Q17 - Rocket engine nozzles

[layamaheshwari]

Can someone explain why D is not the correct answer? I thought that negating it -- showing that the engine gases need not leave the nozzles at the same pressure throughout the rocket's ascent -- did weaken the argument. That's why I picked it. Perhaps the similarity was only needed for one: low altitude or high altitude.

Any help, please? Thank you so much!

[maryadkins]

(D) is wrong because it says "for a rocket to work effectively," while the stimulus says, "for the rocket to work MOST effectively."

The core is:

Rockets are most effective at low altitudes with a short nozzle on their engines and in upper altitudes with a long nozzle.

Therefore, to be most effective throughout their ascent, they should have both.

We are looking for an assumption that is required for this argument to be true, i.e. a necessary assumption.

(A) brings in the difficulty of equipping them with the nozzles, which is out of scope.

(B) must be true! If not all rockets go through the thin upper atmosphere, it doesn't work to say that all rockets need both nozzles to be most effective.

(C) is totally out of scope. There is nothing about not being able to reach high altitudes.

(D) is addressed above.

(E) makes it about one single engine having both a short and long nozzle. But there's no reason these would need to be on the same "engine." The stimulus says "engines." So the nozzles could be on multiple engines on the same rocket!

Hope this helps!

[andrewgong01]

IS "E" also a bit of a premise booster because "E" does make it sound more likely that yes we do need a short and long nozzle.

However, to me I didn't see the flaw with "E" as one that it says it must be on the same engine (as oppose to different engines); rather, I saw it as a premise booster confirming that we need two types of nozzles based of different altitudes. I did not even notice the switch from engines to single engine.

If we insert E into the stimulus it does not get the "ALL" rockets part and I felt like the key to this argument was its extreme language of saying ALL rockets must have the two nozzles; otherwise "E" could have been pretty viable had the conclusion not been that ALL rockets must have both

[tw4jp]

I have a question about answer choice B. I didn't pick it because I think sth else except passing through thin upper atmosphere may require long nozzle. So even though the rocket must have a long nozzle, passing thin upper atmosphere might not be necessary.

[ohthatpatrick]

I didn't understand that second sentence, because of the typo and a missing word, I think.

NEC ASSUMP = which answer, if negated, most weakens

Weakening an argument = accepting the evidence, but arguing against the conclusion.

So the correct answer, when negated, will help us argued that "NOT ALL rockets need short + long nozzles to work most effectively through their ascent."

We win the conversation if we can think of a way to say "at least one rocket does NOT need both a short and a long nozzle".

If you negate (B), it says "at least one rocket does not pass through the thin upper atmosphere during its ascent".

Cool! Since the long nozzle was only needed for the thin upper atmosphere, if we have at least one rocket that never goes through the upper atmosphere during its ascent, then we can argue that at least one rocket doesn't need both a short and a long nozzle (it could just get by with a short, and still be working most effectively throughout its entire ascent).

Does that make sense?

[M.M.]

(D) is wrong because it says "for a rocket to work effectively," while the stimulus says, "for the rocket to work MOST effectively."

The core is:

Rockets are most effective at low altitudes with a short nozzle on their engines and in upper altitudes with a long nozzle.

Therefore, to be most effective throughout their ascent, they should have both.

We are looking for an assumption that is required for this argument to be true, i.e. a necessary assumption.

(A) brings in the difficulty of equipping them with the nozzles, which is out of scope.

(B) must be true! If not all rockets go through the thin upper atmosphere, it doesn't work to say that all rockets need both nozzles to be most effective.

(C) is totally out of scope. There is nothing about not being able to reach high altitudes.

(D) is addressed above.

(E) makes it about one single engine having both a short and long nozzle. But there's no reason these would need to be on the same "engine." The stimulus says "engines." So the nozzles could be on multiple engines on the same rocket!

Hope this helps!

D is also wrong because it is stronger than what we need - let's say there's a middle area between low altituds and the thin upper atmosphere (as there likely is); this answer would account for the fact that we need to have some sort of middle nozzle, but the original argument only talks about what's needed in those two areas. It's more like a sufficient assumption than a necessary one.

[EmilyL849]

Hi,

I am not sure if this is a valid reason to eliminate (D) and want to check!

I did not see the most obvious scope shift. (Most effective -> Effective)
However, I also thought (D) was wrong for enlarging another scope.
By saying "For a rocket to work effectively" (throughout the whole journey) --> exhaust gas leaving pressure = the surrounding atmosphere pressure throughout the ascent.
We know from the conclusion that to work the most effectively "throughout their ascent", all rockets must have both types.
However, we do not know the effectiveness of the rocket's whole journey depends on the effectiveness of its ascent.

Would you say that this is a valid reasoning?

Thank you always!

[smiller]

By saying "For a rocket to work effectively" (throughout the whole journey) --> exhaust gas leaving pressure = the surrounding atmosphere pressure throughout the ascent.
We know from the conclusion that to work the most effectively "throughout their ascent", all rockets must have both types.
However, we do not know the effectiveness of the rocket's whole journey depends on the effectiveness of its ascent.

I can see what you mean about answer choice (D). The statement about working effectively in this answer choice doesn't explicitly limit itself to the ascent. It seems like a rocket would need to ascend effectively in order to be effective at all, but maybe not. One of the big attractions at Kennedy Space Center is the old rockets that they have on display. Those just have to sit on the ground and look like a rocket in order to be effective.

It's not terrible to question this aspect of choice (D). It shows that you're reading carefully and thinking critically, which are very useful skills in LR. But the shift from "most effectively" to "effectively" is a bigger issue, so there are more important questions to ask. Why did you originally miss the shift that you're now seeing as obvious? When working on future questions, what must you do in order to notice that kind of shift? The answers to those questions will do the most to help your LR score.

[StratosM31]

Another fact which weakens (E): it is not explicitly stated in the stimulus that the pressure in the thin upper atmosphere is low! It could just be “lower” than at low altitudes, but compared to a specific reference point they could both be “high”.

Besides, it is not explicitly stated in the stimulus that the amount of pressure is the reason why different lengths of nozzles are needed. The high pressure is stated as a side-fact for low altitudes, and low pressure is never mentioned.

[Laura Damone]

StratosM31, that's a Relative vs. Absolute flaw. Nice catch!