Author Topic: Why stage in Deep Space?  (Read 1191 times)

Offline JasonAW3

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Why stage in Deep Space?
« on: 08/05/2016 06:12 PM »
Ok,  I'll probably get reamed for this, but, with the exception of lander craft and that sort of thing, o9n long range missions that have a return  intended, why bother to stage in space?

   I understand the use of boosters to get to orbit, plus the need for lower atmosphere optimized engines for the initial launch, thus staging there makes sense here, but once out of the atmosphere, the added mass of additional tankage and engines for an Earth Departure Stage and an Earth Return Stage, really doesn't make that much sense on the face of it.

     If it were an issue as to limited number of engine restarts, or possible damage of the engines having been left cold in vacuum after months or years at a time, that too I'd understand, but other than that, I really can't quite fathom the need.

     Could someone explain this to me?
« Last Edit: 08/05/2016 06:12 PM by JasonAW3 »
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Offline S.Paulissen

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Re: Why stage in Deep Space?
« Reply #1 on: 08/05/2016 06:23 PM »
Ok,  I'll probably get reamed for this, but, with the exception of lander craft and that sort of thing, o9n long range missions that have a return  intended, why bother to stage in space?

   I understand the use of boosters to get to orbit, plus the need for lower atmosphere optimized engines for the initial launch, thus staging there makes sense here, but once out of the atmosphere, the added mass of additional tankage and engines for an Earth Departure Stage and an Earth Return Stage, really doesn't make that much sense on the face of it.

     If it were an issue as to limited number of engine restarts, or possible damage of the engines having been left cold in vacuum after months or years at a time, that too I'd understand, but other than that, I really can't quite fathom the need.

     Could someone explain this to me?

I ask you this: Why the mass and additional tankage of an Earth launch stage on a departure stage?  That is the answer.  You'll almost always lose delta-V by not staging.
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Offline JasonAW3

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Re: Why stage in Deep Space?
« Reply #2 on: 08/05/2016 06:28 PM »
Ok,  I'll probably get reamed for this, but, with the exception of lander craft and that sort of thing, o9n long range missions that have a return  intended, why bother to stage in space?

   I understand the use of boosters to get to orbit, plus the need for lower atmosphere optimized engines for the initial launch, thus staging there makes sense here, but once out of the atmosphere, the added mass of additional tankage and engines for an Earth Departure Stage and an Earth Return Stage, really doesn't make that much sense on the face of it.

     If it were an issue as to limited number of engine restarts, or possible damage of the engines having been left cold in vacuum after months or years at a time, that too I'd understand, but other than that, I really can't quite fathom the need.

     Could someone explain this to me?

I ask you this: Why the mass and additional tankage of an Earth launch stage on a departure stage?  That is the answer.  You'll almost always lose delta-V by not staging.

Ok, I'll buy that.

     On the other hand, would reusable systems and improved materials technology, in construction improve this to the point where staging would be no longer an issue?  I'm not talking SciFi types of improvements, but more along the lines of SEP types of engines, newer composites, etc?
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Offline kwan3217

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Re: Why stage in Deep Space?
« Reply #3 on: 08/05/2016 06:30 PM »
In theory, its a question of balancing opposing factors. Let's take for instance the Inertial Upper Stage (IUS) designed as a pure space vehicle. It doesn't ever have to deal with the atmosphere, and all of its engine nozzles are optimized for vacuum. And yet it is a two-stage solid-fueled rocket. Why?

At what point does hauling around the empty tankage (casing) outweigh the mass of another engine? If an upper stage engine weighs significantly less than the lower stage tank, then you can think of two rockets, one with two stages and one with only one. The one-stage rocket is pushing the entire mass of the tank for the entire burn, while the two-stage rocket is pushing the mass of the second engine for the first part of the burn, but dropping the first stage tanks and engine for the second part of the burn. You need the details of how much your engines and tanks weigh, but it is possible to imagine a case where staging makes sense.

In practice, it's because rockets are "lego" in a sense. You don't design a new vehicle custom for every mission. Instead, you go off the shelf and pick the stages which are closest to optimum for your mission. The IUS was designed to take a payload from LEO (dropped off by Shuttle or Titan IV) all the way to geosync, without the payload needing to raise itself from geo transfer to geosync. This requires a two-burn mission. Since the IUS is solid-fueled, it uses two separate motors, one for the boost to geo transfer, and one for circularization in geosync. The burns are several hours apart in this case.

Now that the IUS exists, it can be used for missions which are close but not identical to this. The Ulysses and Galileo space probes used Shuttle/IUS to escape Earth. The stages were burned in close sequence to each other. It might have been more efficient to design a single-stage IUS for this mission, but it would have been more expensive once the design and qualification of the custom stage was factored in.

In many cases, deep space missions don't stage once in space. Most AtlasV/Centaur missions are two-stage only, using a fraction of the Centaur propellant to finish the boost into LEO, and then the rest for all in-space maneuvers. All of the recent Mars missions flew on AtlasV/Centaur and used only two stages. Similarly the TitanIV/Centaur which launched Cassini used the centaur to finish the boost to LEO then the entire Earth escape maneuver. The Centaur (and the similar upper stage on a Delta IV) is quite a capable spacecraft in its own right. Both are capable of going from LEO all the way to geosync, in a similar two-burn mission to that discussed for the IUS, but on only one stage.

New Horizons used a third stage on its Atlas V, but I believe that was the first and only time that an Atlas V flew three stages, and that was because the centaur couldn't quite handle it, so a Star48 solid-fueled upper stage was pulled off the shelf for the final boost.
« Last Edit: 08/05/2016 06:32 PM by kwan3217 »

Online pathfinder_01

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Re: Why stage in Deep Space?
« Reply #4 on: 08/05/2016 06:31 PM »
There are multiple reasons. Mass being the primary reason. All the engines, tank-age and unused propellant have mass. Staging improves the mass fraction of the rocket.  Basically a two stage rocket can achieve a higher end velocity for a given mass than a one stage rocket.

Other reasons include engines designed for different phases of the flight(i.e. A  Hydro lox earth departure stage could boil off by the time the spacecraft reaches the planet or a chemical kick stage to get an Solar Electric Spacecraft out of Orbit of a planet. ). For instance Apollo used a Hydro lox earth departure stage and hypergolic engines to get into/out of and land on the moon.

Offline DMeader

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Re: Why stage in Deep Space?
« Reply #5 on: 08/05/2016 06:32 PM »
I don't think the rocket equation would be impressed by new composites and SEP engines. Mass is mass, it costs fuel, if it serves no purpose get rid of it.

Online pathfinder_01

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Re: Why stage in Deep Space?
« Reply #6 on: 08/05/2016 06:40 PM »
Chemical rockets are very inefficient. They are so inefficient that it is impossible for a chemical rocket to make a round trip from LEO to lunar Orbit and back without refueling. Apollo saved lots of delta v by using direct reentry.


 SEP is efficient enough to so this(given enough time) but that is an totally different question than staging. i.e. Given the right mission it could make sense to stage or drop tanks from an SEP craft.
« Last Edit: 08/05/2016 06:40 PM by pathfinder_01 »

Offline Robotbeat

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Re: Why stage in Deep Space?
« Reply #7 on: 08/05/2016 07:54 PM »
Dry mass of tankage.

Of course, if you get good enough tankage dry mass (and I can imagine tanks that mass just 0.1% of the fuel they contain), then there's not really any reason to stage.
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Offline A_M_Swallow

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Re: Why stage in Deep Space?
« Reply #8 on: 08/06/2016 04:03 AM »
A further reason for staging in space is that the mass of mission payload + Earth Departure Stage + propellant exceeds the maximum payload mass of the launch vehicle. Consequently two or more launches are needed to get the mass into space. The spacecraft is then assembled and fuelled in space. Manned missions to Mars are likely to fit this category.

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