Reaching Altitude

[Janie Leverone]

Givenhow it is significantly easier to achieve suborbital spaceflight, I was wondering about how a rocket spends its energy budget. If you magically could launch a rocket from 0 km/s but from an altitude of 100km, how much energy would you "save" for not having to climb that vertical distance.

I'm wondering this because someone suggested space planes could provide significant savings over rockets due to not having to expend as much energy climbing through the atmosphere, but my understanding is that most of the energy is expended in generating horizontal thrust, so getting to skip 25/50km of altitude but still having to accelerate laterally wouldn't produce savings. I tried finding the answer but I couldn't find much beyond variations on "getting to space: easy, staying there: hard".

One of the things to think about is rockets don't have energy budgets. They have delta-v budgets. Try and think of it that way but: The majority of the energy in a 100km orbit above earth is in 'going sideways'. However besides the lack of numbers this doesn't answer your question.For example how much dV is needed to gain attitude is at least the amount you would need to coast to your target altitude in a vacuum. But doing this as part of a way to get to orbit is only one (inefficient) way of getting there. The dV saving is at most this. There is no good bound in either direction.

How long you have to spend fighting gravity also depends on how long you need to keep the rocket in the air before centripetal force and gravity balance each other and there's no universal answer to that question. Aero-properties and TWR affect what's optimal.

Rockets don't "fish-hop"; they don't go up until they reach orbital height, then sideways until they reach orbital velocity, instead they try and combine the two maneuvers which takes advantage of Pythagoras, but costs some aero-dynamic drag.

Let's do the simplified theory first. If dV was the only concern and there was no atmosphere. With infinite TWR, you could reach a "sea-level" orbit without any energy or dV being expended upwards. Further the theoretically most efficient way to use dV to gain altitude is a Hohmann Transfer, the Hohmann transfer from sea level to 100km around earth is small. Concretely around 60m/s, which compared to 7800m/s for the orbital velocity in the first place is tiny.

So yes, there are many meaningful ways in which your intuition and the saying about staying there are correct. If you simplify things down too much, then especially when you consider that going there then going fast enough to stay there is not a very efficient way of doing things: then 100% lifting rockets up before you launch them is silly, it gains very little for a lot of effort.

However, these are way to big assumptions to ignore. When you add these factors back in things start to make more sense. Drag and gravity losses are a significant factor in current rockets and they are designed to compromise these things. If you didn't need to worry about TWR and being aerodynamic rockets could be made much lighter, which is the holy grail as this means less fuel is need, which makes them lighter. Etc, etc.

I know this is anti-climactic but its really hard to put a number on how much mass/money this would save as it changes a lot of factors, but its hard to imagine this not being at least a 20-30% mass saving.

Another thing is wings etc don't just get you altitude, they might also get you speed. By not needing to use valuable dV just to keep you up, you can fly through the air for long enough to make use of atmospheric engines. Jet engines are a lot more fuel efficient than rocket engines at low speeds, so if you can it would make sense in theory to want to use them when you can.

To achieve a low-earth orbit, you must spend about 9.4 km/s worth of fuel. Low earth orbit velocity is around 7.8 km/s. Most of the energy expended goes into moving sideways, and only about 1.6 km/s or 17% is spent fighting gravity/aerodynamic forces/gaining height.

so i start another fly, this time clicking "to T/O configuration from AFL menu before taking off. the procedure is the same, i also tried to switch off and then on again the trim control. nothing to do, +300/500, -300/500 ft

I have only ever seen it do this when the plane is hitting the overspeed limit. The AP will nose up to slow down, then nose down to get back to altitude, then repeat. You can also see a similar behavior when operating at accelerated time, though for me that tends to be a yawing motion that gradually gets worse and worse.

I am struggling with flight simming itself, specifically the landing, and I would like to ask you all for help and advice. My problem is rather simple: I am unable to land manually, and if I am, then my plane is all over the place. I would like to get to the bottom of this issue, and I am getting back to the basics.

I was taught to set the MCP altitude before T/D at the G/S intercept altitude. As a result of this, I almost always have to go into vertical speed mode, because I will not be able to reach altitude/speed restrictions along the descent path. The problem THEN becomes I am descending too fast, I am picking up speed, and I have to compensate for that with spoilers, which is undesirable for me as I would prefer to last through a flight with minimal warnings (hah!).

Anyway, I would like to know if my initial altitude setting philosophy is plausible, and if you could identify any issues in my little rant, I would appreciate the criticism of my skills (yup) and even moreso a solution.

Secondly, I would like to ask what does what exactly does the MCP altitude selector do? In the instructional video, the narrator set his to 7000 feet, didn't press the button, and the plane started the descent. Upon almost reaching 7000 feet, he reset the MCP altitude to 2000ft WITHOUT pressing the knob. Would it make a difference if he did press it?

Pretty sure this isn't procedurally correct, but I usually set it too the first altitude restriction in my STAR. Or if I'm feeling lazy, the IAF altitude for my landing. Would be curious to hear how it's really done though.

In the video you are seeing, the airplane is flying in VNAV mode. That is the only mode that departs an altitude with only changing the MCP altitude window. When in VNAV, the FMS calculates your top of descent, but it will not descend unless it gets an altitude restriction to go to. That is what he is setting. And that is also why the airplane just continued down when he turned the MCP window further down to 2000. The VNAV did not have the MCP restriction anymore. There are other restrictions VNAV might have. But for you, that is another lesson.

That being said, the 777 almost always finds a way to bring you in hot in VNAV. At 10.000 feet, I'm comfortable with about 45nm to go to touch down (Track miles). This can be read from the progress page. It is the distance remaining to your destination airport's 4 letter ICAO code. Anything less than 45 and you will have to extend flaps early or use speed brakes down low "most probably".

The problem THEN becomes I am descending too fast, I am picking up speed, and I have to compensate for that with spoilers, which is undesirable for me as I would prefer to last through a flight with minimal warnings (hah!).

the speedbrakes are there for a reason, and the vnav path is not really calculated with the idea that you won't be using them. it's not a warning, it is a planned spot where you will focus on reducing airspeed instead of altitude. i am no expert but as i understand it, it's more efficient to descend at 280 or the econ speed until you reach the 10000/250 restriction and then slow down using the brakes when necessary.

now, if you want to minimize the speed difference between your descent speed and the speed you'll be using below 10000 try setting a slower speed in the descent page, if you set it to 250 or 260 you can end up not really needing to use the brakes by the time you reach 10000. you'll also see this is a lot slower, if you spend an extra 5 or 10 minutes on descent, even at idle thrust you are burning more fuel than if you just use the brakes when it's necessary to fly at a slower speed. there are dots that are drawn on the path which indicate where it is expecting a planned deceleration (and that is where the drag required message pops up, along with large deviations above the path in general.)

also making sure to fill out your descent forecast page can be helpful in ensuring that your TOD is calculated to the ideal spot. a strong tailwind can really make it harder to descend in time and this will compensate for that.

Plan the descent to arrive at traffic pattern altitude at flaps up maneuvering speed approximately 12 miles from the runway when proceeding straight-in or about 8 miles out when making an abeam approach. A good crosscheck is to be at 10,000 feet AGL, 30 miles from the airport, at 250 knots.

To the OP, have you verified that the only thing you have engaged is VNAV/LNAV?...no speed in the MCP speed window? AT is engaged all the way down to landing? Are you minitoring the VNAV path in the ND? Also, from a bit further up in the FCTM...

Seems to me that setting the FAF altitude wouldn't be a bad idea...less workload if you know ATC isn't going to assign you any altitudes along the way. On the other hand, even in simpler cases, the MCP can miss altitudes/airspeed restrictions without a bit of help so, putting in the altitudes along the way can help.

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