Does a rocket lift off as soon as it fires?

01先拆开看：它靠什么产生推力？Look inside: what produces the thrust?

左侧结构图用剖面交代壳体、封头与供给路径，并放大发动机。它是原创的通用构型，不对应真实型号；结构尺寸与下面的一维飞行参数不成比例。

The structure view on the left uses a section to show the shell, tank domes and feed path, with the engine enlarged. It is an original, generic layout that matches no real vehicle, and its dimensions are not to scale with the 1D flight parameters below.

结构观察方式（与发射试验相互独立）How to view the structure (independent of the launch experiment)完整外壳Outer shell纵向剖面Lengthwise cutaway轴向分解Exploded along axis

聚焦一个发动机组件Focus on one engine component

两条管路分别进入喷注端与喷管附近。实际发动机还有泵、阀门、控制器等部件，本图没有用装饰件冒充完整发动机。储箱剖面显示空腔与封头，不显示推进剂液位或配比。

Two lines enter at the injector end and near the nozzle. A real engine also has pumps, valves and controllers; this figure does not use decorative parts to pass as a complete engine. The tank section shows cavities and domes, not propellant level or mixture ratio.

几何来源与构型边界Geometry source and design limits

实际调用 Anatomy 的 solidOf、bandsOf、ringBand、ribsOf、dotsOf、tubePieces 与投影工具生成这些表面。喷管采用解析半径与导数，使明暗带沿曲面连续；参考 Raptor 示例的车削件、法兰与分段管路做法。构型、尺寸、剖面和交互为本例另写，没有借用 Raptor 的性能参数。构建源为 scripts/build-rocket-visual.mjs，结果已内嵌，离线无需加载模块。

These surfaces are generated with Anatomy's solidOf, bandsOf, ringBand, ribsOf, dotsOf, tubePieces and projection helpers. The nozzle uses an analytic radius and derivative so shading bands follow the curved surface, following the Raptor example's turned parts, flanges and segmented piping. The layout, dimensions, sections and interaction were written for this lesson; no Raptor performance figures are used. The build source is scripts/build-rocket-visual.mjs; its output is embedded, so nothing loads at runtime.

Anatomy source · pinned revision. The geometry code is MIT licensed, Copyright (c) 2026 Ryan; the full license is embedded in this file. No claim is made that it passed the upstream all-pose geometry audit.

02先预测Predict

03改变推力，观察发射Change the thrust, watch the launch

先比较 40% 与 80% 油门在点火后 1 秒的状态。改变油门或载荷会开始一次新试验。

First compare 40% and 80% throttle one second after ignition. Changing throttle or payload starts a new trial.

油门Throttle80%

额外载荷Extra payload0 kg

点火后时间Time since ignition0 s

把时间推进至少 1 秒，即可检查解释。Advance time by at least 1 s to check the explanation.

04解释，再预测一个新情形Explain, then predict a new case

在发射台上，地面可以支撑火箭。若推力不超过当前重量，火箭不会立即向上加速。离开发射台后，净加速度由 a = F/m − g 决定。

这个模型在燃烧时减少质量。低于初始重量的推力，有时会在燃料消耗后足以让火箭起飞。不能把“开始时没起飞”说成“永远不会起飞”。

燃料耗尽时，推力变为 0，向上的速度仍可大于 0。重力使它逐渐减速，达到最高点后再下降。图中落地后停止，不模拟碰撞。

On the pad, the ground can hold the rocket up. If thrust does not exceed the current weight, the rocket does not accelerate upward. Once it leaves the pad, net acceleration is a = F/m − g.

This model loses mass as it burns. Thrust below the starting weight can become enough for liftoff once some fuel is used. “It didn't lift off at first” does not mean “it never will.”

At burnout, thrust drops to 0, but upward velocity can still be positive. Gravity slows the rocket until it peaks, then it falls. The figure stops at touchdown; it does not model the impact.

用你观察到的两组数据解释起飞条件。Use your two observed runs to explain the liftoff condition.

迁移问题Transfer question

模型、单位与边界Model, units and limits

这是一维教学模型，使用示意参数。干质量 800 kg，初始燃料 400 kg，最大推力 18,000 N，有效排气速度 1,800 m/s，重力加速度固定为 9.81 m/s²。燃烧流量为 ṁ = F / vₑ。滑块表示固定油门的一次试验，不是飞行中途改变油门。

高度和速度来自分段解析解，包含在台上消耗燃料、起飞、燃烧、熄火滑行与落地。忽略空气阻力、风、气压变化、地球曲率、分级和姿态控制。这不是轨道或真实运载火箭的性能预测。

查看物理来源

NASA Glenn 的火箭推力方程说明推力与排气流量、速度及压力有关。本例把压力贡献合并到有效排气速度中。NASA 的推重比说明与理想火箭方程提供模型的基础。数字均为教学选择。

This is a one-dimensional teaching model with illustrative parameters: dry mass 800 kg, initial fuel 400 kg, maximum thrust 18,000 N, effective exhaust velocity 1,800 m/s, and gravity fixed at 9.81 m/s². Mass flow is ṁ = F / vₑ. Each slider setting is one fixed-throttle trial, not a throttle change mid-flight.

Height and velocity come from a piecewise analytic solution covering fuel burned on the pad, liftoff, powered ascent, coasting after burnout, and touchdown. Drag, wind, changing air pressure, Earth's curvature, staging and attitude control are ignored. This is not an orbital or real launch-vehicle performance prediction.

Physics sources

NASA Glenn's rocket thrust equation relates thrust to exhaust mass flow, velocity and pressure; this example folds the pressure term into the effective exhaust velocity. NASA's thrust-to-weight ratio and ideal rocket equation pages underpin the model. All numbers are teaching choices.