Atypical demonstration of ground effect

[.Scott]

TL;DR Summary

How often do you get to see an orbital-class rocket operate in ground effect?

Here is an interesting YouTube video explaining what happened with the most recent Astra launch. One of five rocket engines failed almost immediately.

According to the video, this left the rocket with a thrust-to-weight ratio of 1.00 until the fuel burned off. But it took about 10 seconds before it started rising. So the direct thrust would have been below 1.00 for most of that 10 seconds.

Since the navigation system performed spectacularly well - it is best to assume that it was generating as much thrust as it could to pull itself out of this mess. That leaves only one possible reason for it to hang out that close to the ground without any substantial change in altitude: ground effect.

In fact, I think this might require that some descriptions of ground effect be rewritten - because this is one case where it cannot be described in terms of reduced drag on a lift system.

 

[FactChecker]

The video says that the thrust/weight was 1 at the beginning and that it started up only after enough fuel was burned. Notice that the acceleration is very small even after the altitude is too high for ground effects. I do not think that the ground effects were ever significant unless it somehow was the cause of one engine failing.

 

[.Scott]

What is significant is the amount of time it spent near the ground - roughly 10 seconds. Holding an altitude for that length of time for that long with a constantly changing load requires some sort of feedback system.

At first, one might suspect that the guidance system for such fine control. But to what end? There is no deliberate benefit in stay that close to the ground for that long.

The commentator does not work for Astra Space. His comment that the lift ratio was 1.00 is based on his observation of the trajectory.

There is also direct evidence of the available lift - dirt is being picked up and driven far into the air. So the question is not whether there is ground effect lift but whether its effects are sufficient to be the primary cause of that extended hover.

Notice that the acceleration is very small even after the altitude is too high for ground effects. I do not think that the ground effects were ever significant unless it somehow was the cause of one engine failing.

The reduction in mass will be pretty linear. So the change in acceleration should be pretty linear. And with a roughly constant horizontal component, an initial lift-to-weight ratio of 1.00 would draw out a parabola with the apex very close to where that horizontal correction completed at about t=1sec. But that is not what we see in the video. Instead, it scoots along the ground before starting it parabolic rise.

How else would you explain that extended scoot?

 

[dlgoff]

Summary:: How often do you get to see an orbital-class rocket operate in ground effect?

Here is an interesting YouTube video explaining what happened with the most recent Astra launch. One of five rocket engines failed almost immediately.

According to the video, this left the rocket with a thrust-to-weight ratio of 1.00 until the fuel burned off. But it took about 10 seconds before it started rising. So the direct thrust would have been below 1.00 for most of that 10 seconds.

Since the navigation system performed spectacularly well - it is best to assume that it was generating as much thrust as it could to pull itself out of this mess. That leaves only one possible reason for it to hang out that close to the ground without any substantial change in altitude: ground effect.

In fact, I think this might require that some descriptions of ground effect be rewritten - because this is one case where it cannot be described in terms of reduced drag on a lift system.

You should post this video here: https://www.physicsforums.com/threads/space-stuff-and-launch-info.879196/

 

[sandy stone]

It appears in the video that the attitude control system was vectoring the thrust of the remaining engines for all it was worth, to keep the rocket from tipping over. This resulted in a reduced vertical component of thrust, as well as significant horizontal movement. As the attitude came under control, the thrust vector became more vertical, and that along with reduced propellant weight allowed the rocket to start to gain altitude.
Final grades - attitude control engineering team, A+; propulsion team, D-.

 

[.Scott]

As the attitude came under control, the thrust vector became more vertical, and that along with reduced propellant weight allowed the rocket to start to gain altitude.

It took about a second to correct the attitude and restore the thrust vector to vertical.

It didn't attempt to correct the horizontal velocity component because no one foresaw a problem with it jumping a fence and running across a field.

 

[berkeman]

It didn't attempt to correct the horizontal velocity component because no one foresaw a problem with it jumping a fence and running across a field.

Did you see the looks on those cows' faces?

 

[russ_watters]

Wait, so this thing was only designed for 0.25 g of acceleration at liftoff? That seems impossibly low.

It appears in the video that the attitude control system was vectoring the thrust of the remaining engines for all it was worth, to keep the rocket from tipping over. This resulted in a reduced vertical component of thrust, as well as significant horizontal movement.

Sure, but how much can that really be? It didn't gimbal and hold 60 degrees, did it?

 

[sandy stone]

Sure, but how much can that really be? It didn't gimbal and hold 60 degrees, did it?

Point taken. Obviously, losing one engine out of five was the main problem. Still amazing that the guidance system maintained control under the circumstances.

 

[russ_watters]

One of five rocket engines failed almost immediately.

According to the video, this left the rocket with a thrust-to-weight ratio of 1.00 until the fuel burned off.

Does it really have only a 1.25 thrust to weight ratio at liftoff? That would be shockingly low/slow.

 

[OCR]

Did you see the looks on those cows' faces?

No, but I'm almost certain I could hear them honking their horns. . .Eww. . . .

.

 

[.Scott]

Does it really have only a 1.25 thrust to weight ratio at liftoff? That would be shockingly low/slow.

This was the 6th attempt. On try #5, they just barely missed orbital velocity. So the major change for this try was to add about 5 feet of length to the rocket for additional fuel - but no change to the rockets. Given the results, I would say they were targeting something closer to 1.23 on liftoff.

 

[.Scott]

Did you see the looks on those cows' faces?

The house special: Kodiak Field-Broiled Steak.
Could I have mine medium rare?

 

[sophiecentaur]

Given the results, I would say they were targeting something closer to 1.23 on liftoff.

I would like to know about the control system, to keep the rocket vertical with a full load. How inefficient is it to have one (or two) of the nozzles pointing in a 'strange' direction to do the work of the missing nozzle? With nozzles that close together, there must be a lot of mutual interaction. The remaining nozzles could then need to have been backed off to maintain balance.

When you have intelligent control systems dealing with a fault situation, they will make their own decisions about what's best and, if keeping the rocket upright and getting it away from the launch site was the major criterion, then wouldn't it have been likely to have 'chosen' to do just that? Once it got a safe distance away then I made a good attempt at launching itself, although under powered and under fuelled at that stage.
Would it really have made sense to design a launch with T/W so low as 1.25? Is this a new strategy, compared with normal minimum values of more than 3?

 

[.Scott]

I would like to know about the control system, to keep the rocket vertical with a full load. How inefficient is it to have one (or two) of the nozzles pointing in a 'strange' direction to do the work of the missing nozzle? With nozzles that close together, there must be a lot of mutual interaction. The remaining nozzles could then need to have been backed off to maintain balance.

With a pentagon arrangement of the engines, expect all 5 are gimbaled. Given the velocity (near zero), there is very little inefficiency. You need to keep the thrust vector through the center of gravity. To do that with a missing engine, the body of the rocket needs to be yawed away from the failed engine. This will affect the aerodynamics. At max Q, that could be important. But at elevations between -0.1 and 10 meters above ground level, it is nothing.

Would it really have made sense to design a launch with T/W so low as 1.25? Is this a new strategy, compared with normal minimum values of more than 3?

As I posted earlier, the originall design was certainly targetting a thrust-to-weight ratio over 1.25. This was attempt #6. Attempt #5 just barely failed to reach orbital velocity. So they added about 5 feet to the body for additional fuel. That brought the ratio down - but it would still be an advantage since by the time that extra 5-feet of fuel was burned, the rocket would already have been well on its way.

 

[sophiecentaur]

Given the velocity (near zero), there is very little inefficiency.

OK. I thought that the nozzles are vectored. I was assuming that the 'efficiency' of the engines (or at least the thrust) would be affected if the streams cross and that a missing engine would require this to happen.
Also, the word 'efficiency' that I used may not be applicable. Also, if the efficiency of the whole system is Work Out / Energy In, the efficiency at the start of the launch will be low because the vertical motion is very small at that point and Power is Force times Velocity and he craft is almost stationary. From that point on, the efficiency gets better, of course.
You quoted a low T/W figure but how did they justify it, considering it's so much lower than standard practice? Were they just after a result at any cost? I guess there could be a safety issue for manned launches but why not always squeeze a bit more (non-living) payload in?

 

[.Scott]

You quoted a low T/W figure but how did they justify it, considering it's so much lower than standard practice? Were they just after a result at any cost? I guess there could be a safety issue for manned launches but why not always squeeze a bit more (non-living) payload in?

Obviously, I was not in on the decision - but I can easily understand it.
Getting to orbit with a mass simulator is a major milestone and is critical to the organizations survival.
In this case, Astra Space had just gone public and had raised substantial capital largely on the near-miss on launch #5. So the objective was not to get to orbit efficiently, but to get to orbit at all.

In fact, as Elon Musk has pointed out many times, fuel is not a major cost:
From ARS Technica:

SpaceX's founder, Elon Musk, has said it costs the company about $60 million to build a Falcon 9 rocket. The propellant itself only costs $200,000.

But once you get to say 1.1, you're burning a lot of fuel for very little advantage, but the cost may still be worth it - especially if any other solution would take a year longer while starving you company of development time and funds.

Perhaps the biggest benefit or staying above 1.2 was demonstrated in this launch: On the failure on one engine, it avoids extensive environmental and infrastructure damage.

 

[StandardsGuy]

Summary:: How often do you get to see an orbital-class rocket operate in ground effect? ...

In fact, I think this might require that some descriptions of ground effect be rewritten - because this is one case where it cannot be described in terms of reduced drag on a lift system.

Why do you think ground effect is at work here? Rocket engines do not suck in their own exhaust.

 

[berkeman]

Why do you think ground effect is at work here? Rocket engines do not suck in their own exhaust.

Um, have you read this thread from the beginning? I'm guessing not, since we've gone other places in the discussions

Do you understand what the ground effect is? There is nothing about engines sucking in anything in the ground effect.

Lordy.

 

[.Scott]

Why do you think ground effect is at work here? Rocket engines do not suck in their own exhaust.

As @berkeman suggested, a definition for "ground effect" was established long before the problem that helicopters can have with exhaust. And it applies to both powered aircraft and sail planes - during rotation and especially during the landing phase.

 

[StandardsGuy]

Um, have you read this thread from the beginning? I'm guessing not, since we've gone other places in the discussions

Do you understand what the ground effect is? There is nothing about engines sucking in anything in the ground effect.

Lordy.

Certainly I did. From the link in the OP:

"Early VTOL experimental aircraft operated from open grids to channel away the engine exhaust and prevent thrust loss from HGI.

The Bell X-14, built to research early VTOL technology, was unable to hover until suckdown effects were reduced by raising the aircraft with longer landing gear legs.[17] It also had to operate from an elevated platform of perforated steel to reduce HGI.[18] The Dassault Mirage IIIV VTOL research aircraft only ever operated vertically from a grid which allowed engine exhaust to be channeled away from the aircraft to avoid suckdown and HGI effects.[19]"

Since the rocket is not a winged aircraft or helicopter, there's not much left but this effect from the link provided.

Lordy back to you

 

[berkeman]

Hmm, that's a different version of the ground effect than I'm used to; I'm used to the ground effect adding extra lift to a winged aircraft near the ground/water (even if only gliding). I suppose that there may be some other considerations for air-breathing VTOL engines, as you say. But I'm also not sure that applies to the rocket issue in the OP. I any case, thanks for the thoughtful reply. Sorry if I was a bit offish.

 

[sophiecentaur]

Hmm, that's a different version of the ground effect than I'm used to; I'm used to the ground effect adding extra lift to a winged aircraft near the ground/water (even if only gliding). I suppose that there may be some other considerations for air-breathing VTOL engines, as you say. But I'm also not sure that applies to the rocket issue in the OP. I any case, thanks for the thoughtful reply. Sorry if I was a bit offish.

To me also, Ground Effect refers to the cushioning (increased pressure) of air / gases near the ground, which is common both to aircraft and rockets at take off - when they are both near the ground.
VTOL is not quite like either of those two so things could well be different; a pretty esoteric situation unless you are in that sphere of study. But Physics is full of exceptions so ho-hum.
Speaking of rockets taking off and associated pressures, the local pressure under the rocket will not be increased much, once the gas has spread out over a hemisphere. But perhaps the supersonic flow would change that simple idea.

 

[russ_watters]

Certainly I did. From the link in the OP:

"Early VTOL experimental aircraft operated from open grids to channel away the engine exhaust and prevent thrust loss from HGI.

Hmm, that's a different version of the ground effect than I'm used to;

It seems pretty clear to me from the quote that HGI *isn't* "ground effect", so I agree with the first reaction that it was a very odd thing to bring up.

Since the rocket is not a winged aircraft or helicopter, there's not much left but this effect from the link provided.

I agree that the usage of the term is generally if not exclusively about aerodynamic lift/drag on winged craft. And based on that, the usage of the term here (increase in "lift" of a rocket close to the ground) is a bit of a bastardization. But it's a bastardization that everyone else has seemed to understand and accept.

 

[anorlunda]

I remember reading that these sub-launched missiles have to attain a minimum height above the water before ignition of the rockets. Otherwise, the rocket blows up because of too much backpressure.

From that point of view, it's remarkable that the rocket in this video didn't explode because of the same problem. The launch pad can be excavated below ground to divert the exhaust gasses, but once it drifted away, that's no longer available.