ATK announce Liberty KSC test flights, reveal crew spacecraft with MLAS

[sdsds]

I suppose my question is, if reversing the order mitigates TO, why did not Ares I do that?

Because for the Ares-I booster stage thrust oscillation was officially not a problem?

[Robotbeat]

I suppose my question is, if reversing the order mitigates TO, why did not Ares I do that?

Because for the Ares-I booster stage thrust oscillation was officially not a problem?

What makes us think the situation is different here?

[RocketmanUS]

I suppose my question is, if reversing the order mitigates TO, why did not Ares I do that?

Because for the Ares-I booster stage thrust oscillation was officially not a problem?

What makes us think the situation is different here?

All the more reason to have more than one company designing a LEO crew vehicle. Each team should be looking for private funding to be the first up there instead of waiting for government funding.

We keep hearing from the different companies that if they get government funding they can have crew taxi sooner!

[Downix]

Can someone remind me why the Liberty upper stage is upside-down compared to the Ares I stage? I skimmed the present thread. Hope I didn't miss it.

Which is in front, the LOX or LH2 tank.

I suppose my question is, if reversing the order mitigates TO, why did not Ares I do that?

Simple, because it doesn't.  Ariane's design however might, the jury is still out on that.

[MATTBLAK]

No one should forget that the Ariane V corestage is designed to operate with two rather large solid boosters attached. I should think that would feature in its resistance to T.O.

Also, unless I'm dead wrong, T.O. was never given serious attention until  well into the Ares 1 design cycles. Although it may never have turned out to be worst-case bad, the fact that it's razor-thin payload margins were being eaten into to include hardware to mitigate the problem speaks volumes. Still: water under the bridge - most of us are aware of the Ares 1 saga. Until someone writes a book, probably best not to dwell on it much here.

[fotoguzzi]

Can someone remind me why the Liberty upper stage is upside-down compared to the Ares I stage? I skimmed the present thread. Hope I didn't miss it.

Which is in front, the LOX or LH2 tank.

I suppose my question is, if reversing the order mitigates TO, why did not Ares I do that?

Simple, because it doesn't.  Ariane's design however might, the jury is still out on that.

"Tanks are flipped on the Ariane stage, so heavily mitigated compared to Ares I."

Perhaps what Downix and others are saying is that if putting the hydrogen tank below the oxygen design does not mitigate TO, then something else in the design of Liberty has improved the already good TO situation.

From memory, none of the upper stages in the ESAS study had the hydrogen tank below the oxygen tank. Do we know why the Ariane upper stage originally went with that decision?

[woods170]

Can someone remind me why the Liberty upper stage is upside-down compared to the Ares I stage? I skimmed the present thread. Hope I didn't miss it.

Literally speaking the stage is not upside-down. The tankage is reversed however, when comparing the Ares-1 US to the Liberty US.
The Liberty US has the LOX tank sitting on top of the LH2 tank, separated by a common bulkhead. On the Ares-1 US the order was reversed: the LH2 tank sat on top of the LOX tank, also separated by a common bulkhead.

As to why NASA chose the Ares-1 US configuration: dunno for sure. Might have something to do with center of gravity. But my guess is that someone else on this forum might be able to answer that question for ya.

[woods170]

Can someone remind me why the Liberty upper stage is upside-down compared to the Ares I stage? I skimmed the present thread. Hope I didn't miss it.

Which is in front, the LOX or LH2 tank.

I suppose my question is, if reversing the order mitigates TO, why did not Ares I do that?

Simple, because it doesn't.  Ariane's design however might, the jury is still out on that.

"Tanks are flipped on the Ariane stage, so heavily mitigated compared to Ares I."

Perhaps what Downix and others are saying is that if putting the hydrogen tank below the oxygen design does not mitigate TO, then something else in the design of Liberty has improved the already good TO situation.

From memory, none of the upper stages in the ESAS study had the hydrogen tank below the oxygen tank. Do we know why the Ariane upper stage originally went with that decision?

In the early boost phase, with attached solids, the Ariane 5 vehicle is mostly lifted from the top of the core stage, by the solids. You will want to have your heaviest tank (that being the LOX) closest to the vehicles center of balance. That implied having the LOX tank sitting above the LH2 tank.

[Steven Pietrobon]

You will want to have your heaviest tank (that being the LOX) closest to the vehicles center of balance. That implied having the LOX tank sitting above the LH2 tank.

Actually, you want the centre of gravity (CoG) to be ahead of the centre of pressure (CoP). Arrows do this by having a dense mass at the tip (moves CoG forward) with feathers at the tail (moves CoP back). Having the LOX tank forward moves the CoG forward, increasing stability. Similar for the Space Shuttle ET and the Saturn V S-IC (which also had fins to move the CoP back, giving a few extra seconds of stability for the vehicle in case of an abort). To reduce mass though, you want the heavier tank at the base (which is why the Saturn V S-II and S-IVB had their LOX tanks at the base).

I think the reason for the reduced oscillations is that the EPC uses a double wall structure which could be stiffer than the single wall of the Ares I US. Having the LOX tank forward means that the tank walls need to be stronger, further increasing their stiffness. A stiffer structure implies a higher resonant frequency, taking you away from the lower resonant frequency of the first stage.

[spacejulien]

Can someone remind me why the Liberty upper stage is upside-down compared to the Ares I stage? I skimmed the present thread. Hope I didn't miss it.

Literally speaking the stage is not upside-down. The tankage is reversed however, when comparing the Ares-1 US to the Liberty US.
The Liberty US has the LOX tank sitting on top of the LH2 tank, separated by a common bulkhead. On the Ares-1 US the order was reversed: the LH2 tank sat on top of the LOX tank, also separated by a common bulkhead.

As to why NASA chose the Ares-1 US configuration: dunno for sure. Might have something to do with center of gravity. But my guess is that someone else on this forum might be able to answer that question for ya.

The tank with the higher density fluid should be as near to the CoG, to keep the moment of inertia as low as possible, to reduce the effort needed for piloting (balancing) of the LV. High MoI increases max thrust vector actuation angles, thus more komplex TVA kinematics, more deltaV losses due to TVA, more secondary energy storage needed for  TVA (hydraulic => more high pressure hydraulic oil storage; electric => more high current batteries), less stability of piloting loop, etc. At some point the piloting of the LV get's unfeasible.
During Ares I the fear that piloting of the stick would be impossible was a major reason for the Ares-1X-test, so I can't really believe that they are ok with the even larger MoI for Liberty with LOx-tank @ front of upper stage... Surprising...

[spacejulien]

You will want to have your heaviest tank (that being the LOX) closest to the vehicles center of balance. That implied having the LOX tank sitting above the LH2 tank.

Actually, you want the centre of gravity (CoG) to be ahead of the centre of pressure (CoP). Arrows do this by having a dense mass at the tip (moves CoG forward) with feathers at the tail (moves CoP back). Having the LOX tank forward moves the CoG forward, increasing stability. Similar for the Space Shuttle ET and the Saturn V S-IC (which also had fins to move the CoP back, giving a few extra seconds of stability for the vehicle in case of an abort). To reduce mass though, you want the heavier tank at the base (which is why the Saturn V S-II and S-IVB had their LOX tanks at the base).

Not quite, on Saturn V the S-IC LOx-tank was in front, but for SII and S-IVB the LOx tank was at the stage rear. As I said in my previous post, the reduction of Moment of Inertia is more important than the reduction of distance between CoP and CoG for piloted vehicles.

And the fins on Saturn V were really just needed for the second or two during an abort, when the vehicle is not piloted. The fins were not needed for guided flight, in fact even (slightly) increased drag losses.

[spacejulien]

I think the reason for the reduced oscillations is that the EPC uses a double wall structure which could be stiffer than the single wall of the Ares I US. Having the LOX tank forward means that the tank walls need to be stronger, further increasing their stiffness. A stiffer structure implies a higher resonant frequency, taking you away from the lower resonant frequency of the first stage.

Ariane 5 EPC doesn't have a double wall structure...

[woods170]

I think the reason for the reduced oscillations is that the EPC uses a double wall structure which could be stiffer than the single wall of the Ares I US. Having the LOX tank forward means that the tank walls need to be stronger, further increasing their stiffness. A stiffer structure implies a higher resonant frequency, taking you away from the lower resonant frequency of the first stage.

Ariane 5 EPC doesn't have a double wall structure...

Correct. Single wall structure. Four millimeter thick aluminium.

[edkyle99]

I'm wondering how Liberty can compete, since it carries twice as much payload or more to the ISS orbit as its CCDev competitors.  That means, if fiscal realism is followed, that it should cost twice as much.

NASA does award these things to the lowest bidder, right?

 - Ed Kyle

[A_M_Swallow]

I'm wondering how Liberty can compete, since it carries twice as much payload or more to the ISS orbit as its CCDev competitors.  That means, if fiscal realism is followed, that it should cost twice as much.

NASA does award these things to the lowest bidder, right?

 - Ed Kyle

~ twice the payload.
It can be competitive if 1 launch of a Liberty costs less than 2 launches of its rivals.