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fredinno

Why do GPS IIFs fly with SRBS on Delta IV?

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Why do GPS IIFs fly with SRBs on Delta IV?

I mean, Delta IV without any boosters + 4m diameter upper stage seem to be able to launch GPS sats just fine- The Atlas V 401, also used to launch GPS, only has marginally higher performance, (0.5-0.2 more T to LEO, and 0.05 more T to GTO), and the Centaur used on Atlas V GPS missions even has enough fuel to do a DEORBIT BURN. Also, both were made to launch GPS on their basic configurations. So...why are 2 SRBs used? Why waste money on SRBs when the rocket doesn't really need it to launch GPS?

http://www.spacelaunchreport.com/delta4.html

http://www.spacelaunchreport.com/atlas5.html

http://www.nasaspaceflight.com/2015/03/ula-delta-iv-gps-iif9/

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I have no clue. I thought maybe they launched a whole bunch of them with one rocket but that doesn't seem to be the case either. However, one of the links you listed has this to tell:

Quote

With a mass of 1,630 kilograms (3,590 lb), the new spacecraft are slightly lighter than their predecessors, however this is accounted for by their omission of an apogee motor – instead relying on the more powerful Atlas V and Delta IV rockets to deliver them directly into their operational orbits. Each Block IIF satellite is designed to operate for 12 years

So perhaps that has something to do with it (I’ll admit that, being at work, I don’t have the luxury of doing the math and see if, ΔV-wise, it makes sense).

EDIT:

In addition, the article mentions:

Quote

Wednesday’s mission made use of a Delta IV rocket, flying in the Medium+(4,2) configuration. Consisting of a single Common Booster Core (CBC) first stage, a four-metre Delta Cryogenic Second Stage (DCSS) and a pair of GEM-60 solid rocket motors to augment the CBC’s thrust at liftoff, the Medium+(4,2) is the most-flown version of the rocket with liftoff marking its thirteenth flight. Across all configurations it was the twenty-ninth Delta IV to fly.

These are not the giant boosters that are the same size as the first stage, but rather small boosters. I get the impression that they're added for the same reason we add Flea's or Hammers to a launch vehicle: not so much for additional ΔV but just to boost the TWR at lift-off.

Edited by Kerbart

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3 minutes ago, Kerbart said:

I have no clue. I thought maybe they launched a whole bunch of them with one rocket but that doesn't seem to be the case either. However, one of the links you listed has this to tell:

So perhaps that has something to do with it (I’ll admit that, being at work, I don’t have the luxury of doing the math and see if, ΔV-wise, it makes sense)/

I know that- the Atlas V actually usually has enough fuel to deorbit itself from MEO. That shows they must have a lot of fuel left over.

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You ninja'd my edit :)

I think it has more to do with TWR than with DV.

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18 minutes ago, Kerbart said:

You ninja'd my edit :)

I think it has more to do with TWR than with DV.

That makes little sense either- I thought the Delta IV was rated for 9T? GPS is ~1.5T.

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Just now, fredinno said:

That makes little sense either- I thought the Delta IV was rated for 9T? GPS is ~1.5T.

Well, if my intents to help you make little sense I apologize for wasting your time. Sorry.

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39 minutes ago, Kerbart said:

Well, if my intents to help you make little sense I apologize for wasting your time. Sorry.

Sorry. Thanks though.

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The main reason is that the few GPS IIF birds that went up on Delta IV's rode on Delta IV Medium+ boosters which always have the pair of GEM-60's regardless of payload mass. Only the no-longer-used Medium launched without them.

Another thing to note is that manufacturer quoted mass to orbit specs are mass to GTO, not GEO. Depending on any number of factors it takes an additional 1,700 - 2,000  ΔV to burn and circularize into GEO, thus the payload capability to GEO is nearly half of what it is to GTO. It simply looks better to quote payload to GTO :wink:

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1 hour ago, Rybo said:

The main reason is that the few GPS IIF birds that went up on Delta IV's rode on Delta IV Medium+ boosters which always have the pair of GEM-60's regardless of payload mass. Only the no-longer-used Medium launched without them.

Another thing to note is that manufacturer quoted mass to orbit specs are mass to GTO, not GEO. Depending on any number of factors it takes an additional 1,700 - 2,000  ΔV to burn and circularize into GEO, thus the payload capability to GEO is nearly half of what it is to GTO. It simply looks better to quote payload to GTO :wink:

Why is the No-SRB version no longer used? It seems to require minimal additional infrstructure.

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mmh - a delta IV with no booster with a 4m upper stage and 1,5tons payload has only a TWR of 1,26 at sea level. - the difference between sea level and vacuum ISP for the RS68A is quite big too - (360s SL, 412s vacuum) - so maybe they only propose the delta-IV with boosters so they can limit a bit the gravity losses - and only have limited variations of the same trajectory to compute.

still, guess the main reason could be simply to keep a 'steady' and knowledgeable workforce for both delta-IV and ATK factories, so the DOD can keep two different launchers. (afterall, if you never launch delta-IV except if Atlas has a problem, you would lay off the delta-IV workers, and the day you need delta, you would have to rely on new hires with no experience - bad for reliability.)

 

Edited by sgt_flyer

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14 hours ago, fredinno said:

Why do GPS IIFs fly with SRBs on Delta IV?

I mean, Delta IV without any boosters + 4m diameter upper stage seem to be able to launch GPS sats just fine- The Atlas V 401, also used to launch GPS, only has marginally higher performance, (0.5-0.2 more T to LEO, and 0.05 more T to GTO), and the Centaur used on Atlas V GPS missions even has enough fuel to do a DEORBIT BURN. Also, both were made to launch GPS on their basic configurations. So...why are 2 SRBs used? Why waste money on SRBs when the rocket doesn't really need it to launch GPS?

http://www.spacelaunchreport.com/delta4.html

http://www.spacelaunchreport.com/atlas5.html

http://www.nasaspaceflight.com/2015/03/ula-delta-iv-gps-iif9/

Are they required now to boost/degrade the launch or intermediate stages into graveyard or atmosphere. I understand now that there is some requirement for decluttering space. 

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3 hours ago, Shpaget said:

GPS sats are not in GEO.

They are in 55° inclined 20 000 km orbit

You're right! My memory failed me. 

2 hours ago, fredinno said:

Why is the No-SRB version no longer used? It seems to require minimal additional infrstructure.

The "no-SRB version" was a Delta IV Medium. The Delta IV Medium+ series that does use strap-ons have a handful of other improvements over the older Medium series, mostly consisting of avionics improvements.

Long story short...the Medium didn't use SRB's and the Medium+ does.

Edited by Rybo

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3 hours ago, Rybo said:

You're right! My memory failed me. 

The "no-SRB version" was a Delta IV Medium. The Delta IV Medium+ series that does use strap-ons have a handful of other improvements over the older Medium series, mostly consisting of avionics improvements.

Long story short...the Medium didn't use SRB's and the Medium+ does.

Indeed, but I can't find anything saying Delta IV Medium has been retired. https://en.wikipedia.org/wiki/Delta_IV#Delta_IV_Medium

4 hours ago, sgt_flyer said:

mmh - a delta IV with no booster with a 4m upper stage and 1,5tons payload has only a TWR of 1,26 at sea level. - the difference between sea level and vacuum ISP for the RS68A is quite big too - (360s SL, 412s vacuum) - so maybe they only propose the delta-IV with boosters so they can limit a bit the gravity losses - and only have limited variations of the same trajectory to compute.

still, guess the main reason could be simply to keep a 'steady' and knowledgeable workforce for both delta-IV and ATK factories, so the DOD can keep two different launchers. (afterall, if you never launch delta-IV except if Atlas has a problem, you would lay off the delta-IV workers, and the day you need delta, you would have to rely on new hires with no experience - bad for reliability.)

 

Hmm. It says here many rockets start off at 1.2 TWR so that they will stay in the 1.5-1.7 TWR the longest for higher efficiency.

4 hours ago, PB666 said:

Are they required now to boost/degrade the launch or intermediate stages into graveyard or atmosphere. I understand now that there is some requirement for decluttering space. 

 

 

Those apply for satellites- not upper stages, as those need as much performance as possible. And the Atlas V GPS launches do full deorbit- Delta IV Medium can get away with a graveyard orbit if it is cheaper.

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I'm not entirely sure that they have been officially retired. I just don't think there are many customers interested in using them when they have their choice of  Medium+, Heavy, or Atlas V. Or just ignoring ULA altogether and riding booster from a private venture like SpaceX, to say nothing of the myriad of other boosters they could select from other countries around the world.

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1 minute ago, Rybo said:

I'm not entirely sure that they have been officially retired. I just don't think there are many customers interested in using them when they have their choice of  Medium+, Heavy, or Atlas V. Or just ignoring ULA altogether and riding booster from a private venture like SpaceX, to say nothing of the myriad of other boosters they could select from other countries around the world.

Delta IV isn't even offered commercially since it's so expensive- ULA doesn't bother, since Atlas V also has extreme reliability, and is 1/2 the cost. Only the DOD ever uses Delta IV.

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That's my point exactly. The USAF, USN, NRO, etc., can more or less afford to purchase whatever  they want. Although I think the NRO is really the only consistent Delta user with almost everyone else using Atlas V's

Edited by Rybo

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1 minute ago, Rybo said:

That's my point exactly. The USAF, USN, NRO, etc., can more or less afford to purchase whatever  they want. Although I think the NRO is really the only consistent Delta user with almost everyone else using Atlas V's

So they just buy the extra SRBs just cause?

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More or less. I think they purchase the Medium+ for the improvements in avionics and reliability over the standard Medium and the GEM-60's are just part of the package.

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1 hour ago, Rybo said:

More or less. I think they purchase the Medium+ for the improvements in avionics and reliability over the standard Medium and the GEM-60's are just part of the package.

Wouldn't it be easy to incorporate them into the Delta IV Medium? Avionics are traditionally on the core/second stage...

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2 hours ago, fredinno said:

Delta IV isn't even offered commercially since it's so expensive- ULA doesn't bother, since Atlas V also has extreme reliability, and is 1/2 the cost. Only the DOD ever uses Delta IV.

Well if its DOD you never know, they are still flying B52s, but also should they decide to launch from a dark site, they may not want to advertise the size of their payload.

 

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Maybe the DOD does that simply to keep the dual launcher capability, in case a problem grounds atlas V for months ? - if no one else purchases delta IV, they still need to give to the people working on delta-IV a job to do - to preserve an experienced workforce (these kind of people would be hired elsewhere). - you don't want a workforce entirely made of people with no delta-IV building experience the day they would really need delta-IV - hiring a workforce with no delta-IV experience only in case of atlas V failure would increase building time, increase failure risks. (And before you even restarting building deltas, you need to train those new hires, delaying even more the next launch)

- and the same goes for the whole supply chain behind it - if the suppliers don't have demands for Delta-IV parts, no need to keep the tools and workers at the ready at all time for those. 

Edited by sgt_flyer

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9 hours ago, PB666 said:

Well if its DOD you never know, they are still flying B52s, but also should they decide to launch from a dark site, they may not want to advertise the size of their payload.

 

literally EVERYONE knows the GPS sats size. It's one of the least "military" things the DOD does. https://en.wikipedia.org/wiki/GPS_Block_IIF

6 hours ago, sgt_flyer said:

Maybe the DOD does that simply to keep the dual launcher capability, in case a problem grounds atlas V for months ? - if no one else purchases delta IV, they still need to give to the people working on delta-IV a job to do - to preserve an experienced workforce (these kind of people would be hired elsewhere). - you don't want a workforce entirely made of people with no delta-IV building experience the day they would really need delta-IV - hiring a workforce with no delta-IV experience only in case of atlas V failure would increase building time, increase failure risks. (And before you even restarting building deltas, you need to train those new hires, delaying even more the next launch)

- and the same goes for the whole supply chain behind it - if the suppliers don't have demands for Delta-IV parts, no need to keep the tools and workers at the ready at all time for those. 

What I was asking is why they use GPS on Delta IV Medium+ Variants than on Delta IV Mediums when the latter is likely cheaper, and can launch them. I know why they use Delta IV, just no the variant.

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@fredinno

maybe for the same reasons - GEM60's are only used on delta-IV :) - you need to keep ATK's production lines running to keep the experienced personnel for making the boosters and casting the fuel. (at least until Atlas V GEM63's are operationnal - current Atlas V AJ-60A are built by Rocketdyne, but GEM63's are coming soon, and later GEM63's XL are meant for vulcan.)

and you need your assembly crew to keep the experience of attaching those :)

 

 

Edited by sgt_flyer

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