LR Question 17 Explanation, by LSATHacks
QUESTION TEXT: Rocket engines are most effective when exhaust gases…
QUESTION TYPE: Necessary Assumption
CONCLUSION: Rockets need long and short nozzles for their ascents.
REASONING: At low altitudes, rockets need short nozzles. In the upper atmosphere, rockets need long nozzles.
ANALYSIS: This already sounds like a good argument. But, you know this is flawed. So how could this be wrong? Well, what if some rockets only make short ascents. Maybe some rockets never hit high altitudes. In which case they would only need short nozzles.
So the argument is assuming that every rocket goes high enough to hit the thin upper atmosphere.
(You may have thought that all rockets are space rockets. Nope. Some rockets, such as ICBMs, never leave the atmosphere. All missiles are rockets.)
___________
- It doesn’t matter how difficult it is to equip nozzles. The argument is only about whether rockets need both nozzles.
- CORRECT. If this isn’t true, then not all rockets need long nozzles.
Negation: Some rockets don’t pass through the thin upper atmosphere. - The argument says rockets need both nozzles to be are most effective. But a rocket might be able to reach high altitudes even if it’s not fully effective.
Negation: Less than effective rockets can still reach high altitudes even with only short nozzles. - This is an absurd answer. Absurd answers are almost never correct. Rockets only have two types of nozzle, and air pressure changes constantly as you rise in the atmosphere. So it’s impossible to match pressure exactly through ascent.
Also, this answer talks about working effectively. The stimulus is about what’s required to work most effectively. Those are two very different things.
I worked effectively today, but I could have been far more effective. - This isn’t necessary. The argument only said the engines need long and short nozzles. It’s possible these could be on separate engines.
Negation: A rocket can be most effective if it has zero engines with long and short, as long as there are some engines with long and some engines with short.
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wikipedia: [In order to cover large distances, ballistic missiles are usually launched into a high sub-orbital spaceflight; for intercontinental missiles, the highest altitude (apogee) reached during free-flight is about 4,500 kilometers (2,800 mi). ]
so, ICBM does not only go through thin air, it’s in fact way above the atmosphere.
Hmm, I could have looked up a wrong definition when I wrote this. The exosphere extends to this range and is part of the atmosphere: Exosphere: 700 to 10,000 km (440 to 6,200 miles)
However, the ICBM article does say they leave the atmosphere.
We can leave that debate to the scientists, but there are also other types of rockets which never leave the atmosphere, so the main point stands.
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.
I’d normally be inclined to agree, but this stimulus says that the upper atmosphere is “thin”. This is a reasonable synonym for low in the full context of the passage. Especially since the author says the atmosphere up there is thin enough that a long nozzle is more effective than a short nozzle.
It is important to look for distinctions but you have to be careful when doing so.
Note: This is an old comment but I wanted to clarify the point.