Author Topic: Basic Rocket Science Q & A  (Read 272169 times)

Offline ciscosdad

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Re: Basic Rocket Science Q & A
« Reply #920 on: 06/06/2014 08:12 PM »
Thanks guys, That's just what I'm looking for.
My curiosity was roused by the upcoming EFT-1. That reentry is from High orbit, so significantly below escape speeds. Is there any way to simulate a higher speed by changing the reentry profile (steeper for example).

The info you supplied shows that its a very different flight regime at escape velocities, and is a complex interplay with radiative and convective and both change with altitude.

Another query: Is a higher entry velocity dealt with only by a thicker (ablative) shield? Can the flight profile be shaped to ease the load? Is that why Apollo used the double "dip" style reentry?

Offline scienceguy

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Re: Basic Rocket Science Q & A
« Reply #921 on: 06/20/2014 05:36 PM »
Is it possible for a rocket to go faster than its propellant exhaust velocity?
e^(pi)i = -1

Offline ugordan

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Re: Basic Rocket Science Q & A
« Reply #922 on: 06/20/2014 05:40 PM »
Is it possible for a rocket to go faster than its propellant exhaust velocity?

http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Let's put it this way: if it wasn't possible, no chemical rocket would ever reach LEO.

Offline DarkenedOne

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Re: Basic Rocket Science Q & A
« Reply #923 on: 08/23/2014 10:23 PM »
SpaceX seems to do perform a form of full vehicles integration testing when it performs the static fire tests.  Obviously they can do this at little expense due to the fact that their engines are restartable and relatively reusable.   Obviously this type of test cannot be performed with rockets that can only be started once.

It seems like a pretty valuable test to me.  I was wondering how any other launch systems are tested in such a way?

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #924 on: 08/24/2014 12:29 PM »
SpaceX seems to do perform a form of full vehicles integration testing when it performs the static fire tests.  Obviously they can do this at little expense due to the fact that their engines are restartable and relatively reusable.   Obviously this type of test cannot be performed with rockets that can only be started once.

It seems like a pretty valuable test to me.  I was wondering how any other launch systems are tested in such a way?

It was a practice in the 50's that had diminishing returns as the art of rocket science progressed.  Delta and Atlas are doing good with out doing it.  Even WDR's are becoming a thing of the past.  But that is only because the operators are accepting a schedule risk, if there are an problems that would be uncovered during WDR, they still would be found during the actual countdown.

Offline su_liam

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Re: Basic Rocket Science Q & A
« Reply #925 on: 08/28/2014 07:41 AM »
I'm trying to calculate the Tsiolkovsky delta-V for a parallel stage like the Space Shuttle with greatly different ISP. How would you calculate the "effective" ISP for that kind of arrangement?

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Re: Basic Rocket Science Q & A
« Reply #926 on: 08/28/2014 07:59 AM »
I'm trying to calculate the Tsiolkovsky delta-V for a parallel stage like the Space Shuttle with greatly different ISP. How would you calculate the "effective" ISP for that kind of arrangement?
Isp is a ratio of prop consumed vs thrust produced.

You can add the solid + liquid prop consumed, and add the solid + liquid thrust produced. The ratio of these two sums gives you a blended Isp.

Unfortunately, both the solids and SSME were throttled, which makes this more complicated for Shuttle. Any change in the proportion of thrust from the two systems needs a separate calculation of the blended Isp for that portion of the flight.

But, you may be able to get away with just averaging prop use and thrust over duration of solid operation.

Cheers, Martin

Offline aga

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Re: Basic Rocket Science Q & A
« Reply #927 on: 08/28/2014 08:40 AM »
Isp is a ratio of prop consumed

just to clarify... it is mass/weight consumed, not volume
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Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #928 on: 08/28/2014 09:08 AM »
Yes, assuming Isp is constant.

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #929 on: 08/28/2014 12:19 PM »
Isp is a ratio of prop consumed
just to clarify... it is mass/weight consumed, not volume

Important addition to the original sentence is per unit of time ie. thrust divided by mass flow rate.

For n different engines firing simultaneously with thrusts F1, F2 ... Fn and respective specific impulses of Isp.1, Isp.2 ... Isp.n the math is

Isp.combined = ( F1 + F2 + ... + Fn ) / ( F1 / Isp.1 + F2 / Isp.2 + ... + Fn / Isp.n )
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Online MP99

Re: Basic Rocket Science Q & A
« Reply #930 on: 08/28/2014 02:47 PM »
Isp is a ratio of prop consumed
just to clarify... it is mass/weight consumed, not volume

Important addition to the original sentence is per unit of time ie. thrust divided by mass flow rate.

For n different engines firing simultaneously with thrusts F1, F2 ... Fn and respective specific impulses of Isp.1, Isp.2 ... Isp.n the math is

Isp.combined = ( F1 + F2 + ... + Fn ) / ( F1 / Isp.1 + F2 / Isp.2 + ... + Fn / Isp.n )

Thanks.  And, of course, yes to both.  :-[

cheers, Martin
« Last Edit: 08/28/2014 02:47 PM by MP99 »

Offline liamdoyle

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Re: Basic Rocket Science Q & A
« Reply #931 on: 08/28/2014 03:44 PM »
hi new here but have a question, is there any point to having the dragon v2 make a powered landing? seems to me parachutes work just fine and that thrusters will only add weight and complexity

Offline oiorionsbelt

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Re: Basic Rocket Science Q & A
« Reply #932 on: 08/28/2014 03:51 PM »
Reusability, accuracy of landing and time to access of of time sensitive cargo are all improved with powered landing.
« Last Edit: 08/28/2014 03:51 PM by oiorionsbelt »

Offline dglow

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Re: Basic Rocket Science Q & A
« Reply #933 on: 08/28/2014 03:55 PM »
Parachutes won't work well on Mars.

Offline Nascent Ascent

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Re: Basic Rocket Science Q & A
« Reply #934 on: 08/28/2014 04:07 PM »
The thrusters and fuel have to be there because these serve double-duty.  Used in case of launch abort escape and if not needed for abort, then used for landing.
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Offline rpapo

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Re: Basic Rocket Science Q & A
« Reply #935 on: 08/28/2014 04:16 PM »
You don't need to send ships out to fish the capsule out of the water, and you have much less chance of seawater leaking in.
An Apollo fanboy . . . fifty years ago.

Offline Robotbeat

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Re: Basic Rocket Science Q & A
« Reply #936 on: 08/28/2014 04:19 PM »
Also, the thrusters don't really add complexity since they're needed for abort. Also, parachutes have a way of failing. Powered landing can be done with parachutes as a backup.
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Offline R7

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Re: Basic Rocket Science Q & A
« Reply #937 on: 08/28/2014 04:32 PM »
You don't need to send ships out to fish the capsule out of the water, and you have much less chance of seawater leaking in.

You can avoid those also by landing on ground with parachutes like Soyuz and Shenzhou (albeit using last second retro rockets).
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Offline SpacexULA

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Re: Basic Rocket Science Q & A
« Reply #938 on: 08/28/2014 04:33 PM »
Also, the thrusters don't really add complexity since they're needed for abort. Also, parachutes have a way of failing. Powered landing can be done with parachutes as a backup.

I agree, there are a lot of reasons to do propulsive landing

-increased Landing Accuracy
-decreased impact landing
-Adds no complexity if the LAS is integrated into the capsule
-Adds redundancy if the parachutes fully or partial fail (Soyuz has had partial parachute failures), these lead to high G non fatal impacts at the end of the mission.
No Bucks no Buck Rogers, but at least Flexible path gets you Twiki.

Offline R7

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Re: Basic Rocket Science Q & A
« Reply #939 on: 08/28/2014 04:37 PM »
Soyuz has had partial parachute failures

Apollo 15 also.

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