Author Topic: Staging Q&A - Expendable Launch Vehicle Architechture  (Read 6100 times)

Offline AJA

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Have there been any launch vehicles designed (maybe even prototyped) - that have only one set of engines which fire all the way into orbit? I'm NOT talking about SSTO. Rather, I'm referring to LV architectures where sub-orbital staging takes place, but the discarded stages comprise ONLY fuel tanks: exactly like the Shuttle ET-SSME pairing, but with the tanks being part of the main cylinder (as opposed to being tacked on to the side, and thereby increasing the drag cross section).


The rationale is saved weight - since each stage would be using the same engine.


I figure having the engine up front (Either weird, toroidal tanks, with place for the exhaust through the centre ... or LAS type 'towing' - but for the whole vehicle... with.. with.. some insanely strong spars linking the motor to the tanks) and pumping fuel against g-loads would add a pressure head to the turbopumps. The modifications (and associated weight increases) would probably negate any advantage to be had


So, I guess what I'm asking boils down to this: Have there been any evaluations for LVs where staging happens from the FRONT of the vehicle, and the stack is shortened from the FRONT, and where the orbital segment being located at the aft end of the stack? (The Back-to-front line vector being the thrust-vector).


I imagine that the aerodynamic flow distortions at staging (in such a hypothetical arrangement) would probably lead to unsafe structural loads on the stack - either directly, or by necessitating that the remainder of the stack fly through turbulent areas where guidance and control inputs might get confused, and enter an overcompensation spiral. Additionally, there's the danger of the tanks impacting the payload below - even if you use charges to 'peel' the tank into strips, and jettison these a la payload fairings (requiring explosive bolts in the wall of a fuel tank). Solving all this would still leave the rocket bottom heavy - I don't know if that's a bad thing - control wise.


So... yeah, while this is probably exactly as ridiculous as it sounds - is there any report that's quantified said ridiculousness?
« Last Edit: 09/14/2013 09:47 AM by AJA »

Offline cartman

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #1 on: 09/14/2013 10:54 AM »
Well i think its probably not worth it to carry the weight of the main engines up to orbit.

Offline Jim

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #2 on: 09/14/2013 11:47 AM »

How does it save weight when it needs other structural members (insanely strong spars) that don't carry propellant.  turbopumps that have to pump against the direction of flight, weird tanks that don't support flight loads as efficiently as cylinders, etc.   

If you figured it out in a matter of a few sentences why would there have to be a formal study?

Offline R7

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #3 on: 09/14/2013 12:13 PM »
The rationale is saved weight - since each stage would be using the same engine.

You don't save weight when you take unnecessarily powerful engine all the way to orbit. Consider Falcon 9, it takes off with nine engines but does most of the ascent with just one. If it would fire all nine all the way to orbit it would either have to be beefed up to handle double digit g-forces or do very deep throttle down.
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Offline Oli

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #4 on: 09/14/2013 02:14 PM »

That makes no sense, the heavy stuff is at the bottom. Engines and the tanks which carry all the load.

By the way, there's something I wanted to know for some time: Why do hydrolox launchers like D4 have the LOX tank at the top of the stage? Should it not be at the bottom to minimize structural mass?

Offline baldusi

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #5 on: 09/14/2013 02:19 PM »

That makes no sense, the heavy stuff is at the bottom. Engines and the tanks which carry all the load.

By the way, there's something I wanted to know for some time: Why do hydrolox launchers like D4 have the LOX tank at the top of the stage? Should it not be at the bottom to minimize structural mass?
CG issues?

Offline Jim

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #6 on: 09/14/2013 03:54 PM »

By the way, there's something I wanted to know for some time: Why do hydrolox launchers like D4 have the LOX tank at the top of the stage? Should it not be at the bottom to minimize structural mass?

for controllability.

Online Robotbeat

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #7 on: 09/14/2013 04:19 PM »
Atlas did the opposite it staged off engines and kept the tank.
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Offline AJA

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #8 on: 09/15/2013 06:46 PM »
Atlas did the opposite it staged off engines and kept the tank.

Which Atlas? Are you referring to strap-ons that were solely engines? I'm not talking so much about parallel staging, as much as shortening-the-height-of-the-stack staging. If you are indeed talking about the latter, then are you implying that they had say x engine-stages right on top of each other, with descending TIG values (separated enough to incorporate burn time of lower engine + some coast time) as you descended the stack? In any case, why not split the tank?

You don't save weight when you take unnecessarily powerful engine all the way to orbit. Consider Falcon 9, it takes off with nine engines but does most of the ascent with just one. If it would fire all nine all the way to orbit it would either have to be beefed up to handle double digit g-forces or do very deep throttle down.

1. In the case where you can shed only tanks, you could achieve orbit with a less powerful engine/cluster. Higher Isp. Of course, you've only got limited manoeuvring room with respect to T/W ratio, and maybe time of ascent (Wdrag)

2. We really ought to have an NSF-corollary of Godwin's Law, for SpaceX mentions.


If you figured it out in a matter of a few sentences why would there have to be a formal study?

Variables that I'd perhaps not considered :)

Offline R7

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #9 on: 09/16/2013 06:37 AM »
1. In the case where you can shed only tanks, you could achieve orbit with a less powerful engine/cluster.

Huh? Because you would be taking higher LV inert mass to orbit you'd need larger LV/engines for the same payload than normally staging LV.

Quote
Higher Isp.

Gotta proof that tank-staging concept would somehow enjoy better Isp than others.

Quote
2. We really ought to have an NSF-corollary of Godwin's Law, for SpaceX mentions.

It's already in place  ;D
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Online Robotbeat

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #10 on: 09/16/2013 09:00 AM »
If anything, you'd have lower Isp with this concept since the engine would have to work at sea level so couldn't expand as much.
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Offline kevin-rf

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #11 on: 09/26/2013 11:36 AM »
If anything, you'd have lower Isp with this concept since the engine would have to work at sea level so couldn't expand as much.

There have been concepts that get around that problem. Aerospikes, deploy-able nozzles, chamber throat plugs that jettison, ect... The issue has been studied by many, just never flown by any.
« Last Edit: 09/26/2013 11:37 AM by kevin-rf »
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Online Robotbeat

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Re: Staging Q&A - Expendable Launch Vehicle Architechture
« Reply #12 on: 09/26/2013 09:58 PM »
If anything, you'd have lower Isp with this concept since the engine would have to work at sea level so couldn't expand as much.

There have been concepts that get around that problem. Aerospikes, deploy-able nozzles, chamber throat plugs that jettison, ect... The issue has been studied by many, just never flown by any.
...because these concepts are usually much heavier. And aerospikes still don't get you as high vacuum Isps as a real bell nozzle.
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0