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When we look at various Russian rockets we often see a metal structure looking like a kind of garden fence separating some stages. To my knowledge, only the R7 family (Vostok, Voshkod, Soyuz) and N1 are (were) using it, but nobody else.

All other rockets I know of use a solid fairing to separate stages. So why did the Russian engineers chose (and kept) this solution but nobody else? Apart from being very light-weight, are there any other advantages to these "fences"?

Here are some images to show what I'm talking about:

Soyuz during assembly (source)

N1 (source)

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    $\begingroup$ IIRC Proton uses this design too. On Soyuz, the upper stage engine is ignited before separation, this is only possible with these lattice designs. $\endgroup$ – Hobbes Dec 11 '16 at 18:46
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    $\begingroup$ Titan II wasn't quite as mesh-y, but did have big, noticeable holes in the interstage. This article describes them as vents to allow second stage ignition before separation. popsci.com/… $\endgroup$ – Organic Marble Dec 11 '16 at 18:49
  • $\begingroup$ Used in many early staged vehicles: Wac Corporal, Aerobee, Nike Ajax, etc.... $\endgroup$ – Bill C Nov 12 '18 at 17:01
  • $\begingroup$ Related: space.stackexchange.com/q/5187/26446 $\endgroup$ – DrSheldon Nov 12 '18 at 17:51
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It's all to do with ullage in the fuel tanks. Newton's laws of motion mean that when a rocket is no longer firing and no force is being applied, the rocket receives no acceleration. It continues at its same velocity (if we assume a perfect model). The fuel in the tanks goes into free-fall, just as the astronauts do when they reach orbit. You've seen those soft toys going floating around the cabin when the Soyuz engines cut off.

The fuel also acts a bit like a passenger in a car when you brake hard. Instead of being pushed back in their seats when accelerating, they fly the other way, towards the windscreen. That's why we use seatbelts. The same for the fuel in those huge tanks. It goes whoosh; right to the other end of the tank, the farthest distance from the rocket motor. The problem is, you want it at the rocket motor end. If the fuel is not at the "bottom" of the tank the pumps spin on empty, and can burn out with nothing to suck on.

There are two solutions to this problem. Some craft have ullage motors which are little engines, sometimes just thruster jets or a release of compressed gas. You can see them in many videos of other craft that do not have this mesh. (Not to be confused with the pump exhaust nozzle, but that is another matter entirely!) It often looks like a small exhaust pipe near the engine. They can sometimes just be a small solid fuel engine, much like a firecracker. Their only function is to move (slosh) the fuel down to the pumps. The pumps start, and then the engine fires. Whoosh; rocket starts. Now all this fuel sloshing about can cause stability problems for the craft, so they design the tank with special anti-slosh baffles.

Here we can see those various features in the Saturn V second stage:

Source NASA

Source: https://www.hq.nasa.gov/pao/History/SP-4204/images/m406b.jpg

If you want to avoid the extra complexity, controls, fuel, mechanisms and just plain mass and money in your rocket, you can do it the Russian way. Just turn on the next stage while the previous one is still applying force. No need for ullage equipment. However, you have to make somewhere for all that hot exhaust gas (and its force) to go. The easiest way is to make a slight gap between the stages so you then get less of a baba-boom. It also has the nice advantage of pushing the completed stage away from the craft and back towards the planet, which is usually what you want. It might make the rocket taller, but it keeps the mass and complexity down, which is usually what you want.

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    $\begingroup$ Ah, this related question and its answers provide more context. So there is a timing window (Soyuz: two seconds) but it's still less complex than using ullage motors. But the insulation needed on the previous stage may actually offset the mass gain by the otherwise lightweight stage separation structure and might also increase drag a little. $\endgroup$ – DarkDust Dec 12 '16 at 9:06
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    $\begingroup$ @DarkDust Good point. I think the best way to regard these quite complicated trade-offs is that sometimes the designers do find an optimum, sometimes not. Sometimes the design solution is reached by schedule pressure, vested interests (politics) or simply going with what worked the last time or in a ground test. Against this backdrop the actual facts don't have to add up to a coherent plan! $\endgroup$ – Puffin Dec 12 '16 at 14:16
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    $\begingroup$ You might ant to include that this would be called "hot staging" (staging while the stage below is still attached). $\endgroup$ – Polygnome Dec 12 '16 at 22:43
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    $\begingroup$ Just as an addendum: Not only the Russians do this; the American Titan II does the same thing, it has visible holes about half way up the stack. Here is an article. $\endgroup$ – Hans Apr 16 '18 at 9:35
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    $\begingroup$ I suggest you clarify your "seatbelt" analogy when describing the need for ullage: cutting off thrust is NOT like applying the breaks. It's like putting the car in "neutral". At that point the only force (other than gravity) being applied to the craft is atmospheric friction. I'm not sure about the N1, but on a Soyuz staging of this sort only happens above 100km, long after Max Q. At that point atmospheric friction should only be imparting a small acceleration on the craft. $\endgroup$ – Avi Cherry Nov 14 '18 at 22:04

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