Author Topic: SpaceX: Merlin 1D Updates and Discussion Thread 2  (Read 278896 times)

Offline Joffan

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #20 on: 12/12/2013 10:25 PM »
Yes, it does make a difference. The reason is that with higher thrust you get a quicker ascent and reduced gravity losses. So the answer is that they could increase the maximum payload even without changing the amount of propellant.
But wouldn't that also increase the g forces on the payload?

It could, but even if the g-force were restricted to the same maximum by throttling back, there would still be a significant gain. The high g-force is late in the ascent, where the gravity losses are already getting smaller. Getting off the pad up through max-Q is where you'd get the bulk of the gains.

---------
EDIT to add: If the craft were throttling back for max-Q already, it's possible that you would have to throttle back over a longer altitude range (starting lower/finishing higher) with the higher max thrust.
« Last Edit: 12/12/2013 10:32 PM by Joffan »
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Offline macpacheco

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #21 on: 12/13/2013 12:25 AM »
Considering Elon's statement that they're only using 85% of available thrust, parsing that as a performance envelope not currently used for launches (perhaps only on the test stand):
 100% thrust today =   85% future maximum thrust
 117% thrust today = 100% future maximum thrust
   75% thrust today =   64% future maximum thrust
Not too shabby, ability to throttle from full thrust down to 2/3
Of course you can always wish for more

This has been discussed here:
http://forum.nasaspaceflight.com/index.php?topic=32998.0

The additional consideration is that should this extra thrust be fully available for full time use, this would call for a F9v1.2 with more fuel, increasing both non reuse payload and perhaps allowing for F9v1.2 S1+S2 reusability with the same F9R S1 reusability today, but even without more fuel, this would increase payload (both with and without S1 reusability, just less than with a version with more fuel)
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Offline cleonard

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #22 on: 12/13/2013 12:45 AM »
In general the exhaust velocity will decline as the engine is throttled down.  There are also second order effects.  Take a gas generator cycle engine.  Does the amount of fuel burned to pump the fuel change exactly the same as the throttling.  I doubt it.  It's likely close, but not exact.



Offline dante2308

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #23 on: 12/13/2013 12:47 AM »
Hi everyone,

this question maybe obvious for many of you - please excuse me if this is the case but I'm just in learning mode.

As well known, the Merlin 1D engine can be throttled down from 100% to 60%.

The vacuum thrust of a rocket engine is proportional to the exhaust velocity and to the mass flow rate of the gas exhaust: dm/dt * V_exh.

In vacuum and if also the gravity drag is negligible, when a Merlin 1D engine is throttled down, is dm/dt only changing, OR also V_exh (aka Isp_vacuum) is affected someway, with a consequent reduction in the engine efficiency in using the available propellant?

If the second hypothesis is right (as I would say by looking at the physical principles the engine is based on), does someone know a reasonable way to write the relation Thrust(%) = f(%,dm/dt, V_exh)?

Thanks for any hint,

V_exh is not affected by thrust directly. There is a second order effect because you slightly lower the chamber pressure. This is my impression at least. I may be wrong.

Offline dante2308

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #24 on: 12/13/2013 12:48 AM »
Ok well at least there is some agreement about using the term 'second order effect.'

Online Robotbeat

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #25 on: 12/13/2013 03:45 AM »
Hi everyone,

this question maybe obvious for many of you - please excuse me if this is the case but I'm just in learning mode.

As well known, the Merlin 1D engine can be throttled down from 100% to 60%.

The vacuum thrust of a rocket engine is proportional to the exhaust velocity and to the mass flow rate of the gas exhaust: dm/dt * V_exh.

In vacuum and if also the gravity drag is negligible, when a Merlin 1D engine is throttled down, is dm/dt only changing, OR also V_exh (aka Isp_vacuum) is affected someway, with a consequent reduction in the engine efficiency in using the available propellant?

If the second hypothesis is right (as I would say by looking at the physical principles the engine is based on), does someone know a reasonable way to write the relation Thrust(%) = f(%,dm/dt, V_exh)?

Thanks for any hint,

V_exh is not affected by thrust directly. There is a second order effect because you slightly lower the chamber pressure. This is my impression at least. I may be wrong.
Thrust is proportional to chamber pressure (to first order), so you're right. Good old Sutton.
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Offline ChrisWilson68

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #26 on: 12/13/2013 04:17 AM »
Yes, it does make a difference. The reason is that with higher thrust you get a quicker ascent and reduced gravity losses. So the answer is that they could increase the maximum payload even without changing the amount of propellant.
But wouldn't that also increase the g forces on the payload?

As others have said, you can throttle down near the end of the burn to keep the maximum g load during the first stage burn the same.  But there's also a second stage burn, and, depending on the mass of the payload, the second stage burn could be giving the payload higher g forces than the first stage even with more first-stage thrust.
« Last Edit: 12/13/2013 04:18 AM by ChrisWilson68 »

Offline MP99

Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #27 on: 12/13/2013 07:47 AM »
I have a question for you. If SpaceX were to try the D+, what would they do for propellant? Mass flow goes from 236 to 262 so the burns would be shorter on both stages. Sure, the Isp is up a little but in the wash, it really doesn't make much difference to payload capability. With more powerful engines, will they extend the length of the tanks by 10 % (a guess) to keep the lift-off T/W the same or will they just drive the engines harder for little benefit?

How much extra prop mass could fit within the existing tanking volume by densification (sub-cooling) of the O2?

Obviously this would require the bulkhead between the two tanks to be moved as the RP1 density is unchanged. Aerodynamics would be unaffected, but CoG, etc would be.

Sub-cooled O2 might also help with on orbit lifetime for the upper stage for GTO missions.

Cheers,  Martin


Offline MP99

Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #28 on: 12/13/2013 08:08 AM »
Found an answer.

http://www.grc.nasa.gov/WWW/RT/RT2001/5000/5870tomsik.html

Edit: Seems to be a dead link. Believe this is the same document:-
http://ntrs.nasa.gov/search.jsp?R=20050203875

Quote

This additional loaded mass of 16,100 lb represented on average an 8.9-percent increase in onboard LO2 propellant. Test results also confirmed the presence of thermally stratified oxygen layers inside the tank. These layers varied in the vertical direction from 122 °R for the colder, denser fluid at the bottom to 166 °R for the warmer, less dense LO2 near the top outlet of the STA tank.


(OK,  how many of you had even heard of Rankin as a temperature measure?)

At ~2.5:1,  I believe that's about 6.35% greater kerolox mass in the same total tank volume.

Cheers,  Martin
« Last Edit: 03/10/2014 10:50 AM by MP99 »

Online Robotbeat

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #29 on: 12/13/2013 08:17 AM »
I've heard of Rankine! :) It's like the imperial version of Kelvin. Also, if you're going to subcool the LOx... We could always subcool the kerosene...
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Offline guckyfan

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #30 on: 12/13/2013 09:40 AM »
We could always subcool the kerosene...

Elon Musk does not like solid boosters. ;D

Maybe gel engines?

Offline baldusi

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #31 on: 12/13/2013 12:32 PM »
I've heard of Rankine! :) It's like the imperial version of Kelvin. Also, if you're going to subcool the LOx... We could always subcool the kerosene...
Flows like frozen molasses. Or were you making a joke? Please put a smiley. In the internet you can't say something ridiculous enough not to be taken at face value.  ::)

Offline pagheca

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #32 on: 12/13/2013 01:29 PM »
V_exh is not affected by thrust directly. There is a second order effect because you slightly lower the chamber pressure. This is my impression at least. I may be wrong.

Thanks for your impression, but why a variation of the propellant flow is only "slightly" lowering the chamber pressure? How is the chamber pressure affected by a reduction in the propellant fed into it?

I searched a lot over the internet, for example by reading this http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710019929_1971019929.pdf. I also looked at the Lunar Model Descent Propulsion System, that is someway similar in design to the Merlin, but couldn't find a single reference to the issue "how throttling a rocket engine impacts on the exhaust velocity?".

I was guessing that a rocket engine is essentially working by expanding adiabatically a gas. Now, if I apply the ideal gas law, pV=RT, by reducing p, being V fixed, T reduces LINEARLY. As T is related to the kinetic energy of the molecules expulsed by the nozzle, the exhaust velocity would in turn vary LINEARLY with the pressure, determined by the amount of propellant injected in the gas chamber.

I'm probably wrong here because T is dictated by the chemical reaction, and the chamber pressure is connected through a throat to the nozzle. So within certain limits, the pressure must build up until the gas is expelled through the throat, and the relation be less than linear.

However it is, I would really like to find a single reference explaining this and showing a plot of exhaust velocity vs. throttle to better understand the issue. It must be something very important for a powered descent on the Moon or to Earth.

It may also tell why it is better to use N<<9 engines at full throttle rather than M>N engines at reduced throttle (assuming you have enough throttle range to achieve the required total thrust), despite the fact that, as I suppose, a throttled engine is less stressed and therefore more reliable.

pagheca

p.s. frankly speaking, I do not understand why my original thread labelled "simulating a Merlin 1D throttling down" has been moved here. I'm a newbie here, so I fully accept the decision of the management, but by doing this the replies to my question, that is more a general one about the performances of rocket engines, are now mixed up with many many other posts and topics, making very difficult to follow the discussion. Moreover, I (and everyone reading here, I suppose) am receiving e-mail tell me that "A reply has been posted to a topic you are watching", even it concerns an issue I'm not at all interested in.Can someone please explain me the rationale behind this? Thanks. I wish to adhere the forum rules but I have to understand them first.
« Last Edit: 12/13/2013 02:02 PM by pagheca »

Offline pagheca

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #33 on: 12/13/2013 03:19 PM »
I think that at last I found a reference confirming your impression was right, dante2308:

Only a few applications require a change in thrust during flight. Equations [...] show that the thrust is  directly proportional to the throat area At, the chamber pressure Pl, or the mass flow rate rh, but it is a weak function of CF, which in turn depends on k, the altitude, a pressure ratio, and A2/At. These equations show how the thrust may be varied and imply how other performance parameters may be affected by such variation. For liquid propellant rockets the mass flow to the chamber can be decreased (by throttling valves in  the propellant feed system) while the chamber geometry and the nozzle throat area are unchanged. The reduced mass flow will cause an almost linear decrease in Pl and thus an almost linear decrease of F. The combustion temperature does change slightly but it does not enter into the above relations. The specific impulse would also decrease slightly. (from http://web.mit.edu/e_peters/Public/Rockets/Rocket_Propulsion_Elements.pdf, page 96, paragraph 3.8.)
« Last Edit: 12/13/2013 04:04 PM by pagheca »

Offline aero

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #34 on: 12/14/2013 03:13 AM »
Do the first stage Merlin D engines, as built, throttle?
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Offline Owlon

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #35 on: 12/14/2013 03:26 AM »
Do the first stage Merlin D engines, as built, throttle?

Yes. They are throttled down during a typical launch as the first stage approaches empty in order to limit acceleration. SpaceX has said they throttle to 70%, but some fellow NSFers say the most recent Grasshopper test must have throttled down to about 60%.

Offline Hobbes-22

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #36 on: 12/14/2013 09:32 AM »
On photos of various Merlin 1Ds, I've noticed two different turbopump exhausts:
One, angled to one side like the Merlin 1C was:
http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=26388.0;attach=338595;image

The other is straight down:
http://2.bp.blogspot.com/-XNytk6p2UDk/Up7cbC4ckTI/AAAAAAAAbPQ/ode0klmkBaU/s1600/Captura-de-pantalla-2013-09-29-a-las-20.25.15.png

this reply in another thread (http://forum.nasaspaceflight.com/index.php?topic=32859.msg1103307#msg1103307) suggests that the straight-down version is the production version. Is that correct?

Of course this may be moot since this picture suggests the turbopump exhausts are hidden behind the bottom cover:
http://digitalvideo.8m.net/spacex/new/octaweb2.jpg

But is this a production rocket or just an unfinished engineering model?

I'm working on a 3D printable model of the F9 v1.1...
« Last Edit: 12/14/2013 01:19 PM by Chris Bergin »

Offline cambrianera

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #37 on: 12/14/2013 09:58 AM »
On photos of various Merlin 1Ds, I've noticed two different turbopump exhausts......(snip)

Your first pic is of M1D prototype (later used on GH1).
Other pics are of production M1D.
M1D prototype shares with M1C the thrust structure (then called quadrapod) and a big part of the TPA.
M1D production has a new thrust structure (smaller, to fit into the octaweb) and a new TPA (fixed to the CC).
I attach pictures of M1D prototype (first pic) and M1D production (second pic) on the same test stand.

BTW: it's better to attach pics rather to embed them, Chris is happier!
« Last Edit: 12/14/2013 10:00 AM by cambrianera »
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Offline pagheca

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #38 on: 12/14/2013 12:17 PM »
They are throttled down during a typical launch as the first stage approaches empty in order to limit acceleration.

Do you have any reference for this?
thanks

Offline cambrianera

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Re: SpaceX: Merlin 1D Updates and Discussion Thread 2
« Reply #39 on: 12/14/2013 01:04 PM »
They are throttled down during a typical launch as the first stage approaches empty in order to limit acceleration.

Do you have any reference for this?
thanks
Hmm, SpaceX Falcon9 main page reports this:
"The first stage engines are gradually throttled near the end of first-stage flight to limit launch vehicle acceleration as the rocket’s mass decreases with the burning of fuel."
http://www.spacex.com/falcon9
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