77t, a ratio of 7/5 or a 40% increase.  The dry mass increase should be less than 7/5 (only the sidewalls change dimensions, and they only account for half the area (the rest is the domes and bulkhead) so that's only a 20% increase in steel area).  So the payload should go up by at least 40% for the same delta-V, or up to 7/5*6500 = 9100 kg to GEO (neglecting the decrease in booster delta-V from pushing 20 more tonnes of Centaur fuel).  That should be plenty of margin for Europa Clipper, given any reasonable second stage mass.

So what's going on here?  I can think of two possibilities.  One is that 7200 kg to GEO is the maximum they think they will ever need.  (The public DOD orbits max out at 6590 kg to GEO).  So maybe they set the number conservatively, as it covers the DOD needs with plenty of margin.  They could even state this number without doing any detailed design at all - it's that conservative.

Alternatively, maybe the Centaur Heavy is just a brute force design.  Since they will only make a few, perhaps they tried to minimize design and test costs.  So make it out of non-thinned stainless steel, add lots of margin, don't worry about the last few percent of mass optimization, and so on.  Maybe it's a lot heavier than it might seem.

Whatever the reason, the small increase in payload mass (10%) seems at odds with the large increase in fuel mass (40%).  Does anyone know what's going on here, or are the Heavy numbers just wildly sandbagged?
BTW, you mean Vulcan Centaur Heavy. “Vulcan Heavy” is the tri-core version with even more performance.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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