Author Topic: Launch vehicle fuel margins Q&A  (Read 7033 times)

Offline MP99

Launch vehicle fuel margins Q&A
« on: 04/16/2009 01:36 PM »
I have a question about how launch vehicle margins work, specifically in relation to fuel loads & NASA's SOP.

Take the example of a fictional two-stage vehicle with 120mT payload, using 100mT of fuel during the upper stage burn.

If NASA launched such a vehicle with a 10% reduced payload (120-12=108mT) to provide margins, how much fuel would be loaded into the upper stage?

112mT of u/s fuel would provide for much longer u/s burn in the event of a performance shortfall, but doesn't reduce the total mass to be lifted to orbit in the event of a successful launch. Requires sufficient tanking be available, of course.

There is some u/s fuel load well below 100mT which will just get to orbit (ie no margins).


Between these two limits, what fuel would NASA choose to load in this circumstance?

Is it likely to just be 100mT regardless, since this is the design fuel load?

Is it likely to be somewhere below this, to reduce u/s burn times? If so, how much?

Is it possible that > 100mT fuel would be chosen to maximise the flexibility in case of a performance shortfall? All the way up to 112mT if tanking were available?

Is the choice strongly dependent on thrust-to-weight ratio of the upper stage + payload?

Is there one "obviously right" answer, or is there likely to be a large spread of fuel loads which have very similar risks, eg more fuel allows longer burns, balanced against increased chance of engine failure mid-flight.


Many thanks for any illumination you can provide.

cheers, Martin


PS any similarity between the above example and any vehicle, real-life or proposed, is purely coincidental.
« Last Edit: 05/30/2009 05:47 PM by Chris Bergin »

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 32440
  • Cape Canaveral Spaceport
  • Liked: 11186
  • Likes Given: 331
Re: Launch vehicle fuel margins
« Reply #1 on: 04/16/2009 02:12 PM »
Most launch vehicle's propellant tanks are filled to capacity regardless of payload mass.

Margin is either tabulated as velocity reserve or mass

There is mission margin, and LV margin and what is remaining is called excess margin

Mission margin is based on the trajectory, environment and past LV performance.  It is the margin required to cover 3 sigma variances

LV margin covers differences in actual weights of the vehicle, estimates for mission unique hardware, etc.  This may include "manager's margin".  LV margin is reduced as the launch approaches and things get better defined

Offline Antares

  • ABO^2
  • Senior Member
  • *****
  • Posts: 5201
  • Done arguing with amateurs
  • Liked: 368
  • Likes Given: 226
Re: Launch vehicle fuel margins
« Reply #2 on: 04/16/2009 02:24 PM »
Performance margins are usually carried in terms of equivalent payload mass or delta-V.  In early design of the rocket/payload system, this can be as much as 30% of the payload.  For a new rocket, by the time it gets to first flight, it's down to 10-15%.  As the flights go on, the margin is retired usually.  I don't think there's a universal standard on this.  Ares is not obeying it if there is.  NASA has endorsed the margins on Atlas up through V and Delta up through II (presumably the USAF up through IV), but how they are managed is proprietary (though nothing special).  You can look up the payload planners guides to see what the standard margins are.

Not familiar with Shuttle, though I'm sure they've retired all the margin they're going to long ago.  They always try to shutdown on guidance (hitting the target orbit) rather than running out of gas (STS-93) or running on gas as it were.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

Offline MP99

Re: Launch vehicle fuel margins
« Reply #3 on: 04/20/2009 11:57 AM »
I don't think there's a universal standard on this.  Ares is not obeying it if there is.


OK, those are going to make my supplementary question difficult to answer, then...

Consider an Ares-V cargo-only type flight. The features of this are:-

* Upper stage also performs as EDS.

* Payload is therefore lander&cargo + TLI fuel.

* there is unused US/EDS tank capacity, due to the requirements of other missions.

I'm not sure of the latest Ares-V numbers, so let's re-use the fictional vehicle above, with a 20mT EDS. To make things easier, let's also assume an exact 1:1 burnout-mass-to-TLI-fuel is required (close enough to illuminate an example).

Max IMLEO is 140mT (120mT launch payload + 20mT EDS). Fuel is 70mT & lander+cargo is 50mT (ie 70-20).

If a 108mT payload is flown (as above), the numbers drop to 64mT fuel & 44mT lander+cargo. Any fuel unused in the ascent (as per the previous answers) becomes additional reserve for TLI.


Now for the tricky question...

There is 12mT of unused lift capacity in the example above. Would overall margins be improved if some or all of that capacity was used to load additional fuel? I'm guessing not (components are less likely to fail during a shorter burn), but I'd like to hear any thoughts on the subject.


Alternate version of the same question...

Load the vehicle to capacity (100mT ascent fuel + 70mT TLI fuel + 50mT lander). In the event of a performance shortfall, how likely is it that the lander could at least get to LEO by burning some of the TLI fuel? Is it likely that a lot of TLI fuel would be consumed?

The obvious comparison here is with situations where the "10% light" vehicle (100 + 64mT fuel & 44mT lander) can complete a problematic ascent using only the reserve from the 100mT ascent fuel.

Many thanks.

cheers, Martin

Offline R.Simko

  • Full Member
  • ***
  • Posts: 311
  • Liked: 3
  • Likes Given: 23
Re: Launch vehicle fuel margins Q&A
« Reply #4 on: 07/20/2009 04:23 PM »
Hi, I am posting this question here, but if there is a more appropriate thread let me know and I will repost question.  Thanks  :)

Why is it we choose to launch from Florida, at a site that is almost sea level?  Wouldn't it be better launching from a mountain to, to give us a better starting elevation and thereby increase the carrying capacity of the launch vehicle?

Perhaps one of the Hawaiian islands, or if not there another mountain? 

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7454
  • Martijn Meijering
  • NL
  • Liked: 76
  • Likes Given: 171
Re: Launch vehicle fuel margins Q&A
« Reply #5 on: 07/20/2009 04:34 PM »
Launching to orbit is mostly about gaining velocity, not altitude. A "sea level earth orbit" isn't much cheaper than a 200km one. Launching from a higher altitude might help because of lower drag, but I imagine the logistics would be challenging.
« Last Edit: 07/20/2009 04:36 PM by mmeijeri »
We will be vic-toooooo-ri-ous!!!

Offline R.Simko

  • Full Member
  • ***
  • Posts: 311
  • Liked: 3
  • Likes Given: 23
Re: Launch vehicle fuel margins Q&A
« Reply #6 on: 07/20/2009 04:45 PM »
Thanks for the answer mmeijeri.  :)

Online kevin-rf

  • Elite Veteran
  • Senior Member
  • *****
  • Posts: 8652
  • Overlooking the path Mary's little Lamb took..
  • Liked: 1124
  • Likes Given: 243
Re: Launch vehicle fuel margins Q&A
« Reply #7 on: 07/21/2009 12:59 PM »
Huh? Launching above sea level alows a first stage engine with a higher expansion ration. When done, that leads to slightly higher ISP, which could lead to a more payload or a smaller LV.

You would have to design the rocket to take advantage of it.

Compare the Expansion ratio of the Merlin first stage and second stage engine nozzles. If launching from 200km you would be using a vac optimized nozzle, you could do an SSTO if you had a 200km high launch pad.

About the Merlin from the spaceX site :

Sea Level Thrust :   533 kN (120,000 lbf)
Vacuum Thrust:   569 kN (128,000 lbf)
Sea Level Isp:    275s
Vacuum Isp:    304s

It is more an issue of many launch sites where built to test and develop ICBM's and not dropping downrange on inhabited areas.
If you're happy and you know it,
It's your med's!

Offline mmeijeri

  • Senior Member
  • *****
  • Posts: 7454
  • Martijn Meijering
  • NL
  • Liked: 76
  • Likes Given: 171
Re: Launch vehicle fuel margins Q&A
« Reply #8 on: 07/21/2009 03:40 PM »
Ah, I had hadn't thought of Isp varying with pressure.
« Last Edit: 07/21/2009 03:41 PM by mmeijeri »
We will be vic-toooooo-ri-ous!!!

Online kevin-rf

  • Elite Veteran
  • Senior Member
  • *****
  • Posts: 8652
  • Overlooking the path Mary's little Lamb took..
  • Liked: 1124
  • Likes Given: 243
Re: Launch vehicle fuel margins Q&A
« Reply #9 on: 07/21/2009 06:00 PM »
When launching at a higher altitude

ISP is higher for the same nozzle on the same engine.
You can use a larger Nozzle on the same engine (higher ISP on the same engine, case in point Merlin second stage).
You're MAX Q point shifts upward and the actual Max Pressure Drops ( Less Areo Drag ).

All of which can lead to more payload, or a smaller vehicle...
If you're happy and you know it,
It's your med's!

Online edkyle99

  • Expert
  • Senior Member
  • *****
  • Posts: 12934
    • Space Launch Report
  • Liked: 3992
  • Likes Given: 758
Re: Launch vehicle fuel margins Q&A
« Reply #10 on: 07/21/2009 06:37 PM »
When launching at a higher altitude

ISP is higher for the same nozzle on the same engine.
You can use a larger Nozzle on the same engine (higher ISP on the same engine, case in point Merlin second stage).
You're MAX Q point shifts upward and the actual Max Pressure Drops ( Less Areo Drag ).

All of which can lead to more payload, or a smaller vehicle...

There is a difference, but not much of a difference.  There are mountains in excess of 4 km height in the contiguous U.S., but these are all out west, precluding launch since the rockets would have to fly over populated areas.  Can't use them during winter (multiple meters snowfall depth), and Air Force Range Safety would not approve of stages falling on cities and towns downrange ...

Regardless, the only gain would be the relatively small performance difference of those 4 km.  A rocket launched at sea level is, within a very short time, higher than any mountain and enjoying the advantages of lower atmospheric density and higher specific impulse. 

Air launch provides much more altitude than a mountain top, and adds a bit of initial horizontal velocity, but still only reduces total delta-v requirements by a few percent compared to ground launch.

 - Ed Kyle
« Last Edit: 07/21/2009 06:52 PM by edkyle99 »

Tags: