Author Topic: Basic Rocket Science Q & A  (Read 270762 times)

Offline Remes

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Re: Basic Rocket Science Q & A
« Reply #1060 on: 03/11/2016 09:16 PM »

I find it curious that so many rockets do parallel staging. Apart from ground-starting the engines on the core there don't seem to be any obvious advantages. You save some thrust on the first stage (often little because the core uses hydrolox) but you lose some performance (more dry mass at separation, lower engine ISP).

Anything else?
Ariane 5 was developed to be manrated, so the reliability and abort capability of ground start plus  an opportunity to test the liquid propulsion comes in handy. Same goes for Angara 5.

Soyuz is heritage. Delta 4 requires a lot of helium in order to spin up the pumps. On Atlas 5 you save some tvc for the booster, but aside from that the rd-180 has a significant amount of thrust.

Ariane 5 has a blow down hydraulic tvc system, the Falcon 9 and heavy would need some hydraulic source (like e.g. the APU from the shuttle). (Upper stage for Ariane 5, Atlas 5 and Delta 4 is different, it's electric. And even the Europa rocket could live with a battery driven electrohydrostatic system).

Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #1061 on: 03/12/2016 04:24 PM »
I'd have thought that in the cases Angara, Delta 4 and Atlas 5 (as well as Falcon and the old Titan, for that matter), the attraction of parallel staging is simply the modularity it provides.  It allows one basic design to provide a range of capabilities, depending on what's needed for any particular mission (although it didn't work out so well for the Delta IV, since performance shortfalls have meant that it has several substantially different cores).


Offline sugmullun

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Re: Basic Rocket Science Q & A
« Reply #1062 on: 03/13/2016 04:11 PM »
I have been assuming that the main advantage was less gravity loss early on because
of the T/W, (for a given total "installed thrust?") of this arrangement, as getting through the vertical component quickly is
always preferable.

wrong?
« Last Edit: 03/13/2016 04:22 PM by sugmullun »

Offline Remes

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Re: Basic Rocket Science Q & A
« Reply #1063 on: 03/13/2016 06:51 PM »
the main advantage

the attraction of parallel staging is simply [...]

I think in rocketry as in all other complex systems there is not one reason which is used to make a decision. It's rather a trade off between performance, cost, reliability, development risks, producibility, repeatability, operations, transportability, ground support, etc. Then in addition to technical reasons there are sometimes political issues (who gets to build the thing, who has the biggest power and influence). Most often there is not the one optimum solution, it's rather a set of several solutions and finally (and hopefully) one path is chosen. Decisions are often (on engineering level) influenced by the most influential people. And I believe that every engineer is somehow biased (in good and bad ways) by his/her experience. Then there is heritage: do we use what we have or do we make something better from scratch...

less gravity loss early on because of the T/W, (for a given total "installed thrust?") of this arrangement, as getting through the vertical component quickly is always preferable.
Tradeoff between getting quickly through the vertical component against (e.g., numbers guessed) having a 90% full lox/lh2 tank on an Ariane 5 when staging. Same for D4H. I guess the gravity loss is less influential on performance than having an optimum mass fraction for the center core (e.g. tanks full) after staging.

There is a long discussion on cross feed for F9H.

(And Ariane 5 is a bad example, as in the beginning the thrust/weight ratio is anyway small.)

Offline sugmullun

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Re: Basic Rocket Science Q & A
« Reply #1064 on: 03/14/2016 11:33 AM »
After Googling around I've decided that my question was too simplistic for an inherently complicated problem, so I removed it. 
It seems that parallel staged boosters would be somehow closer to a magical ssto rocket, that sheds dry mass along with propellant, than stacked stages are.

« Last Edit: 03/14/2016 10:12 PM by sugmullun »

Re: Basic Rocket Science Q & A
« Reply #1065 on: 05/17/2016 09:54 PM »
Are rockets designed with the intent of using solid rocket boosters from the beginning?

My assumption was that solids are expensive and are designed only to be used on rare heavy payloads when the rocket needs an extra boost from the pad. That way the standard rocket don't have excesses capacity i.e. lower cost. How wrong am I?

Offline shooter6947

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Re: Basic Rocket Science Q & A
« Reply #1066 on: 05/17/2016 10:10 PM »
Are rockets designed with the intent of using solid rocket boosters from the beginning?

My assumption was that solids are expensive and are designed only to be used on rare heavy payloads when the rocket needs an extra boost from the pad. That way the standard rocket don't have excesses capacity i.e. lower cost. How wrong am I?

Depends on the rocket.  The Space Shuttle and Delta2 both require solids to leave the pad (they're TWR<1 without).  Delta4 and AtlasV, for instance, can optionally use solids if they need extra oomph.

Re: Basic Rocket Science Q & A
« Reply #1067 on: 05/17/2016 10:37 PM »
Are rockets designed with the intent of using solid rocket boosters from the beginning?

My assumption was that solids are expensive and are designed only to be used on rare heavy payloads when the rocket needs an extra boost from the pad. That way the standard rocket don't have excesses capacity i.e. lower cost. How wrong am I?

Depends on the rocket.  The Space Shuttle and Delta2 both require solids to leave the pad (they're TWR<1 without).  Delta4 and AtlasV, for instance, can optionally use solids if they need extra oomph.


When the Atlas V rocket was designed, why did they decide to use 2 rd180 instead of 3? An extra engine would remove the need for solids which in turn could reduce their cost by removing a supplier and increasing their production of the engine bringing the cost down. Does one extra engine cost significantly more than the occasional solid rocket booster?
I've been thinking of the falcon 9 and their "one size fits all" approach and been wondering if, for example, the falcon 9 rocket never was designed and they went with the Falcon 5 instead, would it make economic sense for them to occasionally use solid rocket boosters?

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #1068 on: 05/17/2016 11:07 PM »

When the Atlas V rocket was designed, why did they decide to use 2 rd180 instead of 3? An extra engine would remove the need for solids which in turn could reduce their cost by removing a supplier and increasing their production of the engine bringing the cost down. Does one extra engine cost significantly more than the occasional solid rocket booster?
I've been thinking of the falcon 9 and their "one size fits all" approach and been wondering if, for example, the falcon 9 rocket never was designed and they went with the Falcon 5 instead, would it make economic sense for them to occasionally use solid rocket boosters?

RD-180 is one engine with two nozzles

The EELV program Medium (single core) vehicle basic requirement was to place 10klb into GTO.  Atlas V did this with the RD-180 and latest version of the single engine Centaur.  The EELV program Heavy (3 cores) vehicle basic requirement was to place 10klb into GSO. The Atlas V Heavy (which was designed but not built) did this with 3 RD-180 cores and the same Centaur. 
The need for solids came later as commercial spacecraft grew.  So Atlas V was modified to accept solids after the basic design was finished (that is why it can only carry 5 solids in an unsymmetrical pattern and why the VIF has intermediate levels between floors 1 & 2)
« Last Edit: 05/17/2016 11:09 PM by Jim »

Offline rocx

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Re: Basic Rocket Science Q & A
« Reply #1069 on: 05/18/2016 10:55 AM »
Are rockets designed with the intent of using solid rocket boosters from the beginning?

My assumption was that solids are expensive and are designed only to be used on rare heavy payloads when the rocket needs an extra boost from the pad. That way the standard rocket don't have excesses capacity i.e. lower cost. How wrong am I?

Solid rockets are actually cheaper to produce and operate than liquid stages of similar performance. What also helps is that solids are the best way to launch nuclear bombs at other continents, so most of their development and production investments are already made for the military.
Any day with a rocket landing is a fantastic day.

Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #1070 on: 05/18/2016 12:28 PM »
Are rockets designed with the intent of using solid rocket boosters from the beginning?

My assumption was that solids are expensive and are designed only to be used on rare heavy payloads when the rocket needs an extra boost from the pad. That way the standard rocket don't have excesses capacity i.e. lower cost. How wrong am I?

Solid rockets are actually cheaper to produce and operate than liquid stages of similar performance. What also helps is that solids are the best way to launch nuclear bombs at other continents, so most of their development and production investments are already made for the military.
Not quite. Solids require vertical integration with the LV (Atlas does the whole stack integration vertically, while Delta IV does most horizontally and just the solids are integrated vertically at the pad).
Also, the LV needs to be designed for the extra thrust. It does allows for a "smother" payload/cost curve. Depending on requirements solids might be cheaper or not. In other words, solids were optimal for Atlas V (even if added after the fact) because they had to comply with the EELV requirements. They didn't appear to be optimal for Falcon 9, Antares nor the Russians. For OrbitalATK, since they actually make them, they now want to go all solid. So no straight al generic answers. Each trade results in different optimizations.

Online jabe

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Re: Basic Rocket Science Q & A
« Reply #1071 on: 06/25/2016 01:31 PM »
It seems many companies are working on methane engines.  Would a methane 1st stage be ruled out, or is RP1 too good fuel of choice density/performance wise.

Offline rocx

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Re: Basic Rocket Science Q & A
« Reply #1072 on: 06/26/2016 08:38 PM »
It seems many companies are working on methane engines.  Would a methane 1st stage be ruled out, or is RP1 too good fuel of choice density/performance wise.
http://spacelaunchreport.com/vulcan.html
The successor to the Atlas V and Delta IV rockets is probably going to have a methane first stage.
Any day with a rocket landing is a fantastic day.

Online jabe

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Re: Basic Rocket Science Q & A
« Reply #1073 on: 06/26/2016 11:41 PM »
It seems many companies are working on methane engines.  Would a methane 1st stage be ruled out, or is RP1 too good fuel of choice density/performance wise.
http://spacelaunchreport.com/vulcan.html
The successor to the Atlas V and Delta IV rockets is probably going to have a methane first stage.
is it lack of maturity of the tech for methane that it isn't used more?

Offline rocx

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Re: Basic Rocket Science Q & A
« Reply #1074 on: 06/27/2016 08:01 AM »
is it lack of maturity of the tech for methane that it isn't used more?
Yes. There is currently no large scale methane engine, upper stage nor first stage. SpaceX is building Raptor for both roles, and Blue Origin is building BE-4 as a first-stage engine. Masten is developing the Cutlass for use in planetary landers. This takes many years to fully develop.
Any day with a rocket landing is a fantastic day.

Offline smoliarm

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Re: Basic Rocket Science Q & A
« Reply #1075 on: 07/05/2016 06:52 AM »
Hi everybody,
I have few questions on Earth’s rotation.
Pretty frequently I see statements in press on launch site choice, where low latitude (close to equator) is considered as a simple advantage – because Earth’s rotation contribution is highest at equator. It seems to me, that this is not quite correct:
First, bonus from Earth’s rotation is minor – I estimated it on the order of percent or less.
Second, the most fuel-efficient launch gives an orbit with inclination equal to launch site latitude. In case your target orbit is different, you have to change the orbital plane – and this could cost significantly more fuel, than *Earth rotation savings*.
However, I did not have a proper Orbital Mechanics course, and the one I had, it was 19 years ago. Therefore, I’d like to check my calculations.

So the first question is about the correct numeric estimate for the contribution of Earth’s rotation.
I take as example a launch vehicle Soyuz-like – 305 metric tons at launch, and I assume Earth’s radius = 6371 km. Then I compare launches of this LV from two launch sites – one at 46° N (Baikonur-like) and the other at 5° N (Kourou-like). In both  cases the target orbit is circular at 200 km with the same inclination as launch site latitude.
My result: the launch from Kourou gives ~ 72 kg advantage in payload mass over the launch from Baikonur, of less than one percent.
Is this close enough?

The second question is about inclination change and its hit on payload mass.
Here I take as example Kourou as launch site, Ariane 5ES as LV and two target orbits:
a) ISS orbit (400x400 km at 52°) and
b) a “launch site inclination orbit” – 400x400 km at 5°.
My estimate: in the first case the penalty for orbital plane change will be 15%. Or –
Performance (a) = 0.85 Performance (b)
The same question – is this a close estimate?

Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #1076 on: 07/05/2016 08:30 PM »
If you measured the instantaneous speed of an orbit projected over the surface of the Earth, you could write it as a vector of speeds on the latitude/longitude axis (save at the poles). The Earth rotation at the Equator gives something like 465m/s. So it adds a (0m/s , 465m/s) vector to your launch.
To reach orbit you need something like 9,500m/s of delta-v. If you launch westward, you are adding exactly that amount. If you angle you launch pitch, you will have the cos of the pitch angle.
The math is that at around 86deg of inclination, that's about what you need on the west direction if you launched from the Equator. Anything more inclined and you start loosing performance from sites like Kourou. That's why sites like Plesetsk are so good for polar launches. But there are many other considerations for launch sites.

Online Robotbeat

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Re: Basic Rocket Science Q & A
« Reply #1077 on: 07/09/2016 02:35 AM »
You can do significantly less than 9500m/s (i.e. around 9km/s) if you launch due West and have good thrust/weight ratio and a high ballistic coefficient.
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Offline mobile1

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Re: Basic Rocket Science Q & A
« Reply #1078 on: 08/07/2016 10:51 PM »
How did the Juno space probe reach 165,000 MPH?

Offline joek

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Re: Basic Rocket Science Q & A
« Reply #1079 on: 08/07/2016 11:11 PM »
How did the Juno space probe reach 165,000 MPH?
Gravity.  Specifically, Jupiter's gravity.